Interrelationships Among Water Quality, Lake Morphometry, Rooted Plants and Related Factors for Selected Shallow Lakes of West-Central Minnesota
Part of a Series on Minnesota Lake Water Quality Assessment
March 2005
Interrelationships Among Water Quality, Lake Morphometry, Rooted Plants and Related Factors for Selected Shallow Lakes of West-Central Minnesota
Part of a Series on Minnesota Lake Water Quality Assessment
Steven Heiskary and Matt Lindon
Minnesota Pollution Control Agency Environmental Analysis & Outcomes Division
March 2005
TTY (for hearing and speech impaired only): (651)282-5332 Printed on recycled paper containing at least 10% fibers from paper recycled by consumers Acknowledgments
Study design – This study was a collaboration among MPCA staff (Steve Heiskary, Matt Lindon, Mark Gernes, and Howard Markus; MDNR staff (Donna Perleberg and Nicole Hansel-Welch) and Science Museum of Minnesota (Mark Edlund)
Report contributors - Portions of the document were drawn from analyses and reports from the following persons who collaborated on the production of this report:
Lauren Anderson - MPCA student intern – data analyses, charts;
Nicole Hansel-Welch – Wildlife Section, Minnesota Department of Natural Resources & Donna Perleberg – Ecological Services Section, Minnesota Department of Natural Resources Assessment of aquatic plant communities of study lakes, preparation of related statistics and general lake-specific plant community descriptions;
Dr. Mark Edlund, St. Croix Research Station, Science Museum of Minnesota – diatom reconstruction for study lakes;
Manuscript Review
John Barten – Three Rivers Park District, Hennepin County
Mark Gernes – MPCA, Environmental Analysis and Outcomes Division
Doug Hall - MPCA Environmental Analysis and Outcomes Division
Nicole Hansel-Welch - MDNR
Dr. Howard Markus – MPCA Environmental Analysis and Outcomes Division
Donna Perleberg – MDNR
Dr. David Wright – MDNR Ecological Services
Word Processing – Jan Eckart
Funding: This project was funded in part by an U.S. Environmental Protection Agency Nutrient Criteria Development grant. The study represents a collaborative effort among the Minnesota Pollution Control Agency, Minnesota Department of Natural Resources and Science Museum of Minnesota.
Table of Contents
Page
List of Tables ...... ii
List of Figures...... ii
Executive Summary and Recommendations………………………………………………...iii
Introduction...... 1
Background...... 4 Precipitation ...... 9 Lake levels ...... 10 Methods...... 13 Results...... 15
Lake-specific summaries for 2003 lakes...... 20
Comparisons among lakes for 2003...... 82
Total phosphorus, chlorophyll-a and Secchi relationships ...... 88
Association between submerged macrophytes and water quality...... 103
Sediment Diatom reconstruction – statewide and shallow lakes studies...... 112
Discussion...... 115 Deriving nutrient criteria for shallow lakes ...... 115
References...... 123
Appendix ...... 129
i List of Tables
Page
1. Lake morphometry and watershed data ...... 6 2. Ecoregion percentile distributions based on assessed lakes for 2004...... 8 3. Ecoregion reference lake typical range for summer mean water quality...... 8 4a Summer-mean water quality measurements for 2003 ...... 16 4b Summer-mean, minimum and maximum measurements for field parameters for 2003 ....18 5. Comparison of observed vs. predicted TP, chlorophyll-a and Secchi for CHF lakes….. ..98 6. Summary of morphometric and water quality characteristics for FQI classes ...... 117
List of Figures
1. Total phosphorus concentration as a function by lake mixing type & ecoregion ...... 5 2. Location of study lakes overlain on ecoregion map...... 7 3. Water Year Precipitation: 2003...... 9 4. Lake level measurement for selected lakes ...... 11 5. Total phosphorus concentrations for summer 2003 ...... 84 6. Comparisons of a) TKN + TP and b) TN:TP ratios and TP for 2003 study lake...... 86 7. Chlorophyll-a concentrations for summer 2003...... 87 8. Monthly mean total phosphorus and chlorophyll-a for west central lakes for 2003...... 89 9. Carlson’s TSI...... 90 10. Summer-mean TP vs. chlorophyll-a (log-log) and Secchi a) all west-central lakes (n=31 lakes) compared to statewide and b) CHF WC lakes only (n=19) compared to SW lakes (2002 study)...... 91 11. Summer-mean TKN versus chlorophyll-a for west-central lakes:2003...... 92 12. Bloom frequency relative to TP for west-central lakes...... 93 13. Summer-mean Secchi versus total phosphorus and chlorophyll-a for west-central lakes ...... 95 14. Observed a) TP, b) chlorophyll-a and c) Secchi versus predicted for CHF lakes...... 96 15. Summary of late-summer zooplankton populations...... 99 16. Floristic Quality Index (FQI) values for west-central lakes...... 104 17. Number of submerged and floating-leaf plants species for west-central lakes...... 104 18. Number of plant species (a) & FQI (b) relative to total phosphorus...... 105 19. Number of plant species (a) & FQI (b) relative to TKN...... 106 20. Number of a) plant species and b) FQI relative to TN:TP ratio...... 108 21. Submergent and floating-leaf plants relative to a) Secchi and b) maximum depth of native plant colonization relative to Secchi transparency...... 110 22. Number of plant species relative to TP, chlorophyll-a and Secchi ...... 111 23. FQI relative to total phosphorus and chlorophyll-a ...... 112 24. Comparison of diatom-inferred pre-European and modern-day TP for a) west-central shallow lakes and b) among ecoregions ...... 114 25. Summer-mean TP distribution (IQ range) for CHF ecoregion ...... 119 26. Summer-mean Chlorophyll-a distribution (IQ range) for CHF ecoregion ...... 119 27. Summer-mean Secchi distribution (IQ range) for CHF ecoregion ...... 120
ii
Executive Summary and Recommendations
The Minnesota Pollution Control Agency (MPCA) is developing eutrophication standards for Minnesota lakes and reservoirs. This effort is in response to a U.S. Environmental Protection Agency (EPA) requirement that states develop nutrient criteria for lakes, rivers, wetlands and estuaries. In the course of work on ecoregion reference lakes, assessment of statewide data sets for 305(b), and the development of guidance for listing of nutrient-impaired lakes on Minnesota’s 303(d) list it was apparent that there are some distinct differences in trophic status and potentials of shallow, well-mixed lakes as compared to deeper, stratified lakes. Previous work documented, for instance, the distinct difference in total phosphorus concentrations among dimictic, intermittently mixed, and well-mixed lakes (Fig. 1; Heiskary and Wilson, 1988). From this work it was evident that differences were particularly marked among deep and shallow lakes of the North Central Hardwoods Forests (CHF) and Western Corn Belt Plains (WCP) ecoregions. Also during public comment periods and hearings associated with the establishment of guidance for the listing of nutrient-impaired waters, concerns were expressed that swimming may not be the primary use in many of the states shallow lakes. Among their contentions was shallowness of the lakes, highly organic substrates, and often times over-abundance of rooted submergent and emergent plants. Because of these factors it was recommended that the MPCA consider separate nutrient criteria for shallow lakes that would take these factors into account. This would result in criteria that were more closely attuned to the actual uses of these shallow lakes, which are commonly boating, fishing, aesthetics, wading and waterfowl production, rather than an emphasis on swimming (primary body contact).
The current study of shallow lakes in west central Minnesota built on a previous effort that focused on shallow lakes of southwestern Minnesota (Heiskary et al. 2003). While that study was valuable for characterizing water quality patterns among lakes in that portion of the state it provided minimal insight into relationships among nutrients and rooted plants since most of the lakes were highly eutrophic and had very minimal submerged plant populations. However, that study did provide insights into pre-European trophic status of shallow lakes in that part of the state. These concerns led us to seek a Nutrient Criteria Grant through the USEPA. The overall project, which involved water quality sampling, assessment of submerged vegetation, collection of lake sediment cores for selected lakes in west central Minnesota was a collaborative effort among the MPCA, MDNR and Science Museum of Minnesota.
Of nine lake-sediment cores taken in 2004, from the west-central lakes, diatoms were adequately preserved or in sufficient numbers to allow for estimates of pre-European TP in seven lakes (Fig. 24a). Of these, six are from the CHF ecoregion and one, Red Sand, is from the NLF ecoregion. Based on this comparison CHF pre-European TP ranged from a low of 27 µg/L in Fremont Lake to 51 µg/L in Quamba Lake (Fig. 24a). The average change (modern- day TP minus pre-European) was 60 µg/L (184% increase) and ranged from 20 µg/L (50%) in Johanna up to 132 µg/L (469 %) in Silver Lake. When diatom-inferred TP from these six lakes was compared to those from deeper CHF lakes (Fig. 24b) it is evident that pre-European TP was higher in the shallow lakes and the magnitude of change from pre-European to modern- day was greater as well.
iii This study did not develop a predictive model; rather we characterized linkages among nutrient concentration, algal abundance and composition, macrophyte (submergent and floating-leaf) composition and coverage, fishery composition and management and related factors based on a set of representative shallow lakes from across west central Minnesota. These linkages combined with region-wide patterns in lake trophic status (both pre-European and modern- day), user perception and literature review, provide a basis for establishing nutrient criteria to protect uses such as secondary contact (boating and aesthetics) and fish and waterfowl habitat. In summary, based on the various interrelationships among trophic status variables, rooted plant metrics and other considerations it appears that appropriate ranges for selecting eutrophication criteria values for shallow lakes in the CHF ecoregion are: • Secchi transparency - greater than 0.7 to 1.0 meters; • Chlorophyll-a - less than 20 – 30 µg/L; • Total phosphorus – less than 60 – 80 µg/L;
Given this range of values, and acknowledging that other biotic and abiotic factors can be very significant in determining whether a lake can support a healthy and diverse population of rooted macrophytes, we are inclined to recommend criteria be set at the lower end of each range of the aforementioned values, i.e. maintain summer average Secchi of 1.0 m or greater, summer average chlorophyll-a of 20 µg/L or lower, and summer average total phosphorus of 60 µg/L or lower. While we are not offering nitrogen criteria at this time, it would appear to be beneficial to keep TKN below 2.0 mg/L when possible. Based on the relationship between TP and TKN, maintaining TP below 60-80 µg/L should yield TKN <2.0 mg/L.
Maintaining values at or below these ranges will not absolutely ensure that a shallow lake will remain in a macrophyte-dominated state and support the various uses described for 2b & 2c waters (Minn. Rule Ch. 7050), but should reduce the likelihood that the lake will switch to an algal-dominated state, which as repeatedly noted in the literature can be rather hard to reverse once the change has occurred. Also, maintaining trophic status values at or below these ranges should decrease the likelihood that curly-leaf, a non-native species, will become dominant and further contribute to a shift towards algal dominance. Lakes currently below the TP and chlorophyll-a thresholds should be protected against further increases in TP whenever possible because as these shallow lakes become increasingly nutrient-rich these nutrients will yield distinct increases in chlorophyll-a, which in turn will contribute to reduced transparency and increase the likelihood of a shift from plant-dominance to algal dominance. For lakes currently above these levels reducing TP to 60 µg/L or lower should result in reductions in chlorophyll-a and improved transparency. While this should increase the likelihood of a shift to plant dominance it cannot be guaranteed because of numerous biotic and abiotic factors noted in this study and in the literature on this topic.
This study also provided some insights that may be helpful in structuring future studies to refine these interrelationships in shallow lakes or wetlands. A summary follows: • Water quality (trophic status) will fluctuate temporally in shallow lakes both as a result of external loading and internal nutrient recycling (e.g., wind resuspension, redox- related recycling, rough fish resuspension, curly-leaf senescence, and other factors). These sometimes rapid changes may not be adequately captured by monthly sampling and hence we would recommend that biweekly sampling during the summer months be used to characterize the water quality of the lake and provide improved insight into factors that contribute to changes in water quality. In the case of some of the lakes in
iv this study biweekly sampling may have yielded improved characterization of trophic status that in turn may help to explain some outliers in our analysis or may have helped to refine interrelationships. • While much of our focus was on TP, chlorophyll-a and Secchi, in this study, the role of nitrogen should not be ignored in shallow lakes and / or wetlands based on some recent literature and our findings here. This implies that TKN (and if funds permit nitrate-N) should be measured as a part of future studies. This will provide a basis for examining relationships among plant metrics relative to total nitrogen (TN). This would in turn provide insight as to the need for N criteria as a part of overall efforts to develop nutrient criteria. • Curly-leaf pondweed, a non-native plant, has long been recognized as a significant problem in Minnesota lakes and its impact is particularly pronounced in shallow lakes. Its dominance in a lake often leads to a reduction in native plant species (either directly or indirectly), mid-summer spikes in P that contribute to severe algal blooms and reduced transparency, and other ecological consequences. As such any future studies on shallow lakes should ensure that plant assessments are conducted early enough in the season (e.g., June) to allow for a good characterization of the extent of curly-leaf distribution in the lake, as this information is essential to assessing the condition of the lake and interrelationships among trophic status and rooted vegetation. In several lakes in this study we were unable to accurately describe the extent and dominance of curly- leaf as the surveys were conducted in late summer after the plant typically dies-back. • Fish, in particular common carp, black bullhead and fathead minnows play a significant role in the ecology (including density and diversity of native plant species) and water quality of shallow lakes. While we did not have quantitative estimates of fish composition for many of the lakes, since many were not actively managed as a fishery, we did have some qualitative estimates of the relative abundance of common carp and black bullhead for several of the lakes. While this information was not quantitative it was useful for understanding relationships in several of the lakes. This study and future studies would benefit from having a more detailed assessment of the fishery of each study lake, even if it was somewhat qualitative in nature.
v Introduction
The Minnesota Pollution Control Agency (MPCA) is developing eutrophication standards (also referred to as nutrient criteria development) for Minnesota lakes and reservoirs. This effort is in response to a U.S. Environmental Protection Agency (EPA) requirement that states develop nutrient criteria for lakes, rivers, wetlands and estuaries. As a part of this EPA suggests a variety of approaches and recommends that criteria be develop on an ecoregion-specific basis where appropriate (USEPA, 2000a). MPCA has had extensive experience in assessing lakes within the ecoregion framework and developed ecoregion-based total phosphorus criteria as a part of this effort in the 1980s (Heiskary and Wilson, 1989). Since that time MPCA has been making use of these criteria for goal setting in non point source projects and as a basis for listing lakes on Minnesota’s 303(d) Impaired Waters list (MPCA, 2003).
In the course of work on ecoregion reference lakes, assessment of statewide data sets for 305(b), and the development of guidance for listing of nutrient-impaired lakes on Minnesota’s 303(d) list it has become apparent that there are some distinct differences in trophic status and potentials of shallow, well-mixed lakes as compared to deeper, stratified lakes. Previous work documented, for instance, the distinct difference in total phosphorus concentrations among dimictic, intermittently mixed, and well-mixed lakes (Fig. 1; Heiskary and Wilson, 1988). From this work it was evident that differences were particularly marked among deep and shallow lakes of the North Central Hardwoods Forests (CHF) and Western Corn Belt Plains (WCP) ecoregions. Also during public comment periods and hearings associated with the establishment of guidance for the listing of nutrient-impaired waters, concerns were expressed by several commenters that swimming may not be the primary use in many of the states shallow lakes. Among their contentions was the shallowness of the lakes, highly organic substrates, and often times over-abundance of rooted submergent and emergent plants. Because of these factors it was recommended that the MPCA consider separate nutrient criteria for shallow lakes that would take these factors into account. This would result in criteria that were more closely attuned to the actual uses of these shallow lakes, which are commonly boating, fishing, aesthetics, wading and waterfowl production, rather than an emphasis on swimming (primary body contact).
Work on shallow lakes was first initiated in 2002, with the sampling of several lakes in the Western Corn Belt Plains (WCP) and Northern Glaciated Plains (NGP) ecoregions. The purpose of that effort was to assess trends and condition in several of the reference lakes in these regions, supply baseline data for select lakes that lacked basic water quality data and improve our understanding of the water quality and ecology (e.g., rooted plants, fish, etc.) of these shallow lakes (Heiskary et al. 2003). As a part of the 2002 study, surficial sediment cores were taken to help improve on an existing sediment-diatom model that is used to predict historical water quality based on the composition of the diatom community in lake sediment cores (Ramstack et al. 2003). In addition, six southwest Minnesota lakes had deep cores collected that allowed for the reconstruction of trophic status for lakes in this portion of the state. Since the southwest lakes were quite nutrient-rich and generally lacked submergent or floating-leaf plants, further work was proposed for shallow lakes of west-central Minnesota where there was a wider range in lake trophic state and a more diverse population of rooted plants.
1 These concerns led us to seek a Nutrient Criteria Grant through the USEPA. The overall project (including the work on southwest MN lakes) is intended to provide a basis for developing nutrient criteria for shallow lakes, where swimming may not be the primary use of the waterbody.
The study design and intent draws on the concept of “alternative states” for shallow lakes as described by Moss et al. (1996), Moss (1998) and numerous others, whereby shallow lakes may switch from relatively clear plant-dominated systems to cloudy, algal-dominated systems – both of which can be considered “stable states.” Excess nutrients (eutrophication) is often noted as one of the causes as lakes shift from plant to algal dominance. While exact thresholds (e.g., nutrient or chlorophyll-a) are not frequently noted some studies suggest inverse relationships between macrophyte coverage and phytoplankton chlorophyll-a in shallow lakes. For example, Portielje and Van der Molen (1999) note that when macrophyte coverage exceeds five percent in shallow lakes there is a significant drop in chlorophyll-a. Scheffer et al. (2001) note that shallow lakes often have certain critical levels (often at intermediate nutrient levels) whereby the system shifts catastrophically between two stable states. In other words lake response to excess nutrient loading is not linear; rather the lake may be stable until a threshold is reached and then an alternate stable state is reached quickly. In order to switch back nutrients must be reduced much further than they were previously to some “critical” level. Things that contribute to a loss of resilience in the system increase the likelihood that a drastic switch to an alternative state will occur.
While excessive rooted macrophytes in a lake may impede recreation, macrophytes are essential to the overall ecology and stability of lakes, shallow lakes in particular, and would serve to help provide for “support of aquatic life” as required for Class 2b or 2c waters (Minn. Rule Ch. 7050.0222 subp. 4 & 5; 2002). In terms of stability, Madsen (2001) notes that macrophytes help to slow water velocities, which increases sedimentation, decrease turbidity, thus increase light penetration and growth of macrophytes. Sediment accumulates in areas where macrophyte beds are located. The loss of macrophytes increases the likelihood of sediment resuspension. He notes Marsh Lake, west-central Minnesota, as a good example of this. When macrophytes were absent, resuspension of P occurred >30% of the time in summer. Scheffer (1998) also describes the feedback effects of macrophytes on turbidity whereby as plant biomasss increases sediment resuspension declines resulting in lower turbidity causing hysterisis in plant mass – water turbidity relationships. Moss (1998) notes further that macrophytes store nutrients (luxury uptake) which prevents algal growth, provide zooplankton refuge from predators which increases algal grazing, may also produce allelopathic compounds which inhibit algal growth, and stabilize the sediment which lessens resuspension of nutrients. Radomski and Goeman (2001) describe multiple benefits of macrophytes to the overall ecology of the lake with a particular emphasis on fish spawning and habitat and demonstrated that shoreline areas with diverse submergent vegetation exhibited significantly more fish than shorelines which had little or no vegetation.
Bird use of shallow lakes is often correlated with the abundance of aquatic macrophytes (Mitchell and Perrow, 1998). Changes in waterfowl use have been correlated with presence or absence of aquatic macrophytes in many shallow lakes, including Lake Christina in Minnesota (Hanson and Butler 1994). Many species of aquatic vegetation are consumed by waterfowl. Certain species of Potamogeton are especially important food sources for migrating waterfowl. Submerged aquatic vegetation usually supports a wide array of invertebrates that are consumed
2 by waterfowl. These invertebrates are especially important protein sources for female and juvenile waterfowl. Emergent vegetation such as wild rice can also provide brood cover for waterfowl in addition to being a food source for migratory waterfowl. Aquatic vegetation also provides habitat for other types of aquatic birds such as grebes which nest in dense stands of bulrush.
Numerous attempts have been made to model factors that contribute to macrophytes growth. Herb and Stefan (2003) summarize several of these but note that “…however there is a lack of relationships that characterize the response of macrophyte growth to varying physical conditions.” They go on to present a growth model that relates the rate of production of rooted aquatic macrophytes to basic physiological parameters (growth and respiration rates) and controlling physical parameters (incident irradiance, water temperature, light attenuation by water and phytoplankton). While this work is quite valuable it does not offer specific eutrophication-related thresholds that can be translated into nutrient criteria for shallow lakes.
Our study does not seek to develop a predictive model; rather we will characterize linkages among nutrient concentration, algal abundance and composition, macrophyte (submergent and floating-leaf) abundance and composition, fishery composition and management and related factors based on a set of representative shallow lakes from across west central Minnesota. These linkages combined with region-wide patterns in lake trophic status (both pre-European and modern-day), user perception and literature review, will provide a basis for establishing nutrient criteria to protect uses such as secondary contact (boating and aesthetics) and fish and waterfowl habitat.
A general description of the study that was conducted in conjunction with the Minnesota Department of Natural Resources and Science Museum of Minnesota follows: a) Sampled 31 lakes monthly for water quality during the summer of 2003; b) Collected phytoplankton and zooplankton for qualitative and semi-quantitative measurement; c) Assessed macrophyte (submergent and floating-leaf primarily) composition and distribution were based on existing data or current field assessments by MDNR; d) Compiled fishery composition and management from MDNR records; e) Estimated watershed areas based on USGS web-based minor watershed maps; f) Collected surface sediment samples from all lakes and deep (long) cores from eight lakes to help document pre-European condition.
This report will: • Provide lake-specific descriptions of trophic status and plant communities for shallow west-central Minnesota lakes based on the aforementioned monitoring. • Examine interrelationships among trophic status variables and plant metrics; • Describe pre-European trophic status based on diatom reconstruction from these lakes; • Combine data from this study with that from the prior study of shallow southwest Minnesota lakes and propose potential nutrient criteria ranges for shallow lakes in the CHF, WCP and NGP ecoregions.
3 Background
Lake condition has been described for four of the seven Minnesota ecoregions, based on data from a set of reference lakes and a review of the overall data (water quality, watershed and morphometric) for the larger population of lakes in each region (MPCA, 2004). The reference lakes were deemed to be representative of the region they were located in and minimally impacted by human activities. In some ecoregions, such as the Northern Lakes and Forests (NLF) this was much easier to accomplish than it was for regions such as the Western Corn Belt Plains (WCP) or Northern Glaciated Plains (NGP) where agriculture may comprise over 70-80 percent of the watershed landuse – even for the reference lakes. This approach has provided a basis for comparing lake condition within and across ecoregions. As is evident from land use data from the reference lakes (Heiskary and Wilson, 1988) and ecoregions as a whole, cultivated land use predominates across both regions. This is followed by wetland/marsh land use and pasture/grass land uses. The percentage of pasture and grass uses was slightly higher in the NGP as compared to the WCP ecoregion. While specific data on landuses was not compiled for lakes in this study field observation suggests that land use in these watersheds is quite typical for the given ecoregion (Fig. 2).
As a part of the current (2005) rulemaking effort various definitions are included in the rule language. One definition that is pertinent to this study is “shallow lakes” that is defined as follows: “Shallow lakes are lakes with a maximum depth of 15 feet or less or where 80% or more of the lake is 15 feet or less in depth. Shallow lakes are not generally considered wetlands which are already defined in rule.” As such most of the lakes in this study fit that category, although a few are deeper than 15 feet (Table 1). The lakes in this study were located in three ecoregions in west central Minnesota (Fig. 2). The majority of the lakes studied in 2003 are located in the North Central Hardwood Forests (CHF); however a few from the Western Corn Belt Plains (WCP), Northern Glaciated Plains (NGP), and Northern Lakes and Forests (NLF) were included as well to ensure that we obtained a range in lake trophic status. A previous study in 2002 focused primarily on shallow lakes in the NGP and a few in the WCP (Heiskary et al. 2003) and these data will be used in the discussion of regional patterns and considered as a part of criteria development for shallow lakes in those regions.
Distinct differences in water quality among ecoregions (Table 3) have previously been documented (e.g., Heiskary and Wilson, 1988). These differences are typically a function of lake morphometry and watershed characteristics including landform, soil type and land use. The lakes of the WCP and NGP ecoregions are much more nutrient rich than those of the NLF and CHF ecoregions as is evident in both the assessed and reference lake data (Tables 2 and 3). These data will provide one basis for comparing the trophic status of lakes in this study to other lakes in the ecoregion. Within-region differences in summer-mean TP concentrations have also been described for each ecoregion based on the mixing (temperature stratification) status of lakes: dimictic (deep lake, fully mixes in spring and fall but remains stratified in summer); polymictic (shallow lake, remains well mixed from spring through fall); intermittent (lake with moderate depths, may stratify temporarily during summer, but may mix with strong wind action). The lack of stratification during the summer months contributes to frequent nutrient exchange between the sediments and the overlying water, thus internal nutrient loading in polymictic or intermittently stratified lakes can be high compared to deeper lakes and contribute to elevated epilimnetic phosphorus (illustrated in Fig. 1). Often even when nutrient loading is reduced in the watersheds, water quality in shallow lakes does not improve
4 for some time due to the internal nutrient loading (Scheffer, 1998 and Sondergaard et al. 1993). In this sense it is very important to protect these lakes from excessive loading because recovery can be a long and difficult process.
In general differences in TP were modest among the dimictic, intermittent, and polymictic lakes of the NLF ecoregion but were quite marked in the other two regions (Fig. 1). This and other factors previously noted led us to concentrate most of our shallow lakes efforts in the CHF, WCP and NGP ecoregions (2002 and 2003 studies).
Figure 1. Total phosphorus concentration as a function of lake mixing type and ecoregion. Based on assessed lake data (Heiskary and Wilson, 1988).
Median TP by ecoregion & lake mixing type (Heiskary and Wilson, 1988)
160
140
120
100
80 TP ppb 60
40
20
0 Deep Inter Shallow
NLF CHF WCP
5 Table 1. Lake Morphometry and Watershed Data.
Lake Lake County Ecoregion Area in Depth Depth Watershed Watershed Name ID Acres Mean Ft. Max Ft. Area Acres Lake Ratio Platte 18-0088 Crow Wing NLF 1746 10.0 23.0 19547 11 Clark 18-0374 Crow Wing NLF 343 15.0 31.0 12722 37 Red Sand 18-0386 Crow Wing NLF 502 7.0 23.0 4550 9 Tiger 10-0108 Carver CHF 575 3.0 8.0 4442 8 Diamond 27-0125 Hennepin CHF 406 6.0 8.0 811 2 French 27-0127 Hennepin CHF 352 2.0 3.0 3712 3 Prairie 27-0177 Hennepin CHF 34 3.0 6.0 92 3 Quamba 33-0015 Kanabec CHF 214 6.0 11.0 23241 109 Ringo 34-0172 Kandiyohi CHF 716 5.0 10.0 1471 2 Florida 34-0204 Kandiyohi CHF 772 2.5 4.0 42044 54 Slough Johanna 61-0006 Pope CHF 1584 7.0 12.0 6987 4 Nelson 61-0101 Pope CHF 403 6.0 9.0 820 2 Fremont 71-0016 Sherburne CHF 484 7.0 10.0 2458 5 Silver 72-0013 Sibley CHF 621 4.5 9.0 3747 6 Cedar 73-0226 Stearns CHF 90 20.0 36.0 1428 16 Cedar 73-0255 Stearns CHF 210 5.0 8.0 1225 6 McCormic 73-0273 Stearns CHF 211 7.0 12.0 984 5 Monson 76-0033 Swift CHF 153 12.0 21.0 1164 8 Trace 77-0009 Todd CHF 277 6.0 8.5 672 2 Pelican 86-0031 Wright CHF 2793 5.0 9.0 7705 3 Cedar 86-0073 Wright CHF 147 15.0 47.0 533 4 Smith 86-0250 Wright CHF 226 3.0 5.0 759 3 Jennie 21-0323 Douglas NGP 316 3.0 5.0 2113 7 Shaokotan 41-0089 Lincoln NGP 995 7.0 12.0 8400 8 West Twin 41-0102 Lincoln NGP 216 2.3 4.4 2026 9 East Twin 41-0108 Lincoln NGP 215 2.4 4.5 2026 9 Hattie 75-0200 Stevens NGP 477 6.0 9.0 11662 24 Hollerberg 76-0057 Swift NGP 260 3.5 5.0 3233 12 Hassel 76-0086 Swift NGP 706 4.0 5.0 23335 33 East 34-0246 Kandiyohi WCP 706 9.5 13.0 12907 18 Solomon Titlow 72-0042 Sibley WCP 924 2.0 4.0 35393 38 Mean 570 6.3 12.1 7813 15 25th percentile of 216 3.0 5.0 1195 3 study 75th percentile of 664 7.0 12.0 7705 12 study
1. Lake area from MDNR bathymetric maps; mean and maximum depth based on maps and/or field measurements during water quality or plant surveys. 2. Based on USGS minor subwatershed maps at: http://gisdmnspl.cr.usgs.gov/.
6 Figure 2. Location of study lakes overlain on ecoregion map.
7 Table 2. Ecoregion Percentile Distributions based on Assessed Lakes for 2004. Ecoregion Parameter 5 10 25 50 75 90 95 N NLF & NMW Area (acres) 15 22 49 129 347 835 1,654 1,809 NLF & NMW Depth-max. (feet) 7 10 19 33 54 80 100 1,519 NLF & NMW TP ppb 7 9 13 21 30 45 58 863 NLF & NMW chlorophyll-a ppb 2 2 3 5 8 14 22 521 NLF & NMW Secchi (m) 0.9 1.2 1.8 2.8 4 5.1 5.9 1,394 CHF & RRV Area (acres) 13 22 58 165 400 984 1,754 976 CHF & RRV Depth-max. (feet) 6 8 16 28 46 68 82 829 CHF & RRV TP ppb 15 18 28 51 112 229 351 691 CHF & RRV chlorophyll-a ppb 3 4 8 21 45 89 131 622 CHF & RRV Secchi (m) 0.4 0.5 1 1.6 2.6 3.5 4.2 968 WCP Area (acres) 32 61 143 322 694 1,776 2,222 110 WCP Depth-max. (feet) 4 6 7 10 16 25 33 87 WCP TP ppb 54 62 99 159 234 404 609 89 WCP chlorophyll-a ppb 11 14 32 50 83 125 173 79 WCP Secchi (m) 0.2 0.3 0.4 0.6 1 1.5 2.2 109 NGP Area (acres) 80 108 150 364 658 2,091 4,700 38 NGP Depth-max. (feet) 4 5 8 10 15 18 25 28 NGP TP ppb 46 54 104 148 194 396 405 30 NGP chlorophyll-a ppb 4 9 25 36 52 64 66 27 NGP Secchi (m) 0.3 0.4 0.5 0.7 1.6 1.9 2.1 37 Table 3. Ecoregion reference lake typical range for summer-mean water quality. Parameter Northern Lakes North Central Western Corn Northern and Forests Hardwood Forests Belt Plains Glaciated Plains Total Phosphorus (ug/l) 14 – 27 23 - 50 65 - 150 130 - 250 Chlorophyll mean (ug/l) 4 – 10 5 - 22 30 - 80 30 - 55 Chlorophyll max (ug/l) < 15 7 - 37 60 - 140 40 - 90 Secchi Disk (feet) 8 – 15 4.9 - 10.5 1.6 - 3.3 1.0 - 3.3 (meters) (2.4 - 4.6) (1.5 - 3.2) (0.5 - 1.0) (0.3 - 1.0) Total Kjeldahl Nitrogen (mg/l) 0.4 – 0.75 < 0.60 - 1.2 1.3 - 2.7 1.8 - 2.3 Nitrite + Nitrate-N (mg/l) <0.01 <0.01 0.01 - 0.02 0.01 - 0.1 Alkalinity (mg/l) 40 – 140 75 - 150 125 - 165 160 - 260 Color (Pt-Co Units) 10 – 35 10 - 20 15 - 25 20 - 30 pH (SU) 7.2 - 8.3 8.6 - 8.8 8.2 - 9.0 8.3 - 8.6 Chloride (mg/l) 0.6 – 1.2 4 - 10 13 - 22 11 - 18 Total Sus. Solids (mg/l) < 1 – 2 2 - 6 7 - 18 10 - 30 Total Sus. Inorganic (mg/l) < 1 – 2 1 - 2 3 - 9 5 - 15 Turbidity (NTU) < 2 1 - 2 3 - 8 6 - 17 Conductivity (umhos/cm) 50 – 250 300 - 400 300 - 650 640 - 900 TN:TP ratio 25:1 - 35:1 25:1 - 35:1 17:1 - 27:1 7:1 - 18:1
8 Figure 3. Water Year 2003 Precipitation
Precipitation
In central and southern Minnesota there is a distinct east-west gradient in precipitation and evaporation; annual average precipitation declines as you move westward and evaporation increases. Typical annual precipitation in the eastern portion of the study area (just west of Metro area) is on order of 25-26 inches and declines to about 23-24 inches on the western edge of the study area. Precipitation in 2003 was about average to 2 inches below average in the eastern portion and 2-6 inches below normal in the western portion where the study lakes are located (Fig. 3). Evaporation increases in a northeast to southwest pattern for Minnesota. Typical evaporation in the eastern portion (e.g., Hennepin and Stearns Counties) of the study area is on the order of 35 inches and increases to 39 inches in southwest Minnesota. Thus, in most years, lakes of western and southwestern Minnesota experience a water deficit and declines in lake level are not uncommon.
9 Lake levels
Few of the lakes in the study had long-term lake level records. However, those that did provide some insight on recent and longer term fluctuations in lake level in shallow lakes of this region. Ringo Lake, in Kandiyohi County, is an example of a lake showing an overall decline in lake level over the past seven years – declining from about 1166.25 mean sea level (MSL) in 1995 to 1164.8 in 2002 (Fig. 4). This 1.5 - 2 foot decline in lake level is significant for a lake that is only five feet deep on average. Within-year fluctuations can be of this same order of magnitude. Nearby Florida Slough exhibited significant within-year variation (on order of two-three feet), but no long-term trend was evident. East Solomon Lake exhibited a decline from 1995 to mid 2001, but appeared to rebound in early 2002. Pelican Lake, in Wright County, exhibited higher water levels from 1993 through 1998 with a slight decline through 2002. Also exhibits some decline from the highs of 1993-1995. Fremont and Quamba Lakes do not appear to be as variable; however their individual records are not as extensive as the other lakes in Fig. 4. Lake level declines noted in 2003 (e.g., Kandiyohi County lakes) could be attributed in part to the below-normal precipitation during the 2003 water year in the western portion of the study area (Fig. 3).
From these data it is evident these shallow lakes undergo fairly significant changes in lake level – both over the long-term and seasonally, especially when the changes are considered relative to the mean and maximum depth of the lakes. The drought of 1987-89, for example, reduced lake levels on the order of two-three feet below average which, considering that many of these lakes have average depths on the order of three to six feet (Table 1), can represent a substantial portion of the volume of the lake. In contrast, the high precipitation and runoff that characterized 1993 resulted in lake levels that were two or more feet above average for lakes like Ringo and East Solomon.
Seasonal fluctuation of lake level is natural in shallow lakes and can impact water quality, sediments and rooted plant productivity. During dry periods large expanses of lake beds are often exposed and dry-down and consolidation of lake sediments occurs. In turn this may promote production of emergents like bulrush and cattails. However, declines in lake depth may negatively impact fish communities and the risk of winterkill may increase with loss of volume. If the water is relatively clear submergent plants may prosper under these conditions and as the lake slowly refills. Extended periods of high water level or rapid refilling (large bounce in lake level that may follow huge storms or extended wet periods), in contrast, can negatively impact both emergent and submergent plant stands and negatively impact water quality as well. Shoreline erosion, wind-induced resuspension, and algal production all contribute to low transparency and conditions that are not conducive to emergent or submergent plant growth. Algal dominance is often common under these conditions as there is little competition from rooted plants. Winterkill potential may be slightly reduced with the increased lake level; however in shallow lakes this often favors increases in common carp and bullhead populations.
10
Figure 4. Lake level measurements for selected lakes. Record from 1980 through 2003 included when available.
East Soloman 1140 1139 1138 1137 1136 1135 1134 Mean 1137.4 Ft. 1133 Elevation (Feet) 1132 1131 1130 5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
Ringo 1170 1169 1168 1167 1166 1165 1164
Elevation (Feet) 1163 1162 Mean 1165.4 1161 1160 5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
Florida Slough 1125 1124 1123 1122 1121 1120 1119
Elevation (Feet) 1118 1117 Mean 1120 Ft. 1116 1115 5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
11
Fremont 980 979 978 977 976 975 974
Elevation (Feet) 973
972 Mean 975.2 Ft. 971 970
5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
Quamba 1000 999 998 997 996 995 994 Mean 997.3 Ft.
Elevation (Feet) 993 992 991 990 5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
Pelican 86-0031 960 958 956 954 952 950 948
Elevation (Feet) 946 Mean 952.7 Ft. 944 942 940 5/13/1980 5/13/1982 5/13/1984 5/13/1986 5/13/1988 5/13/1990 5/13/1992 5/13/1994 5/13/1996 5/13/1998 5/13/2000 5/13/2002
12 Methods
Water quality samples were collected monthly from May through September at most lakes. A standard suite of parameters was measured in the lab: total phosphorus (TP), total Kjeldahl nitrogen (TKN), total suspended solids (TSS), total suspended volatile solids (TSV), alkalinity, chlorophyll-a and pheophytin. Typically one mid-lake site was sampled on each lake by means of an integrated sampler that was two meters in length. On very shallow lakes care was taken not to contact the bottom sediments when the sample was collected. In addition a profile of dissolved oxygen (DO), temperature, conductivity, pH and redox was taken as well. Plankton nets are towed vertically, typically 2-3 meters in these lakes, to collect zooplankton samples. These samples are viewed in the field for presence/absence of large daphnids. In addition some samples were retained for a rapid assessment of the relative composition of the zooplankton community by Dr. Bruce Monson. Likewise phytoplankton samples were collected from the integrated samples and were assessed by Dr. Howard Markus by means of a rapid assessment technique. Summaries of these two methods are included in the Appendix.
Watershed areas were estimated based on USGS minor watershed maps on the USGS website and /or MDNR’s watershed delineation project. Lake morphometry including surface area, and mean and maximum depth were derived from MDNR bathymetric maps. Fishery and lake level information was summarized from the MDNR Web site: http://www.dnr.state.mn.us/lakefind/index.html.
Water quality data for the study lakes are summarized in Tables 4a&b. All water quality data are stored in STORET and are available through MPCA’s Environmental Data Access web site: http://www.pca.state.mn.us/data/eda/index.cfm. Laboratory methods and quality assurance are consistent with other recent studies and an example is provided below.
Laboratory methods and precision estimates
(drawn from Heiskary and Markus, 2003).
Parameter Reporting EPA Precision: 1 Difference Limit & Units method mean difference as Percent number of observed
Total Phosphorus 10.0 µg.L-1 365.2 4.8 µg.L-1 2.7 % Total Kjeldahl N 0.1 mg.L-1 351.2 0.05 mg.L-1 2.8 % NO2 + NO3 0.01 mg.L-1 353.1 Total Suspended 0.5 mg.L-1 160.2 2.8 mg.L-1 9.6 % Solids
Total Suspended 0.5 mg.L-1 160.4 -- -- Volatile Solids Turbidity 0.2 NTU 180.1 -- -- BOD5 0.5 mg.L-1 405.1 0.15 mg.L-1 6.6 % Chlorophyll-a 0.16 µg.L-1 446.0 1.7 µg.L-1 7.4 % Pheophytin 0.27 µg.L-1 446.0 -- -- 1 Average of individual means of 10 duplicates and expressed as a % of measured concentrations.
13 Plant assessment and metrics used to evaluate plant data.
Submerged aquatic vegetation (SAV) was assessed by MDNR staff (one-time per lake during summer growing season) using a modified point-intercept method based on Madsen (1999). A minimum of 40 points was sampled on each basin as recommended by Newman (1998). Sampling points are laid out in grid pattern predetermined using GIS software. Points are navigated to using a GPS unit. Vegetation is sampled visually by tossing a grappling hook or double-sided rake. Species present at the sampling point are noted. Frequency of occurrence for each species was calculated as well as various diversity indices for the water body. Such information helps determine type, distribution and relative abundance of submerged vegetation. This information is used to calculate metrics such as the Floristic Quality Index (FQI), which provides a reflection of the relative health and diversity of the SAV community and is based on Nichols (1999) work in Wisconsin lakes. FQI is calculated as the average of the coefficient of conservatism (C) times the square root of the number of native plant species. Conservatism describes the degree to which a species will tolerate disturbance (Perleberg, 2003). C values from Nichols (1999) work have been adopted for use in Minnesota (Perleberg, 2003). Most of the sampling was done during the summer of 2003 (as was the water quality sampling); however in a few instances there was a need to rely on previous assessments of the plant communities. The year of collection, along with various plant community metrics are noted in appendix data.
Sediment Diatom Reconstruction
Sediment and core collection - Surface sediments were collected using a short gravity corer during summer of 2003 by MPCA personnel. The top 0-4 cm of surface sediment were extruded in the field and separated as 0-2 and 2-4 cm sections. Additionally, the bottom 2-cm sample of the gravity corer was collected. Long cores for top-bottom reconstructions were taken from 8 lakes from among the target lakes in central and west central Minnesota using a drive-rod piston corer equipped with a 2.4 m long, 7.5 cm diameter polycarbonate barrel (Wright 1991). When longer cores were needed, one or more Livingston cores could be collected from immediately below the initial 2.4 m drive.
Magnetic susceptibility, sub sampling and sample preparation - The top 20-30 cm of long piston cores were sectioned in the field in 2-cm increments until the core reaches a stiff consistency. The remaining core section were capped and transported to either SCWRS or directly to The Limnological Research Center (U of MN) for cold storage. For magnetics analysis, the cores were brought to room temperature, horizontally extruded in the laboratory, and separated into 1.0-1.7 m sections for magnetic susceptibility logging. Magnetic susceptibility provides a non-destructive measure of ferromagnetic particles within the core and were done using a Bartington MS2 core logging sensor with an automated track feed capable of manipulating up to 1.7-m core sections. Susceptibility measures were taken at 1-cm intervals; this is an integrated signal over a 5-10-cm length of core. Data was subsequently spliced at core breaks for plotting to identify the settlement horizon, which were characterized by an increase in the magnetic signature due to increased erosion following initial land clearance in the region (representing c. 1850-1880). Dates were extrapolated to approximately 1800AD and 1750AD for recovery of two pre-settlement sample intervals. Following susceptibility logging, the down core smearing were removed from the core exterior, the cores
14 sectioned in 2-cm intervals, and intervals returned to 4°C cold storage for possible later analyses. For diatom analysis, a small sample of fresh surface and down core samples were treated with dilute HCl to remove carbonates followed by addition of 30% H2O2 and heating for two hours at 85°C. Following cooling, samples were rinsed once daily for six days to remove oxidation byproducts, the remaining sample dried onto microscope cover slips and the cover slips mounted on microslides using Naphrax.
Microfossil counts and taxonomic harmonization - A total of 400-500 diatoms was counted along random transects from each sample. Diatom counts were converted to relative abundance by lowest taxonomic unit against total diatoms counted.
Numerical Analysis - Diatom species relative abundance were calculated for each surface sample and for bottom samples in each long core. Using harmonized taxonomy, the counts from the 24 additional lakes were appended onto the original Ramstack (1999) data set and reanalyzed as a pooled data set. Canonical correspondence analysis (CCA), a multivariate ordination technique for direct gradient analysis (ter Braak & Prentice, 1988), was used to determine the relationship between water quality variables and diatom distributions after combining the original and expanded MN lakes training set. Environmental variables that independently explain and that are most strongly correlated to diatom species distributions are identified by CCA and used as predictor variables in weighted averaging regression and calibration (Birks et al., 1990). Optima and tolerances were estimated for each species based on their distribution in the calibration data set using the software C2 (Juggins 2003). Reconstructed estimates of TP for each surface and down core sample were determined by taking the optima of each species, weighting it by its abundance in that sample, and determining the average of the combined species optima.
Results
Our presentation of results will be similar to that of the previous report on shallow southwestern Minnesota lakes (Heiskary et al. 2003). We will present a generalized description of each lake that includes: an overview of lake and watershed morphometry, summer 2003 water quality (Table 4a & b), algal composition, macrophyte composition and fishery management. Water quality data will be compared to the overall population of lakes for the ecoregion (Table 2) and to the reference lakes (Table 3) where appropriate. This will be followed by a cross-sectional analysis of all lakes in the study that will examine interrelationships among water quality, algae, macrophytes and related variables with an emphasis on nutrient concentrations. Recommendations on desirable water quality characteristics for shallow lakes, which allow for healthy submerged vegetation communities and consider ecoregion characteristics and constraints, will be offered.
15 Table 4a. Summer-mean Water Quality Measurements for 2003. Standard error (SE)
TP Chl-a Pheo-a TKN Color ChlorideAlk, Tot TSS VSS mg/L µg/L µg/L mg/L Pt-Co mg/L mg/L mg/L mg/L Tiger Mean 0.125 45.0 10.5 2.47 50 48 173 16.3 12.0 10-0108 N 3 3 3 3 3 3 3 3 3 SE 0.019 4.4 3.7 0.42 0 4 38 4.8 4.0 Platte Mean 0.033 11.8 11.9 0.89 35 3 69 3.2 2.5 18-0088 N 4 4 4 4 4 4 4 4 4 SE 0.001 1.9 10.0 0.02 3 0.2 2 0.3 0.3 Clark Mean 0.025 6.6 1.3 0.66 13 3 92 3.1 1.9 18-0374 N 4 4 4 4 4 4 4 4 4 SE 0.004 1.7 0.3 0.02 3 0.2 3 0.8 0.3 Red Sand Mean 0.033 4.6 1.0 0.93 20 12 66 2.5 1.7 13-0386 N 4 4 4 4 4 4 4 4 4 SE 0.005 0.8 0.3 0.06 0.1 0.5 5 0.5 0.3 Jennie Mean 0.214 113.7 23.9 5.45 20 20 160 73.0 58.0 21-0323 N 4 4 4 4 4 4 4 4 4 SE 0.025 13.5 9.7 0.28 0.1 1 7 9.4 3.5 Diamond Mean 0.177 73.1 4.5 2.28 23 41 130 25.7 17.3 27-0125 N 4 4 4 4 4 4 4 4 4 SE 0.083 28.0 1.7 0.50 5 1 7 8.6 7.8 27-0127 Mean 0.372 180.2 33.5 5.25 50 33 100 53.3 42.7 French N 3 3 3 3 3 3 3 3 3 SE 0.178 114.5 15.5 2.41 10 3 10 22.3 21.1 Prairie Mean 0.025 7.6 1.4 0.89 20 1 105 4.6 2.6 27-0177 N 5 5 5 5 5 5 5 5 5 SE 0.006 3.3 0.6 0.06 0.1 0.4 6 2.1 0.8 Quamba Mean 0.089 34.5 4.3 1.53 103 3 78 9.5 6.0 33-0015 N 6 6 6 6 6 6 6 6 6 SE 0.014 30.2 2.9 0.42 46 1 13 5.1 3.6 Ringo Mean 0.092 44.5 5.4 2.36 20 14 187 33.7 23.3 34-0172 N 7 7 7 7 7 7 7 7 7 SE 0.003 5.9 1.2 0.05 0.1 0.5 6 4.0 3.5 Florida Mean 0.136 49.3 13.8 2.13 33 12 193 14.2 9.4 Slough N 3 3 3 3 3 3 3 3 3 34-0204 SE 0.041 19.6 3.8 0.46 3 1 9 7.2 5.4 East Mean 0.095 17.3 3.6 2.24 23 20 275 7.7 4.2 Solomon N 5 5 5 5 4 4 4 4 4 34-0246 SE 0.008 6.7 1.2 0.25 3 0.5 3 1.9 1.4 Shaokatan Mean 0.180 27.0 3.0 3.45 20 9 190 14 6 41-0089 N 3 3 3 3 3 3 3 3 3 SE West Twin Mean 0.323 138.3 22.9 5.05 47 24 230 77.3 62.0 41-0102 N 3 3 3 3 3 3 3 3 3 SE 0.102 31.5 25.4 0.986 20 3 17 16.4 26.2 East Twin Mean 0.207 132.7 18.2 5.72 37 21 213 72.3 67.7 41-0108 N 3 3 3 3 3 3 3 3 3 SE 0.011 9.8 7.1 0.08 2 0.4 2 4.4 3.4
16 TP Chl-a Pheo-a TKN Color Chloride Alk, Tot TSS VSS Johanna Mean 0.083 34.8 6.0 1.86 20 11 138 18.1 9.4 61-0006 N 4 4 4 4 4 4 4 4 4 SE Nelson Mean 0.045 17.5 3.3 1.60 10 11 233 16.2 8.7 61-0101 N 3 3 3 3 3 3 3 3 3 SE 0.005 5.3 1.1 0.119 0.1 1 3 3.6 2.8 Fremont Mean 0.107 49.1 2.4 1.87 28 12 73 19.5 15.5 71-0016 N 6 6 6 6 6 6 6 6 6 SE 0.028 14.2 0.4 0.30 2 6 9 6.3 5.0 Silver Mean 0.159 63.5 5.8 3.31 25 21 162 36.2 25.1 72-0013 N 6 6 6 6 6 6 6 6 6 SE 0.022 12.9 1.7 0.420 2 2 10 6.9 6.7 Titlow Mean 0.229 42.5 12.4 2.14 23 36 223 67.7 11.0 72-0042 N 3 3 3 3 3 3 3 3 3 SE 0.058 12.9 2.3 0.269 3 3 12 21.3 4.0 Pelican Mean 0.020 4.9 1.1 0.66 8 18 115 2.0 1.6 73-0118 N 4 4 4 4 4 4 4 4 4 SE 0.004 1.0 0.3 0.02 1 0.3 3 0.2 0.1 Cedar Mean 0.014 5.7 0.8 0.87 20 5 180 2.8 2.1 73-0226 N 3 3 3 3 3 3 3 3 3 SE 0.013 1.7 0.3 0.049 0.1 0.2 0.0 1.6 0.8 Cedar Mean 0.035 4.1 2.3 1.06 20 13 119 3.1 2.4 73-0255 N 3 3 3 3 3 3 3 3 3 SE 0.006 0.7 1.4 0.057 0.1 1 13 0.8 0.8 McCormic Mean 0.060 12.5 3.0 1.49 18 18 115 5.4 4.4 73-0273 N 4 4 4 4 4 4 4 4 4 SE 0.013 5.1 1.2 0.104 2.5 1 12 1.3 1.1 Hattie Mean 0.315 40.2 11.8 2.27 20 19 250 37.8 10.4 75-0200 N 4 4 4 4 4 4 4 4 4 SE 0.052 17.1 4.2 0.225 0.1 1 11 15.7 3.8 Monson Mean 0.090 45.8 6.8 1.84 18 12 153 12.0 9.9 76-0033 N 4 4 4 4 4 4 4 4 4 SE 0.020 15.3 2.6 0.242 3 0.3 6 3.6 3.2 Hollerberg Mean 0.093 25.8 15.3 2.32 37 13 197 15.3 9.6 76-0057 N 3 3 3 3 3 3 3 3 3 SE 0.005 3.3 10.3 0.056 3 0.3 12 1.5 0.8 Hassel Mean 0.270 91.9 25.3 4.15 35 13 208 141.5 61.3 76-0086 N 4 4 4 4 4 4 4 4 4 SE 0.055 16.5 6.2 0.72 5 1 11 39.0 15.3 Trace Mean 0.119 26.2 13.7 2.07 28 64 153 12.0 10.9 77-0009 N 5 5 5 5 4 4 4 4 4 SE 0.037 13.1 10.4 0.46 3 2 13 5.7 5.3 Pelican Mean 0.169 94.6 6.9 2.78 30 12 82 32.4 27.4 86-0031 N 12 12 12 12 10 10 10 10 10 SE 0.010 10.2 1.4 0.19 0.1 0.1 1 3.4 3.1 Cedar Mean 0.019 4.4 0.9 0.80 23 10 101 2.2 1.5 86-0073 N 4 4 4 4 4 4 4 4 4 SE 0.002 0.9 0.1 0.01 3 0.3 3 0.6 0.2 Smith Mean 0.148 62.9 11.7 3.14 28 29 130 31.5 21.8 86-0250 N 4 4 4 4 4 4 4 4 4 SE 0.083 25.9 16.2 1.47 10 2 12 14.6 5.4
17 4b. Summer Mean, Minimum and Maximum Measurements for Field Parameters for 2003. Multi-parameter probe measures at 0.5 m. Temp DO- pH Spec. Cond. Secchi ORP Station_ID °C mg/L SU µmho/cm M (Redox) Tiger Mean 16.9 12.0 8.9 370 0.6 344 10-0108 Min 6.2 10.3 7.1 308 0.4 309 Max 23.2 13.8 10.1 410 0.8 390 Platte Mean 20.2 8.1 8.1 106 1.5 380 18-0088 Min 9.6 7.1 7.9 86 1.1 363 Max 24.6 9.6 8.4 131 2.3 415 Clark Mean 21.7 8.6 8.4 148 2.4 345 18-0374 Min 12.6 7.8 8.0 124 1.8 286 Max 26.1 8.9 8.6 182 3 376 Red Sand Mean 21.1 8.9 8.8 132 3.3 330 18-0386 Min 10.18 6.5 7.8 104 2.6 6.5 Max 26 11.8 9.3 187 4.3 11.9 Jennie Mean 23.1 10.4 9.6 498 0.3 320 21-0323 Min 18.5 8.0 9.4 438 0.2 284 Max 26.7 14.3 9.8 534 0.3 377 Diamond Mean 20.8 10.9 9.2 240 0.8 360 27-0125 Min 13.6 8.2 8.6 99 0.4 330 Max 25.3 15.1 9.9 359 2.0 415 French Mean 20.5 9.0 9.0 224 0.4 371 27-0127 Min 17.7 6.4 8.5 166 0.1 341 Max 26.3 12.6 9.6 269 0.5 388 Prairie Mean 23.1 9.6 8.5 145 1.0 356 27-0177 Min 19.6 8.3 8.2 108 1.0 345 Max 25.7 12.0 9.1 170 1.0 370 Quamba Mean 16.1 7.0 8.0 95 0.9 408 33-0015 Min 9.9 7.6 6.8 58 0.5 345 Max 22.6 12.7 9.5 144 1.5 471 Ringo Mean 23.4 8.2 8.8 276 0.5 309 34-0172 Min 20.4 7.5 8.5 0.3 269 Max 25.9 8.7 9.1 393 0.8 362 Fl. Slough Mean 22.0 8.6 8.8 344 0.8 323 34-0204 Min 20.0 5.4 8.8 294 0.3 278 Max 23.4 13.2 8.9 379 1.2 411 E.Solomon Mean 23.4 6.6 8.8 454 1.8 373 34-0246 Min 20.5 3.1 8.7 391 1.0 281 Max 26.7 9.8 9.0 543 2.8 460 West Twin Mean 23.0 12.4 9.3 476 0.2 327 41-0102 Min 17.5 10.8 9.3 461 0.2 298 Max 26.1 14.6 9.4 487 0.3 372 East Twin Mean 23.5 14.9 10.2 466 0.2 319 41-0108 Min 18.9 14.2 10.0 455 0.2 299 Max 26.0 15.8 10.3 474 0.3 356 Johanna Mean 23.0 10.7 9.1 272 1.3 323 61-0006 Min 18.5 8.4 8.8 159 1.0 250 Max 27.0 16.5 9.4 442 1.7 379
18 Temp DO- pH Spec. Cond. Secchi ORP Nelson Mean 22.0 9.4 8.9 376 0.8 320 61-0101 Min 19.0 8.7 8.7 323 0.6 250 Max 25.7 10.3 9.1 442 1.2 394 Fremont Mean 21.5 9.5 9.5 112 1.1 285 71-0016 Min 14.6 8.4 9.3 61 0.4 120 Max 24.4 10.5 10.0 135 2.1 391 Silver Mean 16.8 10.4 8.5 277 0.3 339 72-0013 Min 8.0 8.4 7.0 253 0.1 309 Max 23.1 12.4 9.8 304 0.5 378 Titlow Mean 17.7 8.9 8.4 452 0.2 405 72-0042 Min 8.3 6.1 7.6 336 0.1 366 Max 24.3 12.1 8.8 585 0.3 484 Cedar Mean 25.3 8.2 219 3.0 341 73-0226 Min 22.7 6.9 8.4 2.9 279 Max 26.9 9.1 366 3.0 429 Cedar Mean 24.6 10.3 9.3 203 1.4 312 73-0255 Min 19.3 6.4 8.6 141 1.3 256 Max 27.8 13.9 10.4 234 1.5 383 McCormic Mean 24.0 10.0 9.5 226 1.8 322 73-0273 Min 18.6 8.5 8.6 197 0.7 289 Max 27.8 11.0 9.8 257 3.4 364 Hattie Mean 22.5 7.4 8.7 502 0.5 368 75-0200 Min 18.6 6.5 8.4 0.2 316 Max 26.0 8.8 9.0 828 0.9 441 Monson Mean 23.2 8.3 9.0 528 1.5 366 76-0033 Min 20.2 6.5 8.9 204 0.6 280 Max 26.0 11.2 9.4 840 3.5 396 Hollerberg Mean 23.1 11.2 9.7 335 0.6 330 76-0057 Min 20.6 9.5 9.1 332 0.5 281 Max 28.0 14.3 10.0 337 0.7 386 Hassel Mean 23.4 9.3 8.8 376 0.2 339 76-0086 Min 19.8 6.9 8.3 350 0.1 275 Max 27.2 12.2 9.1 419 0.3 417 Trace Mean 24.7 9.2 9.0 423 1.1 337 77-0009 Min 19.9 8.3 8.6 377 0.4 302 Max 28.1 10.5 9.5 463 1.6 389 Pelican Mean 22.3 10.5 9.2 142 0.4 318 86-0031 Min 18.6 7.7 8.7 97 0.4 277 Max 25.6 12.9 9.5 182 0.6 361 Cedar Mean 24.8 8.4 8.5 146 3.6 364 86-0073 Min 22.6 7.5 8.4 77 2.8 324 Max 26.4 9.2 8.6 217 4.8 406 Smith Mean 24.9 10.0 9.0 241 0.4 334 86-0250 Min 17.7 7.6 8.3 179 0.2 273 Max 27.9 11.6 9.4 299 0.5 439 Mean 22.3 9.7 9.0 293 0.9 337 All lakes Min 6.2 3.1 6.8 58 0.1 120 Max 28.1 16.5 10.4 840 4.8 484
19 Lake specific summaries for 2003 lakes
1. Tiger Lake (10-0108) is located in Carver County, one mile west of Norwood Young America, within the Minnesota River basin and in the Central Hardwood Forest (CHF) Ecoregion. It has a surface area of 575 acres. Its 4,442 acre watershed is moderately sized relative to its surface area with a ratio of 8:1. The lake has two distinct bays: a larger, deeper bay to the south and a smaller, shallower bay to the north which is often used by duck hunters. Tiger Lake is surrounded by high, thick cattails and other emergent plant growth, making access and navigation difficult. It has a maximum depth of approximately 8.0 feet, though the average depth is around three feet, placing it within the 10th percentile compared to other lakes within the ecoregion. Water level data has been collected continuously since 1999 and shows an upward trend. There have been no DNR fishery surveys to date, though carp have been observed; fish populations struggle due to winterkills.
Summer-mean total phosphorus (TP) at 120 µg/L is considerably higher than the typical range for the CHF ecoregion (Table 3), with individual measures ranging from 98 to 160 µg/L. This range of concentrations is consistent with data from 1999 when TP averaged 150 µg/L. Chlorophyll-a averaged 21 µg/L, less than predicted by Carlson’s Trophic State Index (TSI) but was similar to the 1999 results of 18 µg/L, with a range of 36 to 50 µg/L. While TP and chlorophyll-a were in the mid range for this study, the levels were in approximately the 75th percentile for all the lakes in this ecoregion. Secchi was relatively low at 2.3 feet. Total suspended solids (TSS) averaged at 16.3 mg/L and volatile suspended solids (VSS) at 12 mg/L with ranges of 11 to 26 mg/L and 7.3 to 20 mg/L, respectively. Few zooplankton were observed based on vertical net-tows.
Tiger Lake TP and chlorophyll-a concentrations:2003
Tiger Lake (10-0108) Total Phosphorus and Chlorophyll-a results 2003 200 162 150 114 98 100 PPB 48 50 50 36
0 10-Jun 16-Jul 1-Oct PPB TP PPB Chl-a
20 The most common algae in July were blue-green algae and algae blooms were likely quite apparent because of their dominance. In October, both blue-green algae and diatoms were the most common forms and the most dominant genus was the yellow brown alga, Dinobryon.
Tiger Lake algal composition
Tiger Lake 10-00108
100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Oct-03 Blue Greens Diatoms Greens Yellow Browns
MDNR vegetation survey
This lake has wildlife lake surveys from 1957, 1985, 1999, and some information from 2002. This lake is divided into two connected basins, a northwest basin and a southeast basin. In both the 1957 and 1985 surveys, submerged vegetation was more abundant in the SE basin. In 1957 and 1985, 40% of the lake was covered with emergent vegetation including cattail and soft stem bulrush. Maximum water clarity was 5 feet as was maximum depth in 1957. Water was shallower in 1985, when the maximum depth was 3.5 feet. Ceratophyllum demersum (coontail) was the most common submerged plant in 1957; sago was the most common in 1985. In 1999, maximum depth found during the survey was 3.3 feet and sago pondweed was the most abundant plant. In 1999, 13 submerged aquatic plants were found, and their floristic quality index (FQI) was 18.028. The lake-wide species richness (number of species identified) was 15. Over the years the coverage of emergent vegetation has fluctuated in abundance depending on water levels. However, emergent vegetation has always been more abundant in the NW basin and the SE basin has a wide fringe of emergent vegetation, primarily cattail now and an open-water area in the center.
21 2. Platte Lake (18-0088) is located in Crow Wing County six miles northeast of Harding in the Northern Lakes and Forests (NLF) Ecoregion and was studied as a part of the 2002 Citizen Lake Monitoring Program Advanced (CLMP +) Program (http://www.pca.state.mn.us/water/clmp- publications.html#reports). The lake has four inlets and one outlet to Sullivan Lake. Platte Lake has a rocky shore dotted with emergent plant growth. Its maximum depth is 23.0 feet and its mean depth is 10.0 feet. With a watershed area of 19,547 acres and a watershed to lake ratio of 11.2:1, Platte’s watershed has a significant impact on lake water quality. Platte Lake itself is 1,746 acres in area, making it the second largest lake assessed in this study and placing it in the 95th percentile in the NLF Ecoregion (Table 2). It is also above the 75th percentile for mean depth, maximum depth, and watershed area when compared to other lakes in this study (Table 1). Due to extensive submergent plant growth, the water has a green appearance. Curly-leaf pondweed was common throughout the lake. MDNR fishery surveys since 1995 have noted an increase in low dissolved oxygen (DO) tolerant species, such as northern pike and bullhead. In a 1997 survey, black and brown bullheads were below typical ranges and yellow bullheads were more abundant than expected. Platte Lake has a low carp population and is aerated occasionally.
Summer-mean TP at 32 µg/L is just above the typical range for the NLF Ecoregion (Table 3). Historically, TP has been higher: in 1981 - 40 µg/L and in 2002 - 35 µg/L. Chlorophyll-a varied somewhat as well: 1981 - 19.2 µg/L, 2002 - 10.6 µg/L, and 2003 - 12.0 µg/L. These levels of chlorophyll-a are similar to that predicted by Carlson’s TSI based on TP value. Average Secchi was 4.8 feet (1.5 m), which is lower than the typical range for the NLF ecoregion. TSS at 3 mg/L and VSS at 2 mg/L were within the typical range for the NLF ecoregion.
Platte Lake TP and chlorophyll-a concentrations:2003
Platte (18-0088) Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB
50 30 34 31 36 10.1 16.1 0.47 9.07 0 4-Jun 8-Jul 20-Aug 9-Sep PPB TP PPB Chl-a
22 Blue-green algae dominated throughout the summer with Anacystis dominant in both June and September, and Anabaena being dominant in August.
Platte Lake algal composition Platte Lake 18-0088 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey
Platte Lake was surveyed in June 2003 and submerged plants were found to a maximum depth of 19 feet but were most common in depths less than 16 feet. The exotic plant curly-leaf pondweed (Potamogeton crispus) dominated the submerged plant community and was found in 39% of the sample sites. It was most abundant in depths of six to fifteen feet where it occurred in about 54% of the sample sites and within this zone, curly-leaf often formed dense mats that reached the water surface. Curly-leaf was the only plant found in water greater than 15 feet.
Despite the extensive growth of curly-leaf pondweed, Platte Lake supports one of the richest native plant communities in central Minnesota. Twenty native submerged species were recorded including coontail (Ceratophyllum demersum), bushy pondweed (Najas flexilis), flatstem pondweed (Potamogeton zosteriformis), Robbin’s pondweed (Potamogeton robbinsii), white-stem pondweed (Potamogeton praelongus) and large-leaf pondweed (Potamogeton amplifolius). Five different floating-leaf species were found including white waterlily (Nymphaea odorata) and yellow waterlily (Nuphar variegata) and a diversity of emergents such as wild rice (Zizania aquatica), bulrush (Scirpus spp.), and cattails (Typha spp.).
Prior surveys Quantitative plant survey data prior to 1994 were not available, however it appears that Platte Lake has historically supported a diverse and abundant aquatic plant community. In a 1955 Dept of Conservation memo, W. Gulbranson discusses the abundant wild rice in the channel between Platte and Sullivan Lakes “It appears to be a very good crop of wild rice covering many acres. There are a large number of people on Platte Lake and they do not see why their lake should be left to grow up into a rice and weed bed.” In recent years many lake users have perceived an increase in dense aquatic vegetation, potentially related to a recent increase in curly-leaf pondweed growth. Past surveys have been conducted in late summer after curly-leaf has died back and hence may have underestimated its abundance.
23 3. Clark Lake (18-0374) is located in Crow Wing County near the town of Lake Hubert in the NLF ecoregion. The lake has two distinct bays, one outlet, and two inlets both from other lakes. It has a maximum depth of 31.0 feet and a mean depth of 15.0 feet. Clark Lake’s area is 343 acres and its watershed area is 12,722 acres, giving it a watershed to lake ratio of 37:1 which is mostly attributed to contributing minor watersheds. Clark Lake is within the 75th percentile for maximum depth, mean depth, watershed, and watershed to lake area ratio for lakes in this study. According to MDNR, aquatic vegetation is quite abundant. In a 1999 fish survey, northern pike of various sizes were found above the typical range. Carp do not seem to be a problem within the lake. Black bullhead numbers were relatively high for this class of lake, but the ratios of black: brown or black: yellow bullheads were quite low. The lake is also subject to partial winterkills.
Summer-mean TP at 24 µg/L is within the typical range for NLF ecoregion lakes (Table 3). Chlorophyll-a averaged 7 µg/L, similar to the value expected by Carlson’s TSI based on TP. Secchi disk averaged 2.5 m based on MPCA data, which was high compared to other lakes within this study. Historically, TP (summarized below) averaged about 48 µg/L in 1974, 1975, and 1976 and about 24 ug/L in 2000, 2001, and 2002. Over these same two periods Secchi averaged 2.3 m and 3.0 m, respectively. This suggests recent TP concentrations are lower and Secchi values are higher in recent years as compared to the mid 1970s.
Clark Lake TP and chlorophyll-a concentrations:2003
Clark (18-0374) Total Phosphorus and Chlorophyll-a Historic Summer-mean Data results 2003 200 Mean Mean Year TP Secchi (m) 150 (ppm) 1974 0.050 2.5 100 PPB 1975 0.030 2.2 1976 0.065 2.4 50 30 32 20 17 2000 0.024 2.8 11 3.2 6.5 5.5 0 2001 0.016 3.2 25-Jun-03 29-Jul-03 25-Aug-03 29-Sep-03 2002 0.031 3.0 PPB TP PPB Chl-a
24 Yellow-brown algae, principally Dinobryon, were the most common algae in June, and these as well as diatoms were common during July. In both August and September, blue-green algae, principally Anacystis, were the most common form of algae; however concentrations were low so only mild blooms may have been evident.
Clark Lake algal composition
Clark Lake 18-03784 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
Clark Lake was surveyed in June 2001, before the annual lake-wide mechanical plant harvesting operations were begun. Submerged plants were found to a maximum depth of 20 feet and vegetation occurred at 89% of the sample points within that vegetated zone. Eighteen native species of submerged plants were recorded and the most common species were flatstem pondweed (Potamogeton zosteriformis), coontail (Ceratophyllum demersum), Canada waterweed (Elodea canadensis), northern watermilfoil (Myriophyllum sibiricum) and whitestem pondweed (Potamogeton praelongus). The exotic plant, curly-leaf pondweed (Potamogeton crispus), was found at a few locations in the northern most bay and the north shore but was not found within any of the sample points. Extensive beds of waterlilies (Nuphar variegata and Nymphaea odorata), wild rice (Zizania aquatica) and cattail (Typha sp.) occurred along undeveloped shores. The exotic plant, yellow iris (Iris pseudacorus) was frequently found along shorelines. Prior surveys Historical surveys (1942, 1951) of Clark Lake found that both submerged and emergent vegetation was abundant and well distributed, and species composition appeared similar to that observed in 2001. As early as 1950, correspondence from lake residents to the MN Dept of Conservation indicated that residents had concerns about low water levels and abundant plant growth and options for plant control were considered.
25 4. Red Sand Lake (18-0386) is located in Crow Wing County within the city of Baxter in the NLF ecoregion. It has three inlets and two outlets, one of which is controlled by a dam. This lake was studied as a part of the 2002 CLMP + program (http://www.pca.state.mn.us/water/clmp- publications.html#reports) and was noted to have dense macrophytes throughout most of the summer. It has a maximum depth of 23.0 feet and an average depth of 7.0 feet, placing it in the 75th percentile for mean and maximum depth for this study. Its area is 502 acres and watershed area is 4,550 acres giving it a watershed to lake ratio of 9:1. According to a 1986 MDNR fishery survey, the lake is subject to occasional winterkill. Thus, low-DO tolerant species such as northern pike and bullhead are favored in Red Sand Lake, but it does not have a carp problem. Black bullheads were within the normal range and brown bullheads (typically associated with good water quality) were uncommonly abundant, pushing the ratio of black to brown bullheads to low levels.
Summer-mean TP at 35 µg/L, was above the typical range based on NLF ecoregion reference lakes (Table 3). CLMP + data for 2002 yielded a slightly lower mean TP of 24 µg/L. Chlorophyll-a averaged 5.0 µg/L which is comparable to the 2002 value. This value is considerably lower than predicted by Carlson’s TSI based on TP readings and is well within the typical range for the NLF ecoregion. Mean Secchi disk was 10.9 feet (3.3 m), which is within typical range for the NLF ecoregion. TSS was 3 mg/L and VSS was 2 mg/L, just above the typical range.
Red Sand (18-0386) Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB
50 39 42 25 25 3 6.6 4 4.9 0 24-Jun-03 29-Jul-03 25-Aug-03 29-Sep-03 PPB TP PPB Chl-a
26 Blue-green algae were the most common form throughout the months of June, July and August, and may have been visible near the surface of the lake. Yellow-browns were dominant in September. The dominant algal genus varied throughout the summer. In June, the blue-green, Anacystis, was dominant. In August, another blue-green, Aphanizomenon, was dominant and in September the yellow-brown alga, Dinobryon, became most abundant. Given the low chlorophyll-a concentration, nuisance blooms most likely did not occur.
Red Sand Lake algal composition
Red Sand Lake 18-0386 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
Red Sand Lake was surveyed in 2001 and vegetation occurred across the entire basin. Submerged plants were found to a depth of 18 feet; fourteen submerged species were identified and dominant species included bushy pondweed (Najas flexilis), Canada waterweed (Elodea canadensis), coontail (Ceratophyllum demersum), flatstem pondweed (Potamogeton zosteriformis), Robbin’s pondweed (Potamogeton robbinsii) and whitestem pondweed (Potamogeton praelongus). Hardstem bulrush (Scirpus acutus) was the most abundant emergent species and was estimated to cover about 48 acres. Wild rice (Zizania aquatica) and cattail (Typha spp.) were estimated to cover about 12 acres and 5 acres, respectively. Intermixed with these emergent beds were areas of white and yellow waterlilies (Nymphaea odorata and Nuphar varigata).
Prior surveys A 1950 survey of Red Sand Lake found abundant vegetation, including many of the species observed in 2001.
27
5. Jennie Lake (21-0323) is located in Douglas County four miles southwest of Brandon on the northern edge of the Northern Glaciated Plains (NGP) ecoregion. Much of the lake is surrounded by agriculture and it has no public access. Jennie Lake has a maximum depth of 7.0 feet and an average depth of 3.0 feet placing it below the 25th percentile for lakes in this study. Its area is 316 acres with a rather small contributing watershed of 2,113 acres, resulting in a low watershed to lake ratio - 6.7:1. The lake was thick with submergents throughout the summer, a mucky bottom, and a yellow/green hue to the water. There have been no MDNR fishery surveys to date completed on Jennie Lake.
Summer-mean TP was at 210 µg/L which is above the typical range (Table 3) and near the 80th percentile based on assessed lakes in the NGP ecoregion. TP values ranged from 160 µg/L in June to a maximum of 280 µg/L in September. Chlorophyll-a averaged 114 µg/L, which is over twice the maximum of the typical range for the NGP ecoregion, placing Jennie Lake into the 95th percentile for chlorophyll-a concentration. Individual chlorophyll-a values ranged from 83 µg/L in August to 137 µg/L in July. These TP and chlorophyll-a results were among the highest in the study. The Secchi disk value was 0.8 feet (0.2 m), which is in keeping with the water’s observed cloudy appearance, is only slightly lower than the typical range for lakes in this ecoregion. TSS was 73 mg/L and VSS was 58 mg/L, both well above the typical range for the NGP Ecoregion. The high VSS can be attributed to high chlorophyll-a (algae) concentrations.
Jennie Lake TP and chlorophyll-a data for 2003.
Jennie Lake (21-0323) Total Phosphorus and Chlorophyll- a results 2003 500
400
300 278 220 200 PPB 200 156 136 137 83 98 100
0 3-Jun 7-Jul 19-Aug 8-Sep
PPB TP PPB Chl-a
28 Jennie Lake was dominated by blue-green algae and blooms were evident throughout the summer. Dominant blue-greens included Anacystis in July and Aphanizomenon in September.
Jennie Lake algal composition
Jennie Lake 18-0374 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns
MDNR vegetation survey
Plant coverage was good throughout the lake in 2003 but species diversity was very low likely due to the low water clarity. The maximum water depth of Jennie Lake was 7 feet and the mean depth was 5.5 feet (sample station N=76). Only five species of submerged plants were found. The 2003 floristic quality index was 10.29. Sago pondweed (Stuckenia pectinata) was found throughout the lake and was the most common species of submerged macrophyte. Myriophyllum sibiricum and Ceratophyllum demersum were found in the northwest bay of the lake. A thin fringe of hardstem bulrush (Scirpus acutus) was present around the shoreline and scattered bulrush beds occurred in the middle of the lake. A fringe of cattail was found along the south shore.
29 6. Diamond Lake (27-0125) is located in Hennepin County one mile east of Rogers in the CHF ecoregion. It has an area of 406 acres. Both its watershed area of 811 acres and its watershed to lake ratio of 2.0:1 are below the 25th percentile for this study. Three feedlots are located within the watershed. Curly-leaf pondweed is present in the lake. Diamond Lake has a maximum depth of 8.0 feet and a mean depth of 6.0 feet. MDNR fishery surveys in 1982 and 1992 found green sunfish and black bullheads to be abundant. No carp were observed but the strong presence of black bullheads most likely has a negative impact on the water quality. Diamond Lake was stocked with walleye in 2001. In severe winters, winterkill occurs.
Summer-mean TP at 180 µg/L is well above the typical range for the CHF ecoregion (Table 3). Individual values varied greatly from 50 to 420 µg/L. Chlorophyll-a averaged 73 µg/L, also well above the typical range. Based on the TP value, the chlorophyll-a concentration is lower than predicted by Carlson’s TSI. Chlorophyll-a concentrations varied from 16 µg/L in May to 140 µg/L in September. The distinct increase in TP and chlorophyll-a over the summer is consistent with curly-leaf dominance. Whereby, concentrations are low early in summer and increase as curly-leaf dies back in mid summer. Historically, Diamond Lake’s TP and chlorophyll-a have been quite variable. In a 1980 assessment, TP was 360 µg/L and the chlorophyll-a was 308 µg/L, and in 1994 TP and chlorophyll-a had dropped to 151 µg/L and 48 µg/L, respectively. Compared to the lake data for this ecoregion, Diamond averaged around the 80th percentile for TP and chlorophyll-a. TSS was 26 mg/L while VSS was 17 mg/L, which is high relative to the reference lakes (Table 3).
Diamond Lake TP and chlorophyll-a data: 2003
Diamond (27-0125) site Total Phosphorus and Chlorophyll-a results 2003 500 421 400
300 221 PPB 200 136 140 100 97 89 100 50 40 16 0 13-May 12-Jun 30-Jun 5-Aug 4-Sep PPB TP PPB Chl-a
30 Blue-green algae were the most common form of algae throughout the summer, and nuisance algal blooms were evident throughout the summer (photo above). Thick, filamentous algae were observed on each sampling event. Blue-green dominants included Aphanizomenon in June, Anacystis in August, and Microcystis in September.
Diamond Lake algal composition
Diamond Lake 27-0125 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms
MDNR vegetation survey
The most common macrophyte species present at the time of the survey in 2001 was Ceratophyllum demersum. Najas flexilis, Stuckenia pectinata, and Elodea canadensis were also present. The lake-wide species richness was five and floristic quality index was 7.5. The maximum water depth was 7.5 feet, and the mean depth was 5.6 feet (sample points N=8)
Prior surveys: 1955: Sago pondweed was the most abundant submerged plant recorded and other species found were narrowleaf pondweed, flatstem pondweed, and bushy pondweed. Emergent plants were confined to a narrow band around the shoreline of the lake and islands and common cattail, reed canary grass, and arrowhead were common. Information from local residents indicates that the lake was dry during 1934 and 1935 and provided good pasture; around 1895 the outlet stream of the lake was “cleaned” and the lake level lowered 2 feet; from 1890-1900 waterfowl were very abundant on the lake and have declined steadily since that time.
31 7. French Lake (27-0127) is located in Hennepin County two miles southeast of Rogers in the CHF Ecoregion. French has one inlet, Diamond Creek, and both larger and smaller bays. There is a single public access on the larger bay. Its watershed is in close proximity to the developing communities of Rogers, Champlin, and Anoka. Its area is 352 acres and a moderately-sized watershed of 3,712 acres with a watershed to lake ratio of 10.5:1. French Lake has a maximum depth of 3.0 feet and a mean depth of 2.0 feet -- below the 25th percentile for this study. There is an abundance of macrophytes, some of which were noted to be covered in sediment from the lake murky bottom. The water level of French Lake seems to be dropping; readings have steadily declined since 1998 and a current level of 902.27 ft., nearly two feet below its OHW. The MDNR evaluated the lake in 2001 for walleye rearing and found that the basin would winterkill frequently due to its shallowness.
Summer-mean TP was 370 µg/L. Individual samplings ranged from 140 µg/L in June to 720 µg/L in September. A nearby mobile home park was reported to have had an unauthorized discharge to the lake that may in part account for the high in-lake TP (Doug Hall, 2004; personal communication). Chlorophyll-a averaged 180 µg/L, ranging from 45 µg/L in June to 480 µg/L in September. Both these averages were over the 95th percentile for the CHF ecoregion based on reference lakes in 2003 and were the highest concentrations found in this study. This lake was studied in 2001 and 2002 by the Metropolitan Council and results were similar to this study. Over the last three years of sampling, the trend of TP and chlorophyll-a rising throughout summer was evident. Average Secchi depth was a rather low at 1.2 feet (0.4 m). In September, the Secchi was 0.3 feet (0.1), the lowest recorded in the study. TSS was 53.3 mg/L and VSS was 42.7 mg/L, with values ranging from 21 to 96 mg/L and 15 to 84 mg/L, respectively. These values are considerably higher than the typical range for the ecoregion.
French Lake TP and chlorophyll-a data: 2003
French Lake (27-0127) Total Phosphorus and Chlorophyll- a results 2003 500 408 400
300 257 721 PPB 200 139 87 100 46 0 12-Jun 30-Jun 4-Sep PPB TP PPB Chl-a
32 In June, green algae were most common, followed closely by blue-greens and diatoms. By September, blue-green algae were most common, though green algae were still quite abundant. Based on the high chlorophyll-a concentrations throughout the summer, it is likely that algal blooms persisted throughout the summer.
French Lake algal composition
French Lake 27-0125 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns
MDNR vegetation survey
Stuckenia pectinata was the most common plant in the 2001 survey. The lake was entirely fringed by cattails. Other species present included Najas flexilis and Ceratophyllum demersum. The lake-wide species richness was five and floristic quality index was 7.5. The maximum water depth of French Lake was 3 feet, and the mean was 2.4 feet (sample point N=4).
33 8. Prairie Lake (27-0177) is located in Hennepin County two miles southeast of St. Michael in the CHF ecoregion. Prairie Lake is located within the Three Rivers Park District’s Crow Hassen Park Reserve and is almost completely surrounded by thick shore vegetation. The lake has been noted to have Trumpeter swans. The lake is adjacent to a horse trail and the traffic has an impact on the lake’s access point. Submerged macrophytes dominate the lake bottom which is otherwise sandy. Prairie is the smallest lake in this study with an area of 34 acres and a small watershed of 92 acres with a ratio of 3:1 relative to its size. Its maximum depth is 6.0 feet and its mean depth is 3.0 feet. Prairie Lake is below the 25th percentile in this study for area, mean depth, watershed acres, and watershed to lake ratio. Its good water quality, despite its size and depth, is likely attributed to its small watershed and isolated location. The MDNR stocked walleye fry for rearing purposed over several years. Based on MDNR fishery survey information, no fish other than walleye were observed from 1981-2003 except for the central mudminnow in 2002. The lake has the potential for annual winterkill due to its relative shallowness.
Summer-mean TP at 30 µg/L is within the typical range for the CHF ecoregion (Table 3) and was among the least nutrient-rich lakes in the study. Values ranged from 20 µg/L in June to 50 µg/L in September. Chlorophyll-a averaged 7.64 µg/L which is also within the typical range. Individual measures ranged from 0.77 µg/L in June to 18.70 µg/L in August. Despite being the smallest and among the shallowest, Prairie Lake’s 2003 TP and chlorophyll-a results were around the 24th percentile compared to data from the CHF Ecoregion. The average Secchi depth was 5.9 feet (1.8 m), but the disk was on the lake-bottom indicating a high level of water clarity. The lake was described as being “very clear.” TSS and VSS averaged 5 and 3 mg/L respectively with individual values ranging from 1.0 to 13.0 mg/L and 1.0 to 5.6 mg/L, a bit above the typical range.
34 Prairie Lake TP and chlorophyll-a data: 2003
Prairie (27-0177) site 101 Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB 50 50 24 19 19 11 16 7 1 0 12-Jun 1-Jul 5-Aug 4-Sep
PPB TP PPB Chl-a
Blue-green and yellow-brown algae were most common during the month of June. In August (when chlorophyll-a values were highest), blue-greens were most common, likely causing mild algal blooms. In September, blue-greens were again most common, though followed closely by yellow-browns. The blue-green alga, Anacystis, was dominant throughout the summer and the yellow-brown, Dinobryon, was also abundant in June.
Prairie Lake algal composition
Prairie Lake 27-0177 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Brow ns Other
MDNR vegetation survey
No survey data were available for the lake.
35 9. Quamba Lake (33-0015) is located in Kanabec County one mile south of Quamba in the CHF ecoregion and is the furthest east of the lakes in this study. The lake has two inlets and one outlet as well as moderate development on the south and northwest shores. Quamba has an abundance of submergent plant growth and curly-leaf pondweed was observed during this assessment. With an area of 214 acres, it was in the lower quartile of the lakes in this study. Its watershed and watershed to lake ratio, however, are in the 75th percentile at 23,241 acres and 108:1 respectively. Quamba is slightly deeper than average with a maximum depth of 11.0 feet and a mean depth of 6.0 feet. According to the MDNR, walleye have been stocked in recent years. Northern pike were stocked until 1995 and are now self-sustained. In a 2000 survey, it was noted that black bullhead populations were low relative to other lakes in this class, while brown and yellow bullhead populations were typical. The ratio of black to brown bullhead was rather low. There is no indication of a carp problem, but winterkill is common in this lake, which often allows for elevated carp and bullhead populations in lakes.
Summer-mean TP was 89 µg/L, ranging from 34 µg/L in July to 132 µg/L in October. chlorophyll-a averaged 34.5 µg/L ranging from 8.6 µg/L in May to 76.9 µg/L in August. Both of these results were above the typical range based on reference lakes for the CHF ecoregion (Table 3) and fell around the 60th percentile in this study. Secchi depth averaged 2.6 feet (0.8 m), which is below the typical range for the ecoregion. The mean TSS and VSS were 9.5 and 6.0 mg/L, respectively, with individual measures ranging from 3.3 in to 17.0 mg/L and 2.7 to 11.0 mg/L. The lake has moderate coloration or “bog-stain” as a result of abundant wetlands in it watershed.
Quamba Lake TP and chlorophyll-a data: 2003
Quamba (33-0015) site 102 Total Phosphorus and Chlorophyll-a results 2003 200
150 132 114 98 100 85
PPB 71 72 73
50 27 9 16 0 20-May 24-Jun 30-Jul 26-Aug 30-Sep
PPB TP PPB Chl-a
36 Blue-green algae were common throughout the summer, with nuisance bloom levels noted in July though September. No specific genera were dominant in any month.
Quamba Lake algal composition
Quamba Lake 33-0015 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns
MDNR vegetation survey In June 2003, submerged plants were found to a depth of 10 feet in Quamba Lake and were most often found in water depths less than six feet. The tannin stained water limits the amount of light that reaches the lake bottom and likely prevent vegetation from growing to deeper depths. The exotic plant, curly-leaf pondweed (Potamogeton crispus) has been present in Quamba Lake since at least 1995 and now dominates the submerged plant community. In 2003, it was most abundant in the shore to five foot depth zone where it was found in 58 percent of the sample sites and formed dense mats that reached the water surface and covered approximately 33 acres. Common native submerged plants included coontail (Ceratophyllum demersum), stonewort (Nitella sp.), flatstem pondweed (Potamogeton zosteriformis) and Canada waterweed (Elodea canadensis). Emergent and floating-leaf plants included bulrush (Scirpus sp.), burreed (Sparganium eurycarpum), wild rice (Zizania aquatica), white waterlily (Nymphaea odorata) and yellow waterlily (Nuphar variegata). Prior surveys 1973: Submerged plants were recorded to 5 feet and coontail was the most common species. Canada waterweed, clasping-leaf pondweed, flatstem pondweed, and wild celery were also present. Waterlilies were found along nearly all shores and emergent plants were distributed around the lake with cattails most abundant. A heavy blue-green algae bloom was present in the summer.
1959: Submerged plants occurred to 10 feet and were very abundant in the shallow northeast and southwest ends of the lake and moderately abundant along the remainder of the shoreline. Clasping leaf pondweed was the most abundant submerged species and was found with flatstem pondweed, wild celery, Canada waterweed and sago pondweed. White and yellow waterlilies were common at the northeast and southwest shores. Wild rice was moderately abundant along the southwest shore and present in a narrow fringe along much of the remaining shoreline. A diversity of other emergent species was found. A slight algal bloom occurred on the lake.
37 10. Ringo Lake (34-0172) is located in Kandiyohi County three miles west of Spicer in the CHF ecoregion. It has three distinct bays and one island. About half of the lake is directly adjacent to Hwy 71 and it has one outlet draining south to Long Lake. Its total watershed area is 1,471 acres. Within this study, Ringo is above the 75th percentile for area at 716 acres and below the 25th percentile for watershed to lake ratio at 2.6 to 1. Water level seems to be decreasing slightly since 1996 to nearly two feet below its OHW of 1166.4 feet (Fig. 4). Ringo Lake’s maximum depth is 10.0 feet and mean depth is 5.0 feet – though both values will change as does lake level. According to MDNR information, the lake has been stocked with walleye in recent years. In a 2000 survey, black, brown, and yellow bullheads were found to be within typical ranges for a lake of this class, though the ratios of black: brown and black: yellow were relatively high. Common carp were also within the typical range for this class of lake. The lake is subject to winterkill, thus winter aeration equipment was installed in 1987.
Summer-mean TP at 90 µg/L was above the CHF typical range (Table 3). Individual measures ranged from 82 to 101 µg/L. Chlorophyll-a averaged 34 µg/L, also above the CHF typical range with individual concentrations ranging from 12.1 to 57.0 µg/L. For the 2003 study, TP was at around the 68th percentile while the chlorophyll-a was at about the 75th percentile in the CHF ecoregion. Secchi depth averaged 2.6 feet (0.8 m), which is below the typical range. TSS and VSS were 10 and 6 mg/L respectively which are both relatively high.
Ringo Lake TP and chlorophyll-a for sites 102 and 101: 2003
Ringo Lake (34-0172) site 101 Total Phosphorus and Ringo Lake (34-0172) site 102 Total Phosphorus and Chlorophyll-a results 2003 Chlorophyll-a results 2003 200 200
150 150
101 102 90 94 100 84 82 89 100 PPB PPB 56 57 52 39 47 49 50 50 12
0 0 5-Jun 9-Jul 21-Aug 10-Sep 9-Jul 21-Aug 10-Sep PPB TP PPB Chl-a PPB Chl-a Series2
38 Blue-green algae were common throughout the summer, and contributed to algal blooms. No specific algal genera were dominant during the 2003 summer.
Ringo Lake algal composition
Ringo Lake 34-0172 100% 80% 60% 40% 20% 0% May-03 Jun-03 Jul-03 Aug-03 Sep-03
Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
Macrophytes for Ringo Lake were last sampled in 1995 and a summary of the plant metrics are noted in the Appendix. Based on that survey five submerged and two floating leaf plants were observed for a FQI of 12. Other metrics were not available.
39 11. Florida Slough (34-0204) is located in Kandiyohi County four miles southeast of Norway Lake in the CHF ecoregion. It is across the street from Florida Lake, which is highly developed. A thick ring of cattails, bulrush and submergents throughout make access very difficult. Its area is 772 acres with a watershed of 42,044 acres and a watershed to lake area ratio of 54:1. Florida Slough has the largest watershed within this study and is over the 75th percentile for lake area and watershed to lake ratio. Its maximum depth is 5.9 feet and its mean depth is 2.5 feet, both below the 25th percentile in this study group. There have been no MDNR fishery surveys completed on Florida Slough Lake to date. Since 2000, water level has fluctuated almost 2-3 feet (Fig. 4), which can have a distinct impact on the macrophyte community.
Summer-mean TP was 140 µg/L, with individual measures ranging from 55 to 189 µg/L. Chlorophyll-a concentrations averaged 49 µg/L but ranged from 10.5 to 73.1 µg/L. Both means are well above the typical range (Table 3). Vertical plankton tows found very small zooplankton. Average Secchi depth was 2.5 feet (0.8 m), below the typical range for the CHF. Mean TSS and VSS were 14 and 9 mg/L respectively, which are considerably higher than the CHF typical range. Individual measures ranged from 3.6 to 28 mg/L for TSS and from 2.8 to 20 mg/L for VSS.
Florida Slough TP and chlorophyll-a data: 2003
Florida Slough (34-0204) Total Phosphorus and Chlorophyll- a results 2003
200 189 165 150
100 73 PPB 55 64 50 11 0 4-Jun 9-Jul 10-Sep
PPB TP PPB Chl-a
40 In June, yellow-brown algae were the most common algal forms with a particular abundance of the genus Dinobryon. In July and September, diatoms were dominant. In July, pennate- diatoms were the most common genus.
Florida Slough Lake algal composition
Florida Slough Lake 43-0204 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
At the time of the 2002 survey Najas marina, a species of “Special Concern” in Minnesota, was found at 34% of the sample points. Chara, a macro-algae characteristic of clear water was abundant. Other common species were Stuckenia pectinata and Myriophyllum sibiricum. The 2002 lake wide species richness was nine and floristic quality index was 15.91. Florida Slough’s maximum water depth was 5.9 feet and its mean depth was 4.6 feet (sample station N=64).
41 12. East Solomon Lake (34-0246) is located in Kandiyohi County five miles northeast of Pennock in the Western Corn Belt Plains (WCP) ecoregion. The lake has three smaller bays and most of its watershed is cultivated or pasture land. Its area is 706 acres with a watershed of 12,907 acres giving it a watershed to lake area ratio of 18:1. Its maximum depth is 13.0 feet and its mean depth is 9.5 feet. East Solomon is in the 75th percentile for area, watershed area, watershed to lake ratio, maximum depth and mean depth among lakes in this survey. It is also noted to have an abundance of curly-leaf pondweed. According to the MDNR, black bullhead are particularly abundant and carp are within typical range for a lake of this class. The lake is stocked with walleye every other year and is subject to winterkill. Winter aeration equipment was installed in 1987.
Summer-mean TP at 90 µg/L was within the typical range for the WCP ecoregion (Table 3). Individual measures ranged from 70 µg/L in June to 111 µg/L in August. Chlorophyll-a averaged 17 µg/L, ranging from 2.8 to 42.1 µg/L with a peak in September. Increased TP and chlorophyll-a over the summer may be related to abundance of curly-leaf pondweed. This value was lower than the typical range for the ecoregion and lower than the value predicted by Carlson’s TSI based on TP readings. TP was around the 24th percentile and the chlorophyll-a was around the 18th percentile compared to all the data in the ecoregion. Secchi depth averaged 4.7 feet (1.4 m), which is above the typical range for the WCP. TSS and VSS averaged 8 and 4 mg/L, respectively.
East Solomon Lake TP and chlorophyll-a data: 2003
East Soloman (34-0246) Total Phosphorus and Chlorophyll-a results 2003 200
150
104 111 109
100 PPB 70 42 50 12 19 3 0 5-Jun 9-Jul 21-Aug 10-Sep
PPB TP PPM Chl-a
42 Blue-green algae dominated East Solomon Lake throughout the summer. The blue-greens Anacystis and Aphanizomenon were particularly abundant in July. It was also noted that Aphanizomenon was thick in September, which coincides with the peak chlorophyll-a concentrations, indicating nuisance algal blooms during this month.
East Solomon Lake algal composition
East Solomon Lake 34-0246 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
Aquatic plant distribution was patchy, plants were not found in water deeper than 9 feet. The maximum water depth in East Solomon was 13 feet, and the mean depth was 9.5 feet (sample station N=103). Only 51% of the sample stations were vegetated. Stuckenia pectinata and Ceratophyllum dermersum were the most common plants. The exotic species, Potamogeton crispus was also present and was likely more abundant earlier in the year. The survey was done in mid-July by which time this plant has usually died back for the season. Other species present were P. zosteriformis, P. friesii and Scirpus acutus. The 2002 lake-wide species richness was seven and floristic quality index was 11.84.
43 13. Lake Shaokotan (41-0089) is located in Lincoln County seven miles southwest of Ivanhoe near the South Dakota border in the Northern Glaciated Plains (NGP) ecoregion. This lake was included as well in the study of southwestern Minnesota lakes (Heiskary et al. 2003) and detailed information on the lake is available in that report. Its surface area is 995 acres with a predominantly agricultural watershed area of 8,400 acres. Its moderate watershed to lake area ratio is 11.2. Lake Shaokotan has a maximum depth of 12.0 feet and a mean depth of 7.0 feet. It was over the 75th percentile for area, mean depth, and watershed area for lakes within this study. The lake has a history of water quality problems including severe nuisance blue-green blooms, summer and winter anoxia, and periodic fish kills. It was involved in a significant nutrient management program in the early 1990’s resulting in a drop in phosphorus concentration from 270 to 89 µg/L by 1994. This resulted in reductions in the frequency and severity of nuisance algal blooms. Transparency has also increased and anecdotal evidence suggested a limited increase in macrophytes in the shallow portions of the lake. The MDNR survey report for 1996 indicates above average populations of walleye, perch, and black bullhead for a lake of its class. Northern pike and bluegill, which typically require rooted vegetation, are well below the typical range for this class of lake. The lake is actively managed for walleye and good natural reproduction of walleye was noted as well. Carp were not noted in a 2000 survey so it is assumed they are not at problematic levels.
Summer-mean TP at 180 µg/L is within the typical range for the NGP. Chlorophyll-a averaged 27 µg/L, which is slightly below the typical range for the ecoregion. Field observations noted that daphnids were quite abundant in the month of July. TSS and VSS averaged 14 and 6 mg/L, respectively.
Shaokotan (41-0089) Site 102 Total Phosphorus and Chlorophyll- Shaokotan (41-0089) Site 103 Total Phosphorus a results 2003 and Chlorophyll-a results 2003 500 500 400 400
300 300 190 203 200PPB 165 174 200PPB 163 125
100 100 62 29.2 10.2 13.1 8.62 10.8 0 0 22-Jul 13-Aug 16-Sep 22-Jul 13-Aug 16-Sep PPB TP PPB Chl-a PPB TP PPB Chl-a
44
Blue-green algae were dominant throughout the summer. The genus, Anacystis, was particularly abundant, though field observations also noted an abundance of filamentous Aphanizomenon in September also, both of which contribute to surface algal blooms. Macrophytes were not sampled in 2003; however other recent surveys of the lake indicated a general absence of SAV.
Lake Shaokotan algal composition
Lake Shaokotan 41-0089 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms
MDNR vegetation survey
Plant distribution during the August 2002 survey was extremely poor. Submerged plants were found only at two of the 80 sample points. There was a fringe of cattails along the shoreline in some less developed areas of the basin. There was an extensive blue-green algae bloom at the time of the survey and there appeared to be a summer kill of fish as dead and dying bullheads were seen. Species of plants found at the two sample stations were Stuckenia pectinata, Lemna trisulca and Ceratophyllum demersum. The average depth at the time of the survey was 7.9 feet and that combined with the poor water clarity likely contributed to the lack of submerged vegetation.
45 14. West Twin Lake (41-0102) is located in Lincoln County seven miles north of Hendricks in the NGP ecoregion. Its surface area is 216 acres with a moderate watershed of 2,026 acres. Its watershed to lake area ratio is 9.4:1. West Twin was below the 25th percentile for maximum and mean depth at 4.4 and 2.3 feet, respectively. It is connected to and shares a common wetland with East Twin Lake, also small and shallow, from which it is separated by a road. The lake had yellow hues throughout most of the summer, an extremely soft bottom, and was thick with submergents throughout the year. There have been no MDNR fishery surveys completed to date on West Twin Lake.
Summer-mean TP at 320 µg/L was around the 87th percentile for the study lakes and was well above the typical range for the NGP ecoregion. Individual values ranged from 266 µg/L in August to 384 µg/L in September. Chlorophyll-a averaged 138 µg/L, placing it into over the 95th percentile and also well above the typical range for the NGP. Both the mean TP and chlorophyll-a were second- highest compared to all other lakes in this study. Secchi depth averaged a low 0.8 feet, which is below the typical NGP range. TSS and VSS averaged 77 and 62 mg/L respectively, both extremely high and well above the typical range for NGP reference lakes (Table 3).
West Twin TP and chlorophyll-a data: 2003
West Twin (41-0102) Total Phosphorus and Chlorophyll-a results 2003
500 384 400 318 300 266
PPB 200 153 144 118 100
0 22-Jul 13-Aug 16-Sep
PPB TP PPB Chl-a
46 Blue-green algae were common and at bloom levels throughout the summer. No specific genera were dominant during any month of the summer.
West Twin Lake algal composition
West Twin Lake 41-0102 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
Plant distribution was very good during the 2002 survey. At the time of the survey plants were found throughout the lake and were especially thick in the north and west parts of the basin. Ninety-five percent of the sample stations were vegetated. Ten species of aquatic plants were found with in the basin. Potamogeton friesii was the most common plant found. Stuckenia pectinata, Ceratophyllum demersum, and Myriophyllum sibiricum were also common. The 2002 floristic quality index was 17.71. It should be noted however, that while the FQI was relatively high the plants that contributed to it were rather tolerant of eutrophic conditions. West Twin Lake’s maximum water depth was 4.4 feet and mean depth was 3.6 feet (sample station N=64) during the vegetation survey. The 2002 survey was in stark contrast to the November 1, 2001 survey where Area Wildlife Manager, Bob Meyer (2003) noted that “water clarity was very poor, the tip of the canoe paddle could not be seen deeper than six inches in the water. Coontail was the only plant observed.”
47 15. East Twin Lake (41-0108) is located in Lincoln County seven miles north of Hendricks in the NGP ecoregion. East Twin is connected to West Twin Lake though the two are separated by a road; both are located in the center of a wetland and share a common watershed. There is no public access to the lake. The lake has yellow hues throughout most of the summer, a very soft bottom, and was thick with submergent throughout the year. The lake’s surface area is 215 acres with a watershed of 2,026 acres, making its watershed to lake ratio of 9.4:1. Its maximum depth is 5.0 feet with a mean depth of 2.4 feet. Compared to other lakes within this th study, East Twin is below the 25 percentile for surface area, mean and maximum depth. There have been no MDNR fishery surveys to date.
TP and chlorophyll-a levels declined throughout the summer. Mean TP at 210 µg/L was within the typical range for the NGP. Individual values ranged from 245 µg/L in July to 178 µg/L in September. Chlorophyll-a averaged 133 µg/L, considerably above the typical range based on NGP reference lakes. Values ranged from 166 µg/L in July to 110 µg/L in September. The 2003 mean TP was near the 80th percentile and over the 95 percentile for chlorophyll-a compared to all the data in the NGP ecoregion. The average Secchi depth was 0.8 feet, which was beneath the typical range for the ecoregion. TSS and VSS were quite high at 72 and 68 mg/L respectively.
East Twin Lake TP and chlorophyll-a data: 2003
East Twin (41-0108) site 102 Total Phosphorus and Chlorophyll-a results 2003 500
400
300 245 199
PPB 178 200 166 122 110 100
0 22-Jul 13-Aug 16-Sep PPB TP PPB Chl-a
48 The algae for East Twin Lake were composed primarily of blue-greens, contributing to elevated levels of chlorophyll-a and nuisance blooms throughout the summer. The genus Anacystis was dominant during the month of July.
East Twin Lake algal composition
East Twin Lake 41-0108
100%
80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03
Blue Greens
MDNR vegetation survey
Compared to West Twin on the opposite side of the road, water clarity and vegetation distribution was poor in this lake. Stuckenia pectinata was the most common species, but distribution was sparse. The maximum water depth in 2002 was 5 feet and the mean depth was 4.1 feet (sample station N=55). The lake wide species richness was 6 and the floristic quality index was10.29.
Based on reports from the Assistant Wildlife Manager, Bill Schuna (Piepgras & Geisen, 2002), the vegetation and water quality in both East and West Twin Lakes is quite dynamic. For example, in the year prior to the 2002 vegetation survey conditions were reversed in the lakes whereby West Twin had very little vegetation and East Twin had a lot of vegetation.
49 16. Lake Johanna (61-0006) is located in Pope County five miles southwest of Brooten in the CHF ecoregion. The lake consists of two larger bays separated by a shallow rock bar; and several adjacent bays connected by wetlands to the south. The lake was thick with macrophytes throughout the year and the curly-leaf pondweed was observed in June, though submergent plants beds were patchy in the northern bay.
Lake Johanna is entirely littoral with a maximum depth of 10 feet as well as a mean depth of 7.0 feet. Water level has been dropping slightly since 1998 to over four feet below the lake’s OHW elevation (Fig. 4). Its area is 1,584 acres, making it the second largest lake in this study. It has a moderate watershed of 6,987 acres and a watershed to lake area ratio of 4.4:1. Based on a 2000 MDNR fishery survey, black bullhead were quite abundant and the ratios of black: brown and black: yellow were very high – indicative of eutrophic conditions. Carp, however were not noted to be abundant. The lake has a past history of winterkill due in part to its heavy vegetation.
Summer-mean TP at 80 µg/L is above the typical range for the CHF ecoregion. Individual measures ranged from 45 µg/L in June to 111 µg/L in July. Chlorophyll-a was also over the typical ecoregion range at a summer average of 35 µg/L. Chlorophyll-a peaked in July with a value of 69 µg/L which was also over the CHF typical range. The 2003 TP and chlorophyll-a averages were around the 60th percentile compared to all the data in the ecoregion (Table 3). The TP to chlorophyll-a ratio was 2.4:1. Secchi depth was 3.1 feet (0.9 m), which was lower than the CHF typical range. TSS and VSS averaged 18 and 9 mg/L respectively, both over the typical range based on CHF reference lakes.
Lake Johanna TP and chlorophyll-a data: 2003
Johanna (61-0006) Total Phosphorus and Chlorophyll-a results 2003 200
150 111 100 86 89
PPB 69 45 50 30 28 12 0 4-Jun 8-Jul 20-Aug 9-Sep PPB TP PPB Chl-a
50 Blue-green algae were dominant throughout the summer, especially in July during which there was a peak in chlorophyll-a concentration. During this month, the genus, Anabaena, was particularly abundant.
Lake Johanna algal composition Lake Johanna 61-0006 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
DNR Fisheries surveyed Lake Johanna in late June, 2000 and found vegetation to a depth of 12 feet. A total of 17 submerged species were found and coontail and Canada waterweed dominated the submerged plant community with sago pondweed, flatstem pondweed, northern milfoil, bushy pondweed and clasping leaf pondweed occurring commonly. Curly-leaf pondweed was found at about 30% of the sample sites and was often recorded as common or abundant. Yellow waterlily, arrowhead, narrowleaf cattail and wild rice were also found.
Prior surveys: In a 1962 survey Sago pondweed, watermilfoil, bushy pondweed, variable pondweed, muskgrass, and clasping leaf pondweed were common. Emergents including cattail, softstem and river bulrush were found around the entire shoreline. There was no evidence of turbidity.
In a 1950 survey both emergent and submerged vegetation were of limited abundance, possibly due to recently established high water levels at that time. Sago pondweed and water marigold were described as common and flatstem, whitestem, clasping leaf, and coontail occurred occasionally. Sedges, arrowhead, softstem bulrush and giant burreed were recorded.
51 17. Nelson Lake (61-0101) is located in Pope County seven miles southeast of Starbuck in the CHF ecoregion. Its surface area is 403 acres and there appeared to be one seasonal resident on the lake. Its watershed and watershed to lake area ration are under the 25th percentile for this study at 820 acres and 2:1. The maximum depth of Nelson Lake is 9.0 feet with an average depth of 6.0 feet. According to the MDNR fishery information, low winter DO is common in this lake and the fishery is reflective of a winterkill lake. Northern pike and yellow perch were found above the normal range for a lake of this class in a 1997 survey. Black bullhead were found in moderate numbers and the survey did not indicate a carp problem. Walleye have been stocked every other year beginning in 1998.
Both summer-mean TP at 50 µg/L and average chlorophyll-a at 18 µg/L were within the typical range based on the CHF Ecoregion reference lakes. Individual TP measures ranged from 40 µg/L in June to 55 µg/L in September. Chlorophyll-a similarly increased throughout the summer with values ranging from 8.3 µg/L in June to 26.6 µg/L in September. The 2003 TP results were in the 54th percentile while chlorophyll-a results were in the 45th percentile (Table 2) compared to other lakes in the CHF ecoregion. Secchi depth averaged 2.0 feet (0.6 m) which was rather low for the CHF Ecoregion. TSS and VSS averaged 16 mg/L and 9 mg/L respectively, which are above the CHF typical range.
Nelson Lake TP and chlorophyll-a data: 2003
Nelson (61-0101) Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB 55 40 41 50 27 18 8 0 4-Jun 8-Jul 9-Sep
PPB TP PPB Chl-a
52 Blue-green algae were the most common algal form throughout the summer, contributing to mild blooms during the summer. The genus, Anacystis, was particularly abundant during the month of July.
Nelson Lake algal composition
Nelson Lake 61-0101 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey
Vegetation distribution was poor. During the 2003 survey, vegetation was found at only 3 of 58 sample points. The basin had a narrow fringe of cattail (narrow-leaf or hybrid) and limited areas of bulrush. Nelson Lake’s maximum water depth was 15 feet, and mean depth was 11.1 feet (sample station N=58) at the time of the survey. The lake wide species richness was seven, and the floristic quality index was 13.23. Lake depth combined with low transparency may account in part for the limited vegetation noted in this survey.
53 18. Lake Fremont (71-0016) is located in Sherburne County on half mile northeast of Zimmerman in the CHF ecoregion. Residences surround two- thirds of the lake and Highway 169 encircles the rest. The lake has one outlet on the southern shore, a uniformly soft bottom, and patchy thick submergents including the Curly-leaf pondweed throughout the summer. Its surface area is 484 acres with a very developed watershed of 2,458 acres and a watershed to lake area ratio of 5.1:1. Its maximum depth is 10.0 feet with a mean depth of 7.0 feet. According to the MDNR, numerous fish species were present in a 1993 survey. Black bullhead and yellow bullhead were quite abundant and the black: yellow ratio was quite high. Black crappie, bluegill and northern pike were stocked in 2001 and there has been no mention of a carp problem in the lake.
Summer-mean TP was 110 µg/L, which is above the typical CHF range. Individual values ranged from 41 µg/L in May to 138 µg/L in September. Chlorohpyll-a averaged higher than the typical range (Table 3) as well at 49 µg/L, with a maximum of 78.2 µg/L in August. The 2003 TP and chlorophyll-a results were around the 74th percentile compared to all the data in the ecoregion. The TP to chlorophyll-a ratio was low. Overall, TP rose steadily throughout the summer while chlorophyll-a peaked in August and declined in September --again, consistent with a lake with abundant curly-leaf. Average Secchi depth was 4.2 feet (1.2 m), just below the typical range for the Ecoregion. TSS and VSS averaged 20 and 15 mg/L respectively, both higher than the typical range based on ecoregion reference lakes.
Fremont Lake TP and chlorophyll-a data: 2003
Fremont (71-0016) Total Phosphorus and Chlorophyll-a results 2003
200
150 138 118
100 78
PPB 68 54 58 55 41 50 5 10 0 13-May 17-Jun 30-Jun 5-Aug 3-Sep PPB TP PPB Chl-a
54 Blue-green algae dominated Lake Fremont throughout the summer and contributed to frequent algal blooms. No specific genera were particularly abundant, though a blue-green algal scum was noted during the June sampling.
Lake Fremont algal composition Lake Fremont 71-0016 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Other
MDNR vegetation survey
Fremont Lake was surveyed in June 2002 and June 2003 and similar results were found in both years. Fremont Lake is a shallow lake where water clarity is sufficient to allow vegetation growth to the maximum depth of eight feet. Curly-leaf pondweed (Potamogeton crispus) and Canada waterweed (Elodea canadensis) dominated the submerged plant community, occurring in 87 and 82 percent of the sample points, respectively.
In many Minnesota lakes where curly-leaf dominates, summer water clarity is low and few native species are present. However, Fremont Lake has a rather diverse native aquatic plant community including several beds of white-stem pondweed (Potamogeton praelongus), large- leaf pondweed (Potamogeton amplifolius), and flat-stem pondweed (Potamogeton zosteriformis). These pondweed beds primarily occurred in the north half of the lake.
Prior surveys Historical surveys of 1947 and 1957 indicate that Fremont Lake has always supported an abundant submerged plant population but historically native pondweed species were once the dominant plants. Emergent vegetation was noted as sparse as early as 1947. In 2002 and 2003, emergent stands were restricted to a bulrush (Scirpus sp.) bed at the north end of the lake and a mixed stand of bulrush, spikerush (Eleocharis sp.) and sedges (Carex sp.) along the southwest shore. Small, isolated beds of white waterlily (Nymphaea odorata) and yellow waterlily (Nuphar variegata) occurred at several locations around the lake.
55 19. Silver Lake (72-0013) is located in Sibley County five miles east of Arlington in the CHF ecoregion. It has two inlets and one outlet. By the end of the summer, it was noted to have a yellow hue. Its surface area is 621 acres with a moderately sized watershed of 3,747 acres, making Silver Lake’s watershed to lake area ratio 6.0:1. The lake’s maximum depth is 9.0 feet with a mean depth of 4.5 feet. Its morphology is typical for this study. According to the MDNR, Silver Lake is considered turbid with minimal vegetation. Silver Lake is subject to low winter DO and has a history of winterkill. Winter aeration equipment has been used by a local sportsman’s club to prevent or minimize its occurrence. Several fish species were present during a 1994 survey with black crappie and walleye within the typical range for the class. Black bullhead were quite abundant, though within the typical range for this class of lake, and no mention was made of carp in that survey. In recent years, walleye fry and adult black crappie have been stocked.
The summer-mean 2003 TP and chlorophyll-a for Silver Lake were above the typical range based on CHF ecoregion reference lakes. Summer-mean TP at 160 µg/L and chlorophyll-a at 63 µg/L was in the 85th and 80th percentile respectively. TP and chlorophyll-a generally increased over the summer with the exception of July, when a decrease was noted. Historically, TP and chlorophyll-a levels have been high. The lake was sampled once in 1981 TP was 130 µg/L and the chlorophyll-a was 123 µg/L. Secchi depth in Silver Lake averaged a rather low 1.5 feet (0.5 m), well below the typical range for the CHF ecoregion based on reference lakes. TSS and VSS averaged 36 and 25 mg/L respectively, both well above the typical range.
Silver Lake TP and chlorophyll-a data: 2003
Silver (72-0013) Total Phosphorus and Chlorophyll-a results 2003 500
400
300 200 213 PPB 200 154 137 101 88 100 51 67 38 21 0 21-May 10-Jun 16-Jul 27-Aug 1-Oct
PPB TP PPB Chl-a
56 In June, yellow-browns were the most common form of algae, though the blue-green Anacystis was particularly abundant. Diatoms were most common in July and by late summer, blue- green algae became dominant, coinciding with increased levels of TP and contributing to algal blooms in these months.
Silver Lake algal composition Silver Lake 72-0013 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Oct-03
Blue Greens Diatoms Greens Yellow Browns
MDNR vegetation survey
During the vegetation survey in 2001, the water was very clear to the bottom. There were large floating mats of filamentous algae along the south shore and the far northeast bay. There was a narrow fringe of cattail around almost the entire shoreline, and in some areas, there was scattered bulrush with cattail. White waterlilies were present, however there were very few. Nine species of submerged plants were present, the most abundant species were Stuckenia pectinata, Najas flexilis and Ceratophyllum demersum. Silver Lake’s maximum water depth was 5.5 feet, and mean depth was 3.8 feet (sample station N=36). The FQI in 2001 was 13.23.
Prior surveys: 1957 survey: Heavy Microcystis bloom occurred throughout the lake. All submerged and floating vegetation was sparse. Sago pondweed was one of the few submerged species found and it was only located twice in the large open water area at the east end, both times in 5 feet of water. A patch of white waterlilies was growing in the west end near the outlet dam. Large floating mats of emergent vegetation had broken loose and were shifting position in the lake according to prevailing winds. Common emergents were reed canary grass (Phalaris arundinaceae), river bulrush (Scirpus fluviatilis), giant burreed (Sparganium eurycarpum), arrowhead (Sagittaria latifolia) and soft stem bulrush (Scirpus validus). The 1957 report noted that past surveys have always reported extensive beds of submerged vegetation; reasons for lack of submerged vegetation are high turbidity and excessive water depth in view of this turbid condition. According to local reports, the lake formerly was up to 35 feet in depth with clear water; sand bars were visible in the lake at several locations during that time. Large portions of the lake were dry during the 1930’s but the lake was never completely dry. 1953: Dam and floating carp screen were installed at the outlet. 1947: Extensive beds of sago pondweed, flatstem pondweed, coontail. Vegetation was very dense throughout the lake except for a small open area near deep water. Arrowhead (Sagittaria latifolia) and bulrush (Scirpus sp.) are very common around the shore.
57 20. Titlow Lake (72-0042) is located in Sibley County at the city of Gaylord in the WCP ecoregion. The lake is extremely shallow with very thick submerged macrophytes, making boating very difficult. Its surface area is 924 acres with a rather large watershed of 35,393 acres. Its watershed to lake area ratio is third highest in this study at 38.8:1. Titlow is over the 75th percentile for area, watershed area, and watershed to lake area within this study. Its maximum depth is 4.8 feet with a mean depth of 2.0 feet. There have been no MDNR fishery surveys of this lake to date. The shallowness of the lake coupled with such a large watershed, contributes to the poor water quality in the lake.
Summer-mean TP was 230 µg/L and well above the typical range based on WCP ecoregion reference lakes. Individual measures ranged from 120 µg/L in June to 320 µg/L in August. Chlorophyll-a mean concentration at 42 µg/L was within the typical range for the ecoregion and concentrations ranged from 17.7 µg/L in June to 82 µg/L in July and stayed relatively high through August until dropping slightly in September. Average Secchi depth for the summer was 0.7 feet (0.2 m), lower than the typical range based on WCP Ecoregion reference lakes. TSS and VSS averaged above the typical range at 68 and 11 mg/L, respectively. The discrepancy between mean TP and chlorophyll-a could be due to the lake’s abundance of macrophytes and/or light limitation from excessive TSS.
Titlow Lake TP and chlorophyll-a data: 2003
Titlow (72-0042) Total Phosphorus and Chlorophyll-a results 2003 500
400 320 300 248
PPB 185 200 120 82 100 61 49 18 0 10-Jun 16-Jul 27-Aug 1-Oct
PPB TP PPB Chl-a
58 Blue-green algae and diatoms were both of approximately equal abundance for the months of June and July. The blue-green genus, Anacystis, and centric diatoms were dominant during June. During July, small filamentous algae were observed while sampling the lake. In August, chlorophyll-a concentrations were still relatively high and blue-greens were the most common form algae.
Titlow Lake algal composition
Titlow Lake 72-0042 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey
Plant diversity and distribution were poor at the time of the survey in 2002. Only half of the sample stations were vegetated. Sago pondweed was the predominant species present. This plant was found mostly in the eastern part of the lake; while the west part was lacking vegetation. The sago found was scattered and not in dense clumps. Secchi readings were poor – 0.7 ft. or less throughout the basin. The water was brown in color. Some siltation is occurring. Two drainage ditches flow into the lake, one on the north side and one on the west side; a lot of silt is accumulating at the mouths of these ditches. Titlow’s maximum water depth was 4.8 feet, and mean depth was 3.6 feet (sample station N=123). In 2002, the lake wide species richness was four, and the FQI was 6.35.
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21. Cedar Lake (73-0226) is located in Stearns County two and a half miles southeast of St. Rosa in the CHF ecoregion. The lake is narrow and deep with abundant submerged aquatic vegetation as well as emergent vegetation around most of the shore making development around the lake somewhat difficult. Its surface area is 90 acres with a watershed of 1,428 acres. The lake has a maximum depth of 36.0 feet and a mean depth of 20.0 feet. Cedar was the second smallest lake in the study, but it has the greatest maximum depth and the second greatest mean depth. Its watershed to lake area ratio is 15.9:1 and was over the 75th percentile for this study.
Based on a 2000 MDNR fishery survey, fishing pressure was noted to be low. Bluegill were the most abundant species within Cedar Lake. Black bullhead were near the lower end of their typical range and the ratio of black: brown and black: yellow bullhead was rather low. Carp were not noted as a water quality problem.
Summer-mean TP was the second lowest recorded within this study at 21 µg/L and was below the typical range based on reference lakes in the CHF ecoregion. Chlorophyll-a averaged 6 µg/L which is on the lower end of the typical range for the Ecoregion. It peaked during the month of July at 7.7 µg/L. Secchi depth averaged 8.0 feet (2.4 m), which is within the typical range. TSS and VSS averaged 3 and 2 mg/L respectively, both within the typical range based on ecoregion reference lakes.
Cedar Lake TP and chlorophyll-a data: 2003
Cedar (73-0226) Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB
50 23 23 20 7 4 6 0 7-Jul 19-Aug 8-Sep
PPB TP PPB Chl-a
Blue-green algae were the dominant algal form throughout the summer at Cedar Lake. The filamentous blue-green, Aphanizomenon, was particularly abundant in July while Anacystis
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was abundant during August and September. However, because chlorophyll-a was so low throughout the summer, these blue-greens, though the “most dominant algal form” did not attain “bloom” levels.
Cedar Lake algal composition
Cedar Lake 73-0226 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Other
MDNR vegetation survey
The macrophyte survey for Cedar was conducted in 2000 and some measurements of the plant community were not collected (Appendix). Based on that survey the lake had a FQI of 22 based on 15 submergent species observed.
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22. Cedar Lake (73-0255) is located in Stearns County two and a half miles northeast of Sauk Centre in the CHF ecoregion. The lake has an abundance of submerged aquatic vegetation, making access with anything larger than a canoe difficult. Its surface area is 210 acres with a relatively small watershed of 1,225 acres and a watershed to lake area ratio of 5.8:1. Cedar was below the 25th percentile for lake area within this study. Its maximum depth is 8.0 feet and its mean depth is 5.0 feet. Historically, Cedar Lake has reared walleye since 1981 but the population has dwindled since the last winterkill in 1997. Bullheads and fathead minnows now dominate the fish population. No carp are present in the lake based on DNR survey data.
Summer-mean TP at 40 µg/L was within the typical range based on ecoregion reference lakes. Individual concentrations ranged from 25 µg/L in June to 46 µg/L in August. Chlorophyll-a averaged 4.1 µg/L, which is below the typical ecoregion range. Concentrations grew throughout the summer, peaked at 5.5 µg/L in the month of August, and dropped. These extremely low levels of chlorophyll-a could be due to the lake’s small watershed and abundance of healthy submerged vegetation. Secchi depth averaged 4.6 feet (1.4 m), which is just below the typical range based on ecoregion reference lakes. TSS and VSS averaged 3 and 2 mg/L respectively, which are both within the typical range.
Cedar Lake TP and chlorophyll-a data: 2003
Cedar (73-0255) Total Phosphorus and Chlorophyll-a results 2003 200
150
94 100 PPB
46 50 34 25 4 7 5 3 0 3-Jun 7-Jul 19-Aug 8-Sep
PPB TP PPB Chl-a
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“Other” (miscellaneous) and yellow-brown algae were the most common algal forms in June. Attached algae (periphyton) were common during this time as well. In August, yellow-brown algae were dominant. By September, blue-green algae became most abundant. No specific genus was dominant during any summer month.
Cedar Lake algal composition Cedar Lake 73-0255 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
Elodea was abundant, coontail was rare, and northern water milfoil is occasional. Whitestem pondweed was seen floating but was not found at any of the sample stations. Almost the entire shoreline possesses a good fringe of emergent vegetation including common arrowhead, cattail (Typha sp.), and hardstem bulrush. The substrate of the basin was sandy along the shore, and soft muck with very fine light brown detritus elsewhere. The water was very clear. The maximum water depth is 6 feet, and the mean is 4.5 feet (sample stations N=58). In 2003, the lake’s species richness was ten, and the floristic quality index was 15.67. Cedar Lake is a game refuge and part of a walleye management project.
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23. McCormic Lake (73-0273) is located in Stearns County two miles southeast of Sauk Centre in the CHF ecoregion. Its surface area is 211 acres with a watershed of 984 acres, both below the 25th percentile for this study. Its watershed to lake area ratio is 4.7:1. The lake has a maximum depth of 12.0 feet with a mean depth of 7.0 feet. Curly-leaf pondweed is present in the lake. Walleye have been in the lake since 1980, according to the MDNR. Fathead minnows are also abundant and are a major fish-rearing and water quality problem in McCormic Lake. They consume walleye fry and zooplankton which lowers fingerling production and increases algal blooms. However, fathead populations seem to have dwindled due to a good production year for walleye in 2003. Winterkill has not occurred since 1997 in the lake.
Summer-mean TP at 60 µg/L was above the typical range based on Ecoregion reference lakes. Individual measures varied from 32 µg/L in June to 95 µg/L in September. Chlorophyll-a averaged 13 µg/L, which was within the typical range, and ranged from 2.7 µg/L in June to 26.7 µg/L in September. TP and chlorophyll-a increased throughout the summer to peak in September – consistent with presence of curly-leaf. Historically, TP has remained relatively constant with a concentration of 56 µg/L noted in 1981. Chlorophyll-a is lower in 2003 than the 28.5 µg/L measured in 1981. The Secchi depth in 1981 was approximately 2.6 feet (0.8 m), whereas it was 4.0 in 2003. Vertical tows indicated the presence of large daphnids; which contributes to the lower than anticipated chlorophyll-a and slightly higher Secchi. TSS and VSS averaged 5 and 4 mg/L, respectively, which are above the typical range based on ecoregion reference lakes.
McCormic Lake TP and chlorophyll-a data: 2003
McCormic (73-0273) Total Phosphorus and Chlorophyll-a results 2003 200
150
95 100 PPB 57 55
50 32 27 10 11 3 0 3-Jun 7-Jul 19-Aug 8-Sep
PPB TP PPB Chl-a
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During the month of June, diatoms were the most common form of algae. Blue-greens became most abundant during the subsequent months of July, August and September. During August and September, the filamentous alga Aphanizomenon was dominant and mild surface blooms may have been evident.
McCormic Lake algal composition McCormic Lake 73-0273 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey
Eleven species of submerged macrophytes were found during the 2003 survey. Eleodea canadensis, Stuckenia pectinata, Potamogeton zosteriformis and P. crispus were the most common plants. Curly-leaf Pondweed was patchy and scattered throughout the entire basin. Chara was absent from the western half of the lake. Emergent vegetation formed a thin (1-2 meters wide) band around the basin with the exception of the eastern shoreline that lacked emergent vegetation entirely. In 2003, the lake’s species richness was 14, and the floristic quality index was 19.97. The maximum water depth is 12 feet, and the mean is 7.6 feet (sample stations N=55).
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24. Lake Hattie (75-0200) is located in Stevens County two miles southwest of Alberta in the NGP Ecoregion. The lake has one inlet and one outlet with little shore development. Its surface area is 477 acres. Hattie was over the 75th percentile for watershed area and watershed to lake ratio compared to the lakes within this study with values of 11,662 and 24.4:1, respectively. Its maximum depth is 9.0 feet with a mean depth of 6.0 feet. A MDNR fishery survey in 2001 characterized Hattie as hypereutrophic but noted that stocked walleye seem to tolerate this “difficult” environment. Black bullheads are quite abundant and, when combined with the carp present in the lake, most likely contribute to the poor water quality. Winter aeration equipment is used to prevent or minimize winterkill on the lake.
Summer-mean TP at 320 µg/L is the second highest within this study and well above the typical range for the NGP Ecoregion. Concentrations grew throughout the summer to peak in August at 456 µg/L and fall in September. TP has historically been high with measures as high as 603 µg/L in 1987. Chlorophyll-a averaged 40 µg/L, below average for this study and within the typical range for the ecoregion. Individual measures grew to 76.1 µg/L in August, after which the level began to drop. Chlorophyll-a varied greatly historically, but seems to be dropping slightly. Compared to the lakes in the ecoregion, summer-mean TP was around the 75th percentile while average chlorophyll-a was about the 40th. The TP to chlorophyll-a ratio was rather high which is comparable to the lake’s historical record. Vertical tows indicated the presence of small and medium zooplankton. Secchi depth averaged a low 0.4 feet (0.1 m), which is below the typical range based on ecoregion reference lakes. Historically, Secchi had been marginally higher even when TP and chlorophyll-a concentrations were higher than those found in 2003. TSS and VSS averaged 38 and 10 mg/L, respectively. VSS is within the typical range for the ecoregion, while the TSS is above it. The water was particularly turbid in September and suspended sediment likely limits light for macrophyte and algae growth in the lake.
Lake Hattie TP and chlorophyll-a data: 2003
Hattie (75-0200) Total Phosphorus and Chlorophyll-a results 2003
500 456 400 333
300 246 225 PPB 200 100 76 62 6 17 0 3-Jun 8-Jul 20-Aug 9-Sep PPB TP PPB Chl-a
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Blue-green algae were the most common form of algae in September. During this month the genus, Anabaena, was particularly abundant. Nuisance blooms probably occurred in August with elevated nutrient levels. It was also noted that there was an abundance of large Aphanizomenon at the surface of the lake in July.
Lake Hattie algal composition Lake Hattie 75-0200 100% 80% 60% 40% 20% 0% Sep-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey The most recent survey for Lake Hattie was conducted in 1996. During that survey only two native plant species were found to yield a FQI of 6. These plants were found to 44 percent (4 feet) of the maximum depth (9 feet) of the lake. No other metric were available from that survey.
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25. Monson Lake (76-0033) is located in Swift County two miles southwest of Sunberg in the CHF ecoregion. Its eastern shores share a border with Monson Lake Memorial State Park and its camp grounds. Monson was under the 25th percentile within this study for lake area and watershed area at 153 and 1,164 acres respectively. Conversely, the lake was over the 75th percentile for maximum depth at 21.0 feet and mean depth at 12.0 feet. Due to its location adjacent to a state park, the lake is used often as a fishery. Walleye are stocked every two or three years. According to a 2001 MDNR fishery survey, several fish species are present within the lake. Northern pike and largemouth bass numbers were found in numbers above historical ranges. There was a moderate number of black bullhead, but the ratio of black to yellow bullhead was rather high, which could be a cause for concern regarding potentially degraded water quality. No mention of carp was made within the survey. Summer-mean TP at 90 µg/L is above the typical range for the CHF ecoregion (Table 3). Individual measures increased from 32 µg/L in June to 119 µg/L in September. Chlorophyll-a averaged 46 µg/L which is also above the typical range. Its concentrations increased throughout the summer to peak in September at 80.4 µg/L. Compared to all lake data for the ecoregion, summer-mean TP was around the 68th percentile while chlorophyll-a was about the 75th. Secchi depth averaged 3.2 feet (1.0 m), which is below the typical range for the ecoregion. TSS and VSS averaged 12 and 10 mg/L respectively. Both of these values were higher than the typical range.
Monson Lake TP and chlorophyll-a data: 2003
Monson (76-0033) Total Phosphorus and Chlorophyll-a results 2003 200
150 119 109 100 100 80
PPB 54 42 50 32 7 0 4-Jun 8-Jul 20-Aug 9-Sep PPB TP PPB Chl-a
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Blue-green algae was dominant throughout the summer and, considering the lake’s elevated levels of chlorophyll-a, probably contributed to frequent algal blooms. During June, balls of algae were observed. Filamentous algae were noted in July. Large algae bodies (colonies) were present in late summer as well. The genus, Anacystis, was particularly abundant in September.
Monson Lake algal composition
Monson Lake 76-0033 100% 80% 60% 40% 20% 0% Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
This lake is located in Monson Lake State Park. Plants were present throughout the lake, except in areas greater than 15 feet deep. Less than 70% of the sample stations were vegetated. Curly-leaf pondweed (Potamogeton crispus) was the most common plant found. It was very dense; often no other species were present where the curly-leaf was growing. Stiff-water crowfoot was present, but not sampled at any of the points. In 2002, the lake’s species richness was nine, and the floristic quality index was 19.97. The maximum water depth is 21.5 feet, and the mean is 12.1 feet (sample stations N=68).
Prior surveys In a 1987 survey, vegetation grew to a depth of 8 feet and curly-leaf pondweed and bushy pondweed were both recorded as abundant. Other submerged species included water milfoil, narrowleaf pondweed, coontail and sago pondweed. A dense blue/green algae bloom was recorded. Emergents included arrowhead, burreed, water smartweed, three-square bulrush and hardstem bulrush. In a 1982 survey submerged plants grew to 8 feet and common species included coontail, and water milfoil. A moderate bloom of blue-green algae was reported. In a 1951 survey common submerged species were coontail, narrow-leaf pondweeds and other species reported were water milfoil, clasping-leaf pondweed, sago pondweed, bulrushes, cattails, and arrowhead. It was noted that lowering the water level would benefit lake for ducks, but it is impractical without an outlet. Secchi disk was 6.5 feet in July. Shoreland was described as 95% ungrazed woods and 5% wild hay with public access through Monson State Park.
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26. Hollerberg Lake (76-0057) is located in Swift County five miles south of Murdock in the NGP ecoregion. It has one inlet to the north. The lake is connected to Hollerberg Lake State Wildlife management area by its southern shores. Reed emergents were patchy throughout the lake during the summer. Thick submergents were noted during the middle to late summer. Hollerberg’s surface area is 260 acres with a watershed of 3,233 acres. Its watershed to lake area ratio is 12.4:1. With a maximum depth of 5.5 feet the lake was below the 25th percentile compared to other lakes in the study. Its mean depth is correspondingly low at ~3.5 feet. There have been no MDNR Fisheries surveys to date completed on Hollerberg Lake.
Summer-mean TP at 90 µg/L was below the typical range for the NGP Ecoregion based on reference lakes, falling around the 22nd percentile. Concentrations rose slightly to peak in August at 101 µg/L, and then began to drop. chlorophyll-a averaged 26 µg/L, which is also lower than the typical range, falling just below the 25th percentile. Chlorophyll-a peaked in September at a concentration of 31.4 µg/L. Secchi depth was within the typical range for the ecoregion, averaging 2.2 feet (0.7 m). TSS and VSS averaged 15 and 10 mg/L respectively which were also within the typical range for the ecoregion.
Hollerberg :Lake TP and chlorophyll-a data: 2003
Hollerberg (76-0057) Total Phosphorus and Chlorophyll-a results 2003 500
400
300
PPB 200 129 101 83 95 100 26 39 20 31 0 4-Jun 8-Jul 20-Aug 9-Sep PPB TP Chl-a
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Yellow-brown algae were the most common algal form during the month of June and the genus Dinobryon, was dominant. Blue-green algae became dominant during August and September coinciding with peaks in TP and chlorophyll-a. No specific genera were dominant during this month.
Hollerberg Lake algal composition
Hollerberg Lake 76-0057 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
The aquatic plant survey was conducted in 2003. The maximum water depth is 5.5 feet, and the mean is 3.6 feet (sample stations N=74). The lake’s species richness was 8, and the floristic quality index was 15.27. The most common plants were Myriophyllum sibiricum and Stuckenia pectinata. The entire northeast bay was choked with sago pondweed (Stuckenia pectinata) and Chara. Filamentous algae was scattered throughout the basin. DNR Fisheries was using the lake as a walleye-rearing pond. The lake water was tinted red near the inlet on the north side of the lake.
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27. Lake Hassel (76-0086) is located in Swift County five miles east of Clontarf in the NGP ecoregion. It has a very soft bottom and its water is colored by readily suspended silt and fine sediments that remained in suspension following disturbance by wind or motor boat activity. Aquatic vegetation was not evident during the water quality sampling events. It has a maximum depth of 5.0 feet and a mean depth of 4.0 feet. Lake Hassel has a surface area of 706 acres with a watershed of 23,335 acres and a watershed to lake area ratio of 33.1:1. Area, watershed area, and watershed to lake area ratio were all over the 75th percentile as compared to lakes within this study. There have been no MDNR fishery surveys to date complete on Lake Hassel.
Summer-mean TP at 270 µg/L was over the typical range of TP based on (Table 3). TP peaked in August at 367 µg/L. Historically, TP has been considerably less with values around 150 µg/L in the mid 1980’s. Chlorophyll-a averaged 92 µg/L, above the typical range for the ecoregion. It peaked in August at 118 µg/L. Past concentrations were lower than this, indicating an upward trend in the levels of both TP and chlorophyll-a. Compared to the current data for lakes in the ecoregion, Hassel’s TP fell around the 83th percentile while chlorophyll-a was above the 95th percentile. Secchi depth averaged a low 0.4 feet (0.1 m), below the typical range for the ecoregion. TSS and VSS were extremely high, averaging 142 and 61 mg/L, respectively. These values are considerably above the typical range for the NGP and indicate a very high level of turbidity within the lake as was observed during sampling. This high turbidity likely suppresses algal and macrophyte growth in the lake.
Hassel Lake TP and chlorophyll-a data: 2003
Hassel (76-0086) site 102 Total Phosphorus and Chlorophyll- a results 2003 500
400 367 301 299 300 PPB 200 111 102 117 105 100 43
0 4-Jun 8-Jul 20-Aug 9-Sep PPB TP PPB Chl-a
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Blue-green algae were the more common form of algae during the months of June and September. In August, both blue-green algae and diatoms were equally abundant. July was the sole summer month when diatoms were more common. The blue-green genus Anacystis was found to be particularly abundant in June, August, and September. The A centric diatom was abundant during July.
Lake Hassel algal composition
Lake Hassel 76-0086 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms
MDNR vegetation survey
This lake has been managed by the DNR to try to improve submerged aquatic plant abundance, however carp and other factors make management difficult. There was a good fringe of bulrush and cattails along most of the shoreline. Vegetation in the main part of the lake was spotty, and primarily sago pondweed (Stuckenia pectinata) and northern watermilfoil (Myriophyllum sibiricum). The southwest bay had the greatest diversity and density of plants. Najas marina, a special concern species in Minnesota, was found in this area. Vallisneria Americana was seen uprooted and floating near the northeast shore, but it was not found at any sample station. Water clarity was poor - the average Secchi disk reading was 0.7 ft. Cattle were seen grazing to the water's edge and into the water near point #134. The basin is subjected to a lot of wind, primarily from the southeast and south (this was the case all three days of vegetation surveying). In 2002, the lake’s species richness was 12, and the floristic quality index was 19.28. The maximum water depth is 4.5 feet, and the mean is 2.4 feet (sample stations N=140).
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28. Trace Lake (77-0009) is located in southeastern Todd County within the city of Gray Eagle in the CHF ecoregion. The lake has a very flat bottom with an abundance of submergent vegetation and one outlet on its northern shore to Lady Lake. Its surface area is 277 acres. Its watershed area is 672 acres, the smallest within this study. Trace Lake has a maximum depth of 9.0 feet and a mean depth of 6.0 feet. The lake receives seasonal discharges from the wastewater treatment ponds for the City of Grey Eagle. The City has a phosphorus effluent limit in their permit and the discharge is treated so that P effluent concentrations are at 1 mg/L or less. The MDNR reports that Trace Lake is the largest walleye rearing area and is stocked every year with walleye fry. A fish barrier was installed in the lake’s single outlet and a microfish barrier will be installed. The lake has a massive black bullhead population. Other species, such as green sunfish and fathead minnows, are present. Carp are not a water quality problem and the plant community experiences frequent changes. Historically, there have been severe winterkills within the basin.
Summer-mean TP at 120 µg/L is well above the typical range based on CHF Ecoregion reference lakes, falling around the 77th percentile for all lake data in the Ecoregion. Individual concentrations ranged from 51 µg/L in June to 238 µg/L in August. The lake has had a history of high TP concentrations. Chlorophyll-a averaged 26 µg/L, above the typical range and at about the 60th percentile based on all the lake data for the Ecoregion. Concentrations varied from 6.94 µg/L in June to 74.7 µg/L in August. Historically, chlorophyll-a concentrations were higher. Secchi depth averaged 2.8 feet (0.8 m), which is still below the typical range but an improvement upon 1992 when the depth was 1.0 feet. TSS and VSS averaged 12 and 11 mg/L respectively which are both higher than the typical range (Table 3).
Trace Lake TP and chlorophyll-a data: 2003
Trace Lake (77-0009) Total Phosphorus and Chlorophyll-a results 2003
500
400
300 238 PPB 200 168
94 75 100 51 33 7 10 0 3-Jun 7-Jul 19-Aug 8-Sep
PPB TP PPB Chl-a
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Blue-green algae remained dominant throughout the summer in Trace Lake. Several genera were abundant during the various months: in June and July, Anacystis was most common; the filamentous Aphanizomenon was most common during August, when TP and chlorophyll-a concentrations peaked most likely causing heavy algal blooms; Anabaena was most common genera during September.
Trace Lake algal composition
Trace Lake 77-0009 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
The lake was surveyed during 2002 by Shallow Lake Program staff. Water levels were extremely high as the access was inundated. At 64 sample stations, the survey maximum lake depth was 8.5 feet and mean lake depth was 6 feet (sample stations N=64). Eighty-seven percent of the sample stations were vegetated. Lake-wide species richness was eight, and the floristic quality index was 14.85. Vegetation was found throughout the basin, but the plants were in poor shape. The basin was dominated by coontail (Ceratophyllum demersum), and sago pondweed (Stuckenia pectinata) was common. Fries pondweed (Potamogeton friesii) winterbuds were found at 48% of the sample stations (31 out of 64 sample stations). Winterbuds were found at 6 points that had no other vegetation. There was extensive blue- green algae throughout the basin. The basin is used as a DNR walleye-rearing pond; however no fish were observed during the plant survey.
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29. Pelican Lake (86-0031) is located in Wright county four and a half miles west of St. Michael in the CHF ecoregion. Pelican has three distinct bays (all of which were sampled) and a uniform, flat bottom. Pelican has extensive emergent and submergent vegetation, making it difficult to traverse. The lake has been subject to extensive flooding, with visible signs such as sunken islands, now-emergent trees, and now-submerged pumping equipment. Pelican was the largest lake in the study at 2,793 acres and its 7,705 acre watershed area was over the 75th percentile. Its watershed to lake ratio is 2.8:1. The lake has a maximum depth of 11.3 feet and a mean depth of 5.0 feet. Water levels, which had been dropping since 1996, began to climb since 2002 to a current elevation of 954.1 feet, two feet above its OHW elevation. There is no MDNR information on the Pelican Lake fishery.
Summer-mean TP at 170 µg/L was well above the typical range based on CHF Ecoregion reference lakes. Individual measures in each of the three bays peaked in September at values of 199-224 µg/L. Chlorophyll-a averaged 95 µg/L, which is also well above the typical ecoregion range. Chlorophyll-a, too, peaked in September with values of 132-156 µg/L. Mean values of TP and chlorophyll-a fell around the 90th percentile when compared to all the lake data for the ecoregion. Secchi depth averaged a low 1.6 feet (0.5 m), well below the typical range. TSS and VSS averaged 32 and 27 mg/L, both extremely high for the CHF ecoregion.
Pelican Lake TP and chlorophyll-a data: 2003
Pelican Lake (86-0031) Total Phosphorus 2003 500
400
300 201 224 PPB 188 187 199 200 170 181 172 143 116 122 123 100
0
12-Jun 1-Jul 6-Aug 3-Sep Site 101 Site 102 Site 103
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Pelican Lake (86-0031) Chlorophyll-a
200 156 132 150 135 104 90.2 107 101 92.4 100 PPB 58.3 58.5 54.7 46.6 50
0 12-Jun 1-Jul 6-Aug 3-Sep
Site 101 Site 102 Site 103
Blue-green algae were by far the most common algal form throughout the summer. The genus, Anacystis, was dominant in the month of June and no other genera were particularly abundant during subsequent months. Based on extremely high chlorophyll-a values it is likely that severe nuisance blooms were common throughout the summer.
Pelican Lake algal composition
Pelican Lake 86-0031 100% 80% 60% 40% 20% 0% Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns
MDNR vegetation survey
In 2001, there was a wide-diversity of plant species, but plants were found primarily in shallower portions where the density was highest. Few to no plants were found in the middle and open parts of the lake. The lake’s species richness was 12, and FQI was 16.67. The maximum water depth was 11.3 feet, and the mean was 5.3 feet (sample stations N=23).
Prior surveys The lake was previously surveyed in 1949, 1957, 1990, and 1999. In 1949, lake wide species richness was 19, and maximum water depth was 6 feet and mean depth was 3.4 feet. In 1957, the lake wide species richness increased to 29. In 1990, when the maximum water depth was 9 feet, and the mean depth was 5.4 feet a significant decrease in species richness occurred when only lesser duckweed (Lemna minor) and three species of emergent vegetation were found in the basin. Recovery took place, as in 1999 twenty-two species of plants occurred when the maximum water depth was 14.5 feet and mean depths was 6.3 feet.
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30. Cedar Lake (86-0073) is located in Wright County five miles southwest of Monticello in the CHF ecoregion The majority of the lake is littoral, but it has two deeper areas in the northern portion of the lake. It has two public accesses which are surrounded by thick macrophytes. The lake drains west through a culvert to a smaller lake, to which it was once connected. During one sampling date, hundreds of small, schooling bullheads surrounded this culvert. Cedar is the deepest lake within this study with a maximum depth of 47.0 feet. Its mean depth is second in the study at 15.0 feet. In contrast, the lake was below the 25th percentile for lake area at 147 acres, watershed area at 533 acres and watershed to lake area ratio at 3.6:1. Cedar Lake was last surveyed in 1991 by the MDNR. Northern pike and walleye were found to be fairly abundant. Black bullhead were below the typical range for a lake of this class, but yellow and brown bullhead were above the typical range. Thus, the ratios of black to brown and black to yellow were rather low. Carp were not mentioned as a water quality problem in that survey.
Cedar Lake had the lowest summer-mean TP and chlorophyll-a in the study. TP averaged 20 µg/L and chlorophyll-a averaged 4 µg/L – both of which are lower than the typical range (Table 3). Cedar was around the 10th percentile for TP and chlorophyll-a compared to all ecoregion data. Secchi depth averaged 12.0 feet (3.6 m), considerably above the typical range for the ecoregion. TSS and VSS averaged on the lower end of the typical range at 2 and 1 mg/L, respectively.
Cedar Lake TP and chlorophyll-a data: 2003
Cedar (86-0073) Total Phosphorus and Chlorophyll-a results 2003 200
150
100 PPB
50 23 20 19 15 4 3 5 7 0 17-Jun 1-Jul 6-Aug 3-Sep
PPB TP PPB Chl-a
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Yellow-brown algae were the most common form of algae in the early summer months of May and June. During these months, the genus Dinobryon was particularly abundant. Throughout middle to late summer, blue-green algae dominated Cedar Lake. The genus, Anacystis, was abundant during July and Anabaena was abundant in August and September. Judging by the lake’s high level of clarity and extremely low levels of TP and chlorophyll-a, nuisance algal blooms most likely did not occur during the summer.
Cedar Lake algal composition Cedar Lake 86-0073 100% 80% 60% 40% 20% 0% May-03 Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Yellow Browns Other
MDNR vegetation survey
No plant surveys have been conducted on this lake.
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31. Smith (86-0250) Smith is a moderate sized lake at 226 acres located west of Howard Lake. It is quite shallow and considered 100 percent littoral. Based on a 1980 fisheries survey black bullhead and carp were excessively high in the lake – both greatly exceeding the statewide medians.
Summer-mean TP and chlorophyll-a were both relatively high at 150 ppb and 63 ppb, respectively. This combined with high TSS – 36 mg/L, resulted in very low mean transparency of 1.3 feet (0.4 m).
Smith Lake TP and chlorophyll a for 2003
Smith (86-0250) Total Phosphorus and Chlorophyll-a results 2003 500
400
300 268
PPB 200 126 122 78 100 76 74 76 24 0 17-Jun 23-Jul 14-Aug 15-Sep
PPB TP PPB Chl-a
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Blue-green algae were dominant in Smith Lake throughout the summer of 2003 and other forms were poorly represented. Late summer zooplankton was dominated by small daphnids (Fig. 12) and no large daphnids were noted. Daphnids will crop smaller algal forms, such as greens or diatoms but generally do not feed on blue-green algae.
Smith Lake algal composition
Smith 86-0250 100%
80% 60% 40% 20%
0% May-03 Jun-03 Jul-03 Aug-03 Sep-03 Blue Greens Diatoms Greens Yellow Browns Other
MDNR vegetation survey
During the 2001 survey more than 90% of the sample stations were vegetated. Common plants included Ceratophyllum demersum, Najas flexilis, Potamogeton friesii, and Stuckenia pectinata. Potamogeton zosteriformis also occurred in the basin. Good sago pondweed beds occurred in the southwest bay. Also, extensive beds of coontail (Ceratophyllum demersum) and bushy pondweed (Najas flexilis) occurred throughout the lake. In 2001, the lake’s species richness was ten, and the floristic quality index was 11. The maximum water depth was 10 feet, and the mean was 4 feet (sample stations N=16).
Prior surveys Smith Lake was surveyed in 1950, 1978, 1980, and 1990. In 1990, about 40% of the sample stations (N=13) were vegetated by submerged plants, most commonly by sago pondweed (Stuckenia pectinata) and flat-stem pondweed (Potamogeton zosteriformis). Cattails were also found on the perimeter of the basin. Maximum depth was 3.5 feet and median depth was 2.5 feet.
In 1978, a lush 20-foot fringe of cattail occurred around lake and in the south-central and southwest bays. Muskgrass and sago pondweed were also present. Maximum lake depth was 5.7 feet and the mean 4.5 feet, and water levels were considered high due to heavy summer rains. Conditions were very similar in 1980, when about 10% of the lake was covered with cattail, which was found as a fringe on the shoreline and in the southwest bay. About 15% of the lake had submerged vegetation that was dominated by coontail and sago pondweed. Water conditions in 1980 were considered stable, and the maximum water depth was 5 feet and median water depth was 3.8 feet.
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Comparisons among lakes for 2003
Ranges and patterns in total phosphorus, total Kjeldahl N, chlorophyll-a and Secchi
Summer-mean TP ranged from about 20 µg/L in Cedar (73-0255) to 370 µg/L in French Lake (Table 4a) with a median concentration of 108 µg/L for the lakes in this study. This range in TP was by design to allow us to evaluate a variety of factors and relationships across a broad spectrum of trophic state.
In deep, dimictic lakes in northern and central Minnesota there are often distinct seasonal patterns in TP, whereby TP is often high in spring (April or May) and then tends to slowly decline from June through September. With the onset of fall mixing surface concentrations may increase as nutrient-rich hypolimnetic waters mix with the surface water. In the shallow southwest MN lakes in 2002 (WCP & NGP ecoregions) there were no consistent seasonal patterns in TP (Heiskary et al., 2003). Wind mixing and resuspension of bottom sediments were thought to be a significant factor in the within-lake variability in TP over the sample season. In the west-central lakes we tend to see a more distinct pattern in TP concentrations over the summer with peak TP in either August or September in most of the lakes (Fig. 5). This is in contrast to deeper dimictic lakes that often exhibit declines in TP over the summer. This pattern is more evident if we calculate monthly mean values for all of the west-central study lakes for 2003 (Fig. 7). In this fashion a steady increase in TP over the summer if quite evident. However, if we consider the standard error of these estimates the data fall into two groups: spring and early summer (May & June) and mid to late summer (July – September).
While many factors such as wind mixing and sediment resuspension may contribute to this, most of these lakes are much more vegetated than the southwest lakes, which should serve to minimize resuspension. However, many of the west-central lakes had significant beds of curly-leaf pond weed (Potamageton crispus). It is well documented that this macrophyte, which prospers in the cool waters of spring and early summer, begins to senesce in mid summer as the lake warms. As the curly-leaf dies back and decomposes substantial amounts of nutrient (P and N) are released – which may contribute to mid-summer pulses in TP and algal growth. Monson and Fremont Lakes, which had curly-leaf reported at a high percentage of sites, both exhibit mid summer increases in TP (Fig. 5). High water temperatures and pH values that averaged 22.3 C and 9.0, respectively (Table 4b), also encourage internal recycling of phosphorus from the sediments, which can be an additional contributor to mid-summer TP increases.
Nitrogen (N), while not considered the limiting nutrient in most cases for freshwater lakes, is nonetheless an essential nutrient for algal and rooted plant growth. And, as EPA has requested states to consider development of N criteria, some brief discussion of N, as related to TP, chlorophyll-a and trophic status is merited here. Total nitrogen (TN), for purposes of discussion, is essentially equal to total Kjeldahl nitrogen (TKN, includes organic-N & ammonia-N) as nitrite + nitrate – N is at or below detection (0.01 mg/L) for most NLF and CHF lakes (Table 3). For WCP and NGP lakes, summer-mean nitrate-N typically ranges between detection and 0.1 mg/L. As such, the nitrate contribution to TN is rather low in most Minnesota lakes. TKN values, overall in the 2003 lakes, ranged from a low of 0.7 mg/L (Clark Lake) to 5.72 mg/L (East Twin Lake). Distinct regional patterns in TKN were evident as well with the 19 CHF lakes exhibiting an interquartile (IQ) range of 1.28 – 2.32 mg/L, which is high relative to reference lake data for
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the CHF (Table 3). Individual values in the CHF ranged from a low of 0.80 (Cedar 86-0073) to a high of 5.25 mg/L (French Lake). The three NLF lakes ranged from 0.70-0.098, which likewise is a bit high relative to reference lakes for the region (Table 3). The seven NGP & WCP lakes, collectively exhibited an IQ range of 2.27 – 5.05 mg/L, which is above the typical range exhibited by reference lakes for these two regions (Table 3). TP and TKN are highly correlated in the shallow CHF lakes in this study (Fig. 6a), which is consistent with our findings for other Minnesota lakes (Heiskary and Wilson, 2005, in prep.).
TN:TP ratios are often used as a basis for defining “P-limitation (when TN:TP >17:1)” vs. “N- limitation (when TN:TP<10:1)” with lakes in between being either P or N-limited (Smith, 1983). Smith (1983) also notes that low TN:TP ratios often favor dominance by blue-green algae. Blue- greens are favored, in part, at low TN:TP because some species can fix N from the atmosphere under “N-limiting” conditions (Wetzel, 2001). As noted in the preceding discussion of individual lake results blue-green algae were common in all the 2003 study lakes and were dominant in many of the lakes. Though we refer to “N-limitation,” from a lake water-quality management standpoint, this often means that very large P reductions are required to bring the lake closer to “P limitation.”
The IQ ranges for the CHF, NLF and NGP/WCP TN:TP ratios in the 2003 study were: 19-26, 28-29 and15-26 respectively. While the TN:TP ratios for the NLF and NGP/WCP lakes were comparable to their reference lake ranges (Table 3), the CHF lake TN:TP ratios tended to be lower than the CHF reference lakes. Based on a comparison of TN:TP ratios and TP these lakes will generally be considered “P-limited” when TP is at or below about 0.100 ppm (100 µg/L) – which was the case for most of the CHF lakes (Fig. 6b).
Summer-mean chlorophyll-a ranged from a low of 4 µg/L in Cedar Lake (73-0255) to a high of 180 µg/L in French with a median of 35 µg/L. Summer maxima ranged from ~ 5 µg/L to 408 µg/L in these two lakes. Again, as with TP, having a range in trophic status was important to the overall goals of the study.
Chlorophyll-a often exhibits a seasonal increase in dimictic lakes with peak chlorophyll-a observed in August or September (not including the large diatom bloom that may occur following spring overturn). Chlorophyll-a in the study lakes was highly variable between sample dates. But for most lakes, peak concentrations were observed in August or September – coinciding with peak TP for many of the lakes. Across the entire data set there was a steady increase in chlorophyll-a from May through September (Fig. 7 & 8), with May and June being significantly lower than July through September measurements (Fig. 8).
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Figure 5. Total phosphorus concentrations for summer 2003. Lakes sorted by mean TP. a) mean TP < 100 ppb
200
150
100 TP ppb 50
0 Cedar 86- Cedar 73- Clark Prairie Platte Red Sand Cedar 73- Nelson McCormic Johanna Ringo Site 0073 0226 0255 101 May June July Aug Sept. b) mean TP = 90 – 160
200 213
150
100 TP ppb
50
0 Monson Hollerberg Fremont East Solomon Ringo 102 Quamba Tiger Silver Florida Slough May June July Aug Sept.
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c) mean TP 120-170 ppb
500
450
400
350
300
250 TP ppb 200
150
100
50
0 Trace Smith Pelican Site 103 Shaokotan 102 Pelican Site 102 Shaokotan 103 Pelican Site 101 May June July Aug Sept.
d) mean TP 230-320 ppb
500 721 450
400
350
300
250 TP ppb 200
150
100
50
0 Titlow Eest Twin Jennie Diamond Hassel French Hattie West Twin May June July Aug Sept.
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Figure 6. Comparisons of a) TKN & TP and b) TN:TP ratios & TP for 2003 study lakes. a)
TKN vs. TP by ecoregion
7.00
6.00
5.00
4.00
3.00 TKN ppm 2.00
1.00
0.00 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 TP ppm TKN = 12.325 TP + 0.6664 CHF NGP NLF Linear (CHF) R2 = 0.86 b)
TN:TP ratios vs. TP for west-central lakes 2003
45 40 35 30 25 P-lim ited 20 TN:TP 15 10 5 0 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 TP ppm
CHF NGP NLF
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Figure 7. Chlorophyll-a concentrations for summer 2003. Lakes sorted by mean chlorophyll-a a) mean chl-a < 15 ppb
50
40
30
20 ppb Chl-a
10
0 Cedar 73- Cedar 86- Red Sand Cedar 73- Platte Clark Prairie Shaokotan McCormic 0255 0073 0226 102
May June July Aug Sept.
b mean chl-a 15-35 ppb
100
90
80
70
60
50
Chl-a ppb Chl-a 40
30
20
10
0 Nelson East Shaokotan Hollerberg Trace Johanna Ringo Site Quamba Solomon 103 101
May June July Aug Sept.
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c) mean chl-a 35-65 ppb
100 101 90 80 70 60 50
Chl-a ppb Chl-a 40 30 20 10 0 Hattie Fremont Tiger Monson Florida Titlow Ringo 102 Silver Smith Slough
May June July Aug Sept.
d) mean chl-a >65 ppb
200 408 180 160 140 120 100
Chl-a ppb Chl-a 80 60 40 20 0 Diamond Pelican Site Hassel Pelican Site Pelican Site Jennie Eest Twin West Twin French 103 102 101
May June July Aug Sept.
Total phosphorus, TKN, chlorophyll-a and Secchi relationships
There are various methods of describing the interrelationships among the trophic state variables: TP, chlorophyll-a and Secchi. Empirically-derived regression techniques that rely on data from previously studied lakes are a common method. These regressions can then be used to predict changes in the “response” variables (chlorophyll-a, bloom frequency and Secchi transparency) as a function of changes in the “causative” variable (TP). Carlson’s Trophic State Index (TSI) (Carlson, 1977; Fig. 9) is one such example of the use of empirical data and equations.
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While Carlson’s TSI is quite useful for describing trophic status and interrelationships among these three trophic status variables for individual lakes it is oftentimes used to examine patterns within groups of lakes. The relationships among these variables have been described numerous times in the literature for lakes from across North America (e.g., Canfield and Bachman, 1981), Europe (e.g., Ryding and Forsberg, 1980) and other locations. Specific regression equations for Minnesota lakes were derived from the ecoregion reference lake data and are as follows (Heiskary and Wilson, 1988): 2 Eq. 1) Log10 chlorophyll-a = 1.16 Log10 TP - 0.76 (R = 0.73, n=103) 2 Eq. 2) Log10 Secchi = -0.57 Log10 chlorophyll-a + 0.87 (R =0.82, n=103)
These equations are similar to those found elsewhere in the literature (e.g., Jones and Bachman, 1976). Heiskary and Wilson (1988) found that the correlations (Eq. 1 and 2) were strengthened somewhat if the data were limited to lakes with lower TP concentrations (< ~ 100 µg/L). While shallow lakes were included in deriving Eq. 1 and 2, we thought it might be instructive to look at these relationships for shallow lakes specifically and compiled summer-mean data for lakes from the 2003 study. This yielded 31 lakes with paired TP, chlorophyll-a, and Secchi data (Fig. 9a). While there is scatter in the TP and chlorophyll-a relationship at the high TP concentrations the regression equation (Eq. 3) is similar to the statewide ecoregion-based equation (Eq. 1) and actually exhibits a higher R2 value. The R2 for the west-central lakes is much higher than that derived from 62 lake-summers of TP & chlorophyll-a data compiled for WCP and NGP lakes (Eq. 4, Heiskary et al., 2003). The between-region difference in TP: Chl-a relationship is even more pronounced if we compare the 26 SW MN lakes sampled in 2002 (Heiskary et al., 2003) with the CHF west-central lakes (n=19) (Fig. 19b). 2 Eq. 3) Log10 chlorophyll-a = 1.08 Log10TP – 0.660 (R =0.80, n=31) (west-central lakes); 2 Eq. 4) Log10 chlorophyll-a = 0.84 Log10TP – 0.002 (R =0.51, n=62) (southwest lakes);
Figure 8. Monthly mean total phosphorus and chlorophyll-a for west-central lakes 2003.
Monthly mean TP & Chlorophyll-a for w est-central lakes 2003
250
200
150 ppb 100
50
0 May June July August September Mean month
TP Chl-a
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Figure 9. Carlson’s Trophic State Index
Carlson’s Trophic State Index RE Carlson
TSI < 30 Classic Oligotrophy: Clear water, oxygen throughout the year in the hypolimnion, salmonid fisheries in deep lakes.
TSI 30 - 40 Deeper lakes still exhibit classical oligotrophy, but some shallower lakes will become anoxic in the hypolimnion during the summer. TSI 40 - 50 Water moderately clear, but increasing probability of anoxia in hypolimnion during summer. TSI 50 - 60 Lower boundary of classical eutrophy: Decreased transparency, anoxic hypolimnia during the summer, macrophyte problems evident, warm-water fisheries only. TSI 60 - 70 Dominance of blue-green algae, algal scums probable, extensive macrophyte problems. TSI 70 - 80 Heavy algal blooms possible throughout the summer, dense macrophyte beds, but extent limited by light penetration. Often would be classified as hypereutrophic. TSI > 80 Algal scums, summer fish kills, few macrophytes, dominance of rough fish.
Oligotrophic Mesotrophic Eutrophic Hypereutropic
20 25 30 35 40 45 50 55 60 65 70 75 80 Trophic State Index
15 10 8 7 6 5 4 3 2 1.5 1 0.5 0.3 Transparency (m)
0.5 1 2 3 4 5 7 10 15 20 30 40 60 80 100 150 Chlorophyll-a (ppb)
3 5 7 10 15 20 25 30 40 50 60 80 100 150 Total Phosphorus (ppb)
After Moore, 1. And K. Thornton, [Ed.]1988. Lake and Reservoir Restoration Guidance Manual. USEPA>EPA 440/5-88-002.
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Figure 10. Summer-mean TP vs. chlorophyll-a (log-log) and Secchi a) all west-central lakes (n=31 lakes) compared to statewide regression and b) CHF WC lakes only (n=19) compared to SW lakes (2002 study). a)
TP vs. Chlorophyll-a for shallow West Central MN Lakes. Statewide regression noted. 2.50
2.00
1.50
1.00 Log Chl-a ppb Chl-a Log 0.50
0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 Log TP ppb y = 1.08x - 0.66 West-Central Statew ide Linear (West-Central) R2 = 0.80 b)
West-Central (WC) vs. Southwest (SW) MN Lakes TP vs. chlorophyll-a
3.00 WC-Chl = 1.15TP - 0.75 R2 = 0.84 (n=19) 2.50 SW-Chl = 0.44 TP+ 0.712 R2 = 0.10 (n=26) 2.00
1.50
Log Chl-aLog ppb 1.00 73-0226 73-0255 0.50
0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 Log TP ppb
SW-Chl WC-Chl Linear (WC-Chl) Linear (SW-Chl)
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A significant relationship is also evident for TKN and chlorophyll-a (Fig. 11) based on the CHF lakes. This is not too surprising given that TKN is includes organic forms of N such as that found in algae. As with TP the relationship appears to be tighter at the lower end (TKN <~2 mg/L) and becomes more variable at higher concentrations. Based on Fig. 11 it appears that the nutrient-rich lakes of the NGP & WCP ecoregions yield lower chlorophyll-a per unit TKN as compared to the CHF lakes.
Figure 11. Summer-mean TKN vs. chlorophyll-a for west-central lakes: 2003
Chlorophyll-a vs. TKN for West-central lakes 2003
200 180 160 140 120 100 80 chl-a ppb chl-a 60 40 20 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 TKN ppm
Chl-a = 36.3 TKN - 29.20 CHF NGP NLF Linear (CHF) R2 = 0.88
In addition to comparing summer-mean concentrations of TP, chlorophyll-a and Secchi it has proven useful to describe the relationship between TP and the frequency and magnitude of nuisance algal blooms. This relationship was previously established for Minnesota lakes based on data from the ecoregion reference lakes (Heiskary and Walker, 1988; Heiskary and Wilson, 1988). While that work described the relationship for oligotrophic to mildly hypereutrophic lakes it was data-poor at the higher TP concentrations. Data from the west-central and southwest Minnesota lake-studies allow us to examine this relationship specifically for shallow lakes (Fig. 12). At TP concentrations less than 40 µg/L “nuisance blooms” (previously defined as chlorophyll-a>20 µg/L) occur less than 5% of the summer. As concentrations increase to the 60 µg/L range nuisance blooms occur about 60% of the summer and “severe nuisance blooms” (previously defined as chlorophyll-a 30-60 µg/L) occur about 20% of the summer. Based on Fig. 7 and 12 lakes with summer-mean TP less than 60 µg/L and chlorophyll-a less than 15 µg/L did not tend to experience severe nuisance blooms. However, at TP concentrations in the 80 – 100 µg/L range severe nuisance blooms occur about 70 % of the summer (Fig. 12). Summer-mean chlorophyll-a was generally greater than 20 µg/L as well in these lakes (Fig. 7). As concentrations rise further severe nuisance blooms can be anticipated for the vast majority of the summer and there is a distinct increase in the very severe nuisance blooms (chlorophyll-a >60 µg/L) over the TP range from 100-165 µg/L. Based on results from the individual lakes we
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anticipate these blooms will be dominated by blue-green algae and will occur most frequently during the months of July to September.
Figure 12. Bloom frequency relative to TP. Based on 223 pairs of observations from west- central & southwest Minnesota lakes.
Bloom frequency for shallow lakes. Based on 223 observations.
100%
80%
60%
40% % of summer of %
20%
0% 0 22 40 60 80 100 120 140 165 195 225 >250 TP ppb (interval midpoint)
Chl-a >60 Chl-a 31-60 Chl-a 21-30 Chl-a 11-20 Chl-a <10
The relationship among Secchi and TP and Secchi and chlorophyll-a in these shallow lakes is similar to that of deeper lakes, though the R2 values, 0.69 and 0.71, respectively, are slightly lower than that reported for the ecoregion reference lakes (Heiskary and Wilson, 1990). As with deeper lakes the greatest relative change in Secchi is noted over a TP range below 50 ppb (Fig. 13) and at TP concentrations above 150 ppb the relationship appears to reach an asymptote. For chlorophyll-a the greatest change in Secchi occurs at chlorophyll-a below 20 ppb and as chlorophyll-a increases above about 40-60 ppb the change in transparency is rather minimal. However there is a fair amount of variability in the relationships over a TP range from about 70 – 150 ppb and chlorophyll-a from about 30 – 50 ppb.
Reviewing a few of the “outliers” in Fig.13 can provide some insight on factors (other than TP and chlorophyll-a) that may be influencing the relationships. Red Sand, East Solomon, Fremont and Shaokatan Lakes exhibit a higher Secchi than anticipated based on their TP concentration; Nelson and Ringo exhibit lower than anticipated Secchi based on TP; Fremont, Monson, and Diamond exhibit deeper Secchi than anticipated based on chlorophyll-a and Nelson and Ringo exhibit lower than anticipated Secchi based on chlorophyll-a. Notes on each follow:
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• Red Sand Lake is characterized by an extensive population of macrophytes found at 97% of the plant survey points and occurred to 78% of the maximum depth of the lake. Chlorophyll-a (5 ppb) was lower than anticipated based on TP but the algal community was dominated by blue-green algae. Extensive macrophyte growth, low chlorophyll-a and blue-green dominance likely account for the high transparency of the lake. • East Solomon Lake was well vegetated with macrophytes at 50% of the sites assessed and occurred to 97% of the maximum depth of the lake. The extensive macrophytes combined with abundant daphnids and blue-green dominance noted in 2003 likely account for higher than expected transparency. • Fremont Lake had macrophytes at 99% of the sites assessed and were found to 100% of the maximum depth. The extensive vegetation combined with blue-green dominance likely accounts for the higher than anticipated transparency. • Lake Shaokatan has few if any macrophytes in most years based on previous surveys. In this case blue-green dominance throughout the summer and abundant daphnids, which preferentially graze on other algal forms, combine to account for the higher than anticipated transparency. • Nelson Lake had macrophytes at 52% of the sites assessed and they were found to 56% of the maximum depth of the lake. TSS concentrations were above the typical range for the ecoregion and chlorophyll-a was slightly lower than anticipated based on TP. It is possible that inorganic turbidity (resuspended sediment) may be a partial cause of the lower than anticipated transparency. • Ringo Lake’s macrophyte survey was conducted in 1995 and at that time plants were found over 70% of the maximum depth of the lake. Carp and bullhead were present in moderate numbers (for a lake of this class) based on the most recent fishery survey. TSS was quite high in 2003 (Table 4a) and could very likely account for a lower than anticipated transparency. • Monson Lake had macrophytes at 68% of the sites and were found to 63% of the maximum depth. Carp and bullhead are present in the lake but TSS was not excessive. In 2003 blue-green algae were the dominant algal form and daphnids were found to be quite abundant. The combination of extensive vegetation, blue-green dominance and abundant daphnids likely account for higher than anticipated transparency. • Diamond Lake had macrophytes at 77% of sites assessed and they were found to 75% of the maximum depth. Black bullhead were abundant based on the most recent survey. TSS was high in 2003, which could be attributed to sediment resuspension by fish, wind, and/or power boating activities that were noted. In 2003 blue-greens were dominant and daphnids were abundant. Chlorophyll-a was lower than anticipated based on TP and Secchi was higher -- likely as a combination of the aforementioned factors.
While the above observations are not completely conclusive on the causes for higher or lower than anticipated transparency in some of the lakes they do suggest the multiplicity of factors that may contribute to the variation. Also these observations suggest the significance of submerged macrophytes as they influence these relationships and related factors.
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Figure 13. Summer-mean Secchi vs. TP and chlorophyll-a for west central lakes.
Summer-mean TP vs. Secchi: 2003 y = 26.985x-0.7861 R2 = 0.69 4.0
3.5
3.0
2.5
2.0
Secchi m 1.5
1.0
0.5
0.0 0 50 100 150 200 250 300 350 400 TP ppb
Summer-mean chlorophyll-a vs. Secchi: 2003 y = 7.077x-0.6623 R2 = 0.71 4.0
3.5
3.0
2.5
2.0
Secchi m Secchi 1.5
1.0
0.5
0.0 0 20 40 60 80 100 120 140 160 chl-a ppb
95
MINLEAP – predicted vs. observed TP, chlorophyll-a and Secchi for CHF lakes
Numerous complex mathematical models are available for estimating nutrient and water budgets for lakes. These models can be used to relate the flow of water and nutrients from a lake's watershed to observed conditions in the lake. Alternatively, they may be used for estimating changes in the quality of the lake as a result of altering nutrient inputs to the lake (e.g., changing land uses in the watershed) or altering the flow of amount of water that enters the lake. To estimate the in-lake water quality of the CHF lakes, the model MINLEAP (Wilson and Walker, 1989) was used. The "Minnesota Lake Eutrophication Analysis Procedures" (MINLEAP), was developed by MPCA staff based on an analysis of data collected from the ecoregion reference lakes. It is intended to be used as a screening tool for estimating lake conditions with minimal input data and is described in greater detail in Wilson and Walker (1989). For this application MINLEAP can provide an estimate of anticipated TP, chlorophyll-a and Secchi for these shallow CHF lakes based on mean depth, surface area, watershed area and ecoregion-based precipitation, evaporation, runoff and stream-TP concentration.
Based on 19 CHF lakes, ten exhibited higher observed TP (means ± standard error) as compared to that predicted by MINLEAP (Fig. 14a): Diamond, French, Ringo, Johanna, Florida Slough, Fremont, Silver, Trace, Pelican and Smith. These ten lakes generally exhibited higher observed chlorophyll-a as compared to predicted as well (Fig. 14b); however results for Secchi were not as consistent (Fig. 14c). Large watershed area (e.g. Florida Slough), shallowness of the lake, presence or absence of rooted plants, dominance of roughfish, or point source discharges (Trace) can all be important factors that may contribute to higher than anticipated TP concentrations in these lakes. A summary of means and medians for predicted and observed values is presented in Table 5. Based on these 19 lakes the median predicted TP & chlorophyll-a are approximately one-half of the observed median for this set of lakes.
Figure 14. Observed versus predicted: a) TP, b) chlorophyll-a, and c) Secchi for CHF lakes. a) CHF Lakes: Obs. TP vs. MNLEAP Predicted 350
300
250
200
TP ppb 150
100
50
0 10-0108 27-0125 27-0127 27-0177 33-0015 34-0172 34-0204 61-0006 61-0101 71-0016 72-0013 73-0226 73-0255 73-0273 76-0033 77-0009 86-0031 86-0073 86-0250
TP MINLEAP TP
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b) CHF: Observed Chl-a vs. MNLEAP Predicted 160
140
120
100
80 Chl-a ppb Chl-a 60
40
20
0 10-0108 27-0125 27-0127 27-0177 33-0015 34-0172 34-0204 61-0006 61-0101 71-0016 72-0013 73-0226 73-0255 73-0273 76-0033 77-0009 86-0031 86-0073 86-0250
Chl-a MINLEAP Chl-a
c) CHF Observed Secchi vs. MNLEAP Predicted
-1
-2 Secchi m
-3
-4 10-0108 27-0125 27-0127 27-0177 33-0015 34-0172 34-0204 61-0006 61-0101 71-0016 72-0013 73-0226 73-0255 73-0273 76-0033 77-0009 86-0031 86-0073 86-0250
Secchi observed Secchi Predicted
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Table 5. Comparison of observed vs. predicted TP, chlorophyll-a and Secchi for CHF lakes.
N=19 Observed Predicted mean median mean median TP (µg/L) 109 90 60 45 Chl-a (µg/L) 44 35 29 17 Secchi (m) 1.1 0.8 1.2 1.2
Zooplankton and Fish – influence on TP, chlorophyll-a and Secchi
Zooplankton are an essential part of the overall “food or energy” web in lakes as they consume algae and in turn are an important food source for larval and some adult fish (e.g., panfish). It is also well documented that high zooplankton populations, in particular large daphnids (e.g., Shapiro and Wright, 1984); Sondergaard, M., et al., 1990), can play a significant role in the cropping of algae in lakes such that lake transparency may be much deeper than anticipated based on TP concentrations and a significant effect on the chl-a:TP relationship (Pace 1984). In other words, where large daphnids are common to abundant there will be less chl-a per unit of TP than in lakes with predominately small daphnids. Spring or early summer peak transparency often coincides with peaks in populations of large zooplankton. With respect to shallow lakes, suppression of algae and increases in transparency could allow for more extensive growth of rooted submergent plants than might otherwise be possible in the absence of zooplankton. Hence qualitative and semi-quantitative measures of zooplankton were conducted as a part of the 2003 survey. The semi-quantitative data from late summer sampling dates, typically August and September, are represented in Fig. 15. This summary allows for comparison of the relative numbers of zooplankton observed in the samples where ratings range from: 1 (present) to 4 (very abundant).
In terms of relative abundance samples were often dominated by rotifers and copepods -- both of which are inefficient grazers of algae (Moss et al., 1996), though James and Forsyth (1990) suggest that copepods can graze more effectively on filamentous algae than can passive filter- feeding daphnids. While rotifers serve as a food source for very small fry they are not eaten by adult fish and copepods while being an important food source for larger fish are more able to avoid capture as compared to daphnids (Moss et al., 1996). In contrast, large daphnids, which feed extensively on algae, were found infrequently during this late summer timeframe (Fig. 15). This is not uncommon as fish (both fry and adult) feed heavily on daphnids and even in deeper lakes where the daphnids have daytime refuge in the upper portion of the hypolimnion (Hembre and Megard, 2003), we often find very few large daphnids in lake samples by late summer.
The lakes that had some large daphnids included: Cedar (73-0226 & 86-0073), Diamond, East Solomon, Shaokatan, and Trace (Fig. 15). Small daphnids were much more common than the large forms on most of the lakes. While not as efficient at grazing on algae Turner and Mittelbach (1992) noted that in enclosure experiments they were quite effective at cropping algae (in the absence of fish).
Interactions among fish, particularly planktivorous forms like sunfish and crappie, and their impact on zooplankton, algae and rooted plants are well documented. Shapiro and Wright
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(1984) noted significant increases in zooplankton populations and declines in chlorophyll-a following fish removal in Round Lake. Fish can have a larger positive effect on blue-green algal dominance through nutrient recycling than by elimination of zooplankton (Attayde and Hanson, 2001). They note further that fish predation on zooplankton does have significant effect on other algal species, but not blue-green algae, which are not favored by zooplankton as a food source.
Hanson and Butler (1994) note that following a fish kill in a small shallow eutrophic Minnesota lake, larger species of Daphnia (galeata, and pulex) began to dominate. These contributed to increased grazing on phytoplankton thus lowering chlorophyll-a and increasing transparency. Larger zooplankton were also able to graze larger sized phytoplankton. They noted further that the increased transparency was also due to fish not being there to resuspend nutrients from the sediments. The increases in transparency also lead to increases in macrophytes which in turn helped maintain the transparency. Likewise, the negative influence of benthivorous fish like common carp and black bullhead on rooted plants and internal recycling of phosphorus has been demonstrated as well.
Our information on fish composition of several of the lakes in this study is rather sketchy, since many are marginal as fisheries and no recent surveys were available for several (about 1/3rd). We were able to draw on web-based MDNR fishery survey data for about 2/3rd of the lakes and did some additional survey of area fishery managers for their insights on those without survey data. As noted in the general description for each lake most lakes with fish survey data supported a variety of species including panfish. Also because they are shallow and eutrophic many were prone to winterkill and winter aeration was used to maintain the fishery. A qualitative list of lakes and information on the relative populations of black bullhead and carp are noted in the Appendix. Based on that information the following lakes had moderate to high populations of black bullhead or carp or both: Diamond, Ringo, East Solomon, Shaokatan, Johanna, Fremont, Silver, Hattie, Trace, and Smith (86-0250) Lakes. These data and the individual descriptions may help explain anomalies in trophic status, rooted plant abundance and other relationships explored in this study.
Figure 15. Summary of late-summer zooplankton populations; expressed in terms of relative abundance. Lakes sorted alphabetically and by sample date (yr-mo-da). Minnesota Zooplankton Rapid Assessment Cedar 73-0226, Cedar 73-0225, Cedar 86-0073 and Clark 18-0374 20
18 16
14
12
10
8
Abundance 6
4 2
0 030819 030908 030819 030908 030806 030729 030825 030929
73-0226 73-0226 73-0255 73-0255 86-0073 18-0374 18-0374 18-0374 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
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Minnesota Zooplankton Rapid Assessment Diamond 27-0125, Dunns 47-0082, E. Solomon 34-0246, E Twin 41-0108, Florida Slough 34-204
20 18 16 14 12 10 8 Abundance 6 4 2 0 030805 030904 030814 030915 030821 030910 030813 030910
27-0125 27-0125 47-0082 47-0082 34-0246 34-0246 41-0108 34-204 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
Minnesota Zooplankton Rapid Assessment Freemont 71-0016, French 27- 0127, Hassel 76-0086, Hattie 75-0200, Hollerberg 76-0057
20 18 16 14 12 10 8
Abundance 6 4 2 0 030805 030903 030904 030820 030909 030820 030909 030820 030909
71-0016 71-0016 27-0127 76-0086 76-0086 75-0200 75-0200 76-0057 76-0057 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
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Minnesota Zooplankton Rapid Assessment Jennie 21-0323, Johanna 61.0006, McCormic 73-0273, Monson 76-0033, Nelson 61-0101
20 18 16 14 12 10 8
Abundance 6 4 2 0 030819 030908 030820 030909 030819 030820 030909 030909
21-0323 21-0323 61-0006 61-0006 73-0273 76-0033 76-0033 61-0101 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
Minnesota Zooplankton Rapid Assessment Pelican 86-0031, Platte 18-008, Prairie 27-0177, Monson 76-0033, Nelson 61-0101
20 18 16 14 12 10 8 6 4 Relative Abundance Abundance Relative 2 0 030806 030806 030806 030730 030826 030930 030805 030904 030904
86-0031 86-0031 86-0031 18-0088 18-0088 18-0088 27-0177 27-0177 27-0177 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
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Minnesota Zooplankton Rapid Assessment Quamba 33-0015, Red Sand 18-0386, RIchardsons 47-0088, Ringo 34-0172
20 18 16 14 12 10 8 Abundance 6 4 2 0 030730 030826 030729 030825 030929 030814 030915 030821 030821 030910 030910
33-0015 33-0015 18-0386 18-0386 18-0386 47-0088 47-0088 34-0172 34-0172 34-172 34-0172 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
Minnesota Zooplankton Rapid Assessment Shaokatan 41-0089, Silver 72-0013, and Smith 86-0250 20 18 16
14
12 10
8 Abundance 6
4 2
0 030813 030813 030916 030827 031001 030814 030915
41-0089 41-0089 41-0089 72-0013 72-0013 86-0250 86-0250 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
Minnesota Zooplankton Rapid Assessment Titlow 72-0046, Trace 77-0009, and West Twin 41-0102 20
18
16
14
12
10
8 Abundance 6
4
2
0 030827 031001 030819 030908 030813 030916
72-0046 72-0042 77-0009 77-0009 41-0102 41-0102 Date / Lake ID Large Daphnia Small Daphnia, Cladocera and Leptodora COPEPODA ROTIFERA
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Association between submerged macrophytes and water quality
A primary focus of this study of shallow lakes was to improve our understanding of the relationship between submerged and floating-leaf macrophytes and water quality of lakes across a broad spectrum of trophic status. This section of the report will pursue potential relationships between the macrophyte populations (based on a variety of metrics) and water quality (with a principal focus on TP, chlorophyll-a and Secchi). This analysis is intended to provide an improved understanding of these relationships and help define thresholds for transitions from plant-dominated to algal-dominated states for shallow lakes in Minnesota. Relationships among trophic status variables and a variety of plant metrics including number of submerged aquatic plant species, number of floating-leaf species, floristic quality index, percent of sample sites with native vegetation, percent of stations with curly-leaf pond weed, and maximum depth of native plant colonization are considered in this section.
The Floristic Quality Index (FQI) provides one basis for describing the integrity of rooted plant populations in lakes. As described in the Methods section FQI is derived based on the number of native macrophyte species in the lake and the “coefficient of conservatism” for a lake macrophyte community. Conservatism describes the degree to which a species will tolerate disturbance (Nichols, 1999). Nichols (1999) assigned coefficients of conservatism to macrophyte species found in Wisconsin with values ranging from 1 for taxa tolerant of disturbance to 10 for intolerant taxa. These values were adopted for use in Minnesota lakes and relative rankings of “high, medium, and low” were developed based on assessment of Minnesota lakes (Perleberg, 2003). Based on data for 27 lakes in this study 22% were classified as high, 44% as medium and 34 % as low (Fig. 16).
Though the number of plant species is factored into the FQI, we thought it might be instructive to examine the distribution of the number of plants (submergent and floating-leaf plant species relative to FQI) (Fig. 17). Lakes having a “high” FQI tended to have several species of floating- leaf plants while floating-leaf plants were generally absent from lakes classified as having a low or medium FQI. All lakes classified as having a high FQI had 10 or more submergent species along with about two to five floating leaf species for a total of 15 or more species. Those classified as medium generally had ten or fewer species and those classified as low generally had seven or fewer species. One exception was Cedar Lake (73-0255), which had a medium FQI but had the largest number of floating-leaf plants (Fig. 14). This lake had native vegetation at 98% of the sites sampled and exhibited plants up to 75% of its maximum depth of eight feet.
Next we elected to examine patterns in TP, TKN, chlorophyll-a, and Secchi relative to both FQI and the number of rooted plants (Figs. 16 and 17). In general, lakes classified as having a high FQI had TP concentrations of 90 µg/L or less with the exception of Fremont Lake, which had a mean TP of 110 µg/L but a high FQI (Fig. 16). In contrast, lakes classified as low FQI had TP concentrations of 60 µg/L or more. Those classified as medium spanned a wide range of TP concentrations (Fig. 16). As for Fremont Lake its high FQI is a function of high species richness, however a majority of the species were present in low numbers and the lake is dominated by curly-leaf pondweed and Canada waterweed, two fairly tolerant plant species.
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Figure 16. Floristic Quality Index values for West-Central Lakes
Floristic Quality Index (FQI) for West-Central Lakes
75-0200 72-0042 27-0125 27-0127 Low Medium GoodHigh 41-0108 21-0323 34-0172 34-0246 61-0101 77-0009 76-0057 73-0255 33-0015 76-0033 34-0204 72-0013 86-0031 86-0250 41-0102 76-0086 73-0273 73-0226 61-0006 71-0016 18-0386 18-0374 18-0088
0 5 10 15 20 25 30 FQI
Figure 17. Number of submergent and floating-leaf plant species.
Number of submergent and floating-leaf plants. Lake sorted by FQI & FQI categories noted..
# Sub #FL 75-0200 72-0042 27-0125 27-0127 41-0108 Low Medium High 21-0323 34-0172 34-0246 61-0101 77-0009 76-0057 73-0255 33-0015 76-0033 34-0204 72-0013 86-0031 86-0250 41-0102 76-0086 73-0273 73-0226 61-0006 71-0016 18-0386 18-0374 18-0088
0 5 10 15 20 25 30 # of plants
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Figure 18. Number of plant species (a) andFloristic Quality Index (b) relative to total phosphorus. Lakes sorted by mean TP. a)
Submerged and floating leaf plants sorted by TP.
30
25
20 Fre m ont
15
Hassel West Tw in # of plants # of 10
Ne ls on 5
0 10 24 32 40 60 90 90 90 110 140 160 180 210 230 320 370 TP ppb
# Sub #FL
b)
Floristic Quality Index vs. TP
30
25
Fremont high 20 Hassel W. Tw in medium 15 FQI low 10
5
0 10 24 32 40 60 90 90 90 110 140 160 180 210 230 320 370
TP ppb
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Figure 19. Number of plant species (a) and Floristic Quality Index (b) relative to TKN. Lakes sorted by mean TKN. a)
Number of submergent & floating-leaf species relative to TKN
30
25
20 Fremont Johanna
15 Ne ls on
# of species # of 10
5
0
0 7 0 6 4 7 2 8 1 5 5 2 7 8 6 8 1 .2 .3 7 3 1 0. 0. 0.89 1.02 1.49 1. 1. 2.07 2. 2 2 2. 3. 4. 5.2 5.7 TKN ppm
# Sub #FL b)
FQI relative to TKN
30
25
20
15 FQI
10
5
0 0.7 0.9 0.9 1.0 1.5 1.6 1.9 2.1 2.1 2.3 2.3 2.8 3.3 4.2 5.3 5.7 TKN ppm
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Comparisons with TP and the number of rooted plant species show a slightly more consistent pattern. In general as TP increases the total number of species tends to decline, though there were some notable exceptions in Fremont, West Twin and Hassel Lakes for example. All three of these lakes are quite shallow (mean depths of 7.0, 3.6, & 2.4 feet, respectively) and the SAV was dominated by tolerant plant species. Also, with the exception of three lakes (Pelican, Titlow & Fremont), floating-leaf plants were absent at TP concentrations greater than 90 µg/L (Fig. 18). If we use 15 plant species as a “benchmark” for a lake with a high FQI or generally healthy and diverse rooted plant population we find that all lakes with this number of plants (with the exception of Fremont Lake) had TP concentrations less than 90 µg/L. In the case of Fremont, summer-mean Secchi transparency (1.1 m) is high relative to its TP and likely provides adequate light for SAV and floating-leaf plants to flourish across most of the basin given that the mean depth of lake (Fig. 21) is about 2.2 meters (7 feet).
It is important to note that the reported mean concentrations (e.g., TP) used to sort the lakes and assess relationships were derived based on three to four samples. More frequent monitoring, e.g., bi-weekly, could result in slightly different mean values and affect the relative ranking of lakes inn Figs. 18-23.
Comparisons with TKN show some similar patterns. In general high number of species are associated with low TKN values and there appears to be an abrupt decline in the number of species at TKN above about 2.0 mg/L (Fig. 19a). At TKN above 2.0 mg/L no lake had 15 or more species and most had 10 or less species. Comparisons with FQI revealed that all lakes having a high FQI had TKN less than 2.0 mg/L. The majority of the lakes with TKN > 2.0 mg/L had low FQI; however there was no distinct correlation between FQI and TKN for those lakes > 2.0 mg/L. Sagrario et al. (2004) noted, based on enclosure experiments, that a shift to a turbid state with low plant coverage occurred at TN >2.0 mg/L. They state further that this concurs with findings for Danish shallow lakes as well.
In an attempt to better understand the relative roles of TP and TN we reviewed relationships between the number of plant species and FQI relative to TN:TP ratios (Fig. 20). No linear relationship was evident in the comparison of number of species and TN:TP ratios (Fig. 20). However, we note that all lakes with 15 or more plant species would be considered “P-limited.” Likewise all lakes with a high FQI were “P-limited as well (Fig. 20). All lakes with a medium FQI were either “P-limited” or were intermediate between P and N limitation. As noted previously, Fremont and Nelson Lakes tend to be “outliers” suggesting that factors other than nutrients contribute to the higher than expected number of species in Fremont and the lower than expected number of species in Nelson.
We pursued similar comparisons with summer-mean Secchi transparency (Fig. 21a). In this case floating-leaf species were generally absent at summer-mean Secchi values of less than one meter. Further, the only lakes that supported 15 or more species were those with Secchi values of one meter or greater (Fig. 21a). However, as was the case with TP, a low TP and relatively high Secchi does not guarantee a diverse plant population. For example, Nelson Lake with a TP of 50 µg/L and Secchi of 1.2 m had only 6 species and a FQI of 13. However, Perleberg (2004) noted that lake level was rather high at the time of the plant survey (average depth about 11 feet as compared to an estimated maximum depth of 9 feet based on a 1997 MDNR fisheries survey). Increased depth and low transparency (2 feet, 0.6 m), at the time of the 2003 survey, likely
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Figure 20. Number of a) plant species and b) FQI relative to TN:TP a)
Number of plant species relative to TN:TP
N-limited P-limited 25
20
Fre m ont 15
10 # of species # of
5 Ne ls on
0 0 1020304050 TN:TP b)
FQI vs . TN:TP r a tio
N-limited P-limited 30
25 high Fre m ont 20 medium 15 FQI Ne ls on low 10
5 Titlow Hattie 0 0 1020304050 TN:TP
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combined to limit the number of species and extent of plant growth in the lake. With respect to floating-leaf plants, spring transparency may play an important role in floating-leaf survival, since they are in a submerged state at that time. This might explain why lakes such as Fremont, Pelican and Titlow, with summer-mean Secchi of 1.1, 0.5 and 0.2 m each had floating-leaf plants (Fig. 21a); whereas lakes with similar Secchi values did not. Fremont, for example, had a maximum Secchi of 2.1 m (Table 4b) which would allow light to reach the bottom of the lake throughout the basin.
Secchi transparency can also influence the depth to which rooted plants may be found in the lake. Canfield et al. (1985) and Chambers and Kalff (1985) offer equations as follows for predicting depth of submersed aquatic vegetation (SAV) colonization as a function of Secchi depth:
Eq. (5) Log (y) = 0.79 log (Secchi) + 0.25 (Canfield et al. 1985) Eq. (6) Y0.5 = 1.33 log Secchi + 1.4 (Chambers and Kalff 1985) Eq. (7) Y = 1.6 Secchi + 1.10 (MPCA, based on 27 west-central lakes)
As with these two studies a significant relationship was noted (R2=0.78) for the west-central MN lakes and a simple linear regression is offered (Fig. 21b). The Canfield et al. (1985) relationship was included on Fig. 21b as a basis for comparison. Based on Eq. 7, as Secchi falls below about 0.7 m the depth for rooted plant colonization will be limited to about 2 m (6.6 feet) or less in most lakes, which for many of the shallow lakes in this study could constitute an appreciable portion of the lake basin (Table 1).
Charting the number of plant species (Fig. 22) and FQI (Fig. 23) relative to TP, Chlorophyll-a, and Secchi provides a further basis for examining patterns and relationships. In general, the total number of submerged and floating-leaf plant species tended to decline as TP increased and Secchi decreased (Fig. 22b). At TP concentrations less than 60-70 µg/L there were typically 10 or more species found in each lake; whereas as TP increased to about 90 µg/L or greater there were generally less than 10 species and in several instances five or fewer species were found (Fig. 22). Likewise, as Secchi fell below about 1 meter there were generally 10 or fewer species found in the lakes (Fig. 22b). Further, floating-leaf species were quite uncommon in lakes where TP was 90 µg/L or greater and Secchi was below one meter. As TP increases from about 60 to 90 µg/L chlorophyll-a often averages 30 µg/L or greater, a concentration typically characterized as severe nuisance blooms. Secchi generally declines below one meter as TP increases above 60 µg/L; however some variability is noted over the 60-80 µg/L range, which is often attributed to the blue-green algae that tend to dominate as TP and chlorophyll-a increase. The blue-greens often form colonies at the surface of the lake that allow for deeper than anticipated transparency.
Based on Fig. 23 lakes with high FQI had TP less than 90 µg/L and chlorophyll-a less than 30 µg/L, with the exception of Fremont Lake. We believe several factors contribute to Fremont Lake’s high FQI. As noted in the vegetation survey the shallowness of the lake (~ 2 m mean depth) and relatively high summer Secchi (~1.4 m) allow SAV to grow over essentially the whole basin. Perleberg (2004) describes this as a lake in transition whereby native species, while yet present are represented by very small numbers and increasingly curly-leaf pondweed is becoming the dominant plant being found at 87% of the sample stations.
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Figure 21. Submerged and floating-leaf plants relative to a) Secchi & b) maximum depth of native plant colonization relative to Secchi transparency. a)
Submerged and floating-leaf plants sorted by Secchi
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Mean Secchi vs. Max. Depth of Native Plant Colonization Depth (m) = 1.60x + 1.10 2 7.0 R = 0.78
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Figure 22. Number of plant species relative to TP, chlorophyll-a, and Secchi. A “benchmark” of 15 plant species is noted. a) Number of rooted plant species vs. chlorophyll (algae). Based on 2003 data from west-central MN lakes. Sorted by TP
30 200 180 25 160 140 20 Ne ls on Fremont 120 Johanna 15 100 80 Chl-a(ppb)
# of species 10 60 40 5 20 0 0 10 24 32 40 60 90 90 90 110 140 160 180 210 230 320 370 TP ppb # plants Chl-a b)
Number of rooted plant species vs. Secchi. Sorted by TP.
30 4.0
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0 0.0 10 24 32 40 60 90 90 90 110 140 160 180 210 230 320 370 TP ppb
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Figure 23. FQI relative to TP and chlorophyll-a. Lines indicate high, medium and low FQI.
Floristic Quality Index vs. chlorophyll-a
30 200 180 25 160 140 20 120 15 100 FQI 80
10 Chl-a ppb 60 40 5 20 0 0 10 24 32 40 60 90 90 90 110 140 160 180 210 230 320 370 TP ppb
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Sediment diatom reconstruction – statewide and shallow lakes studies
Diatom reconstruction provides an opportunity to examine both temporal and spatial trends in lake trophic status and related factors. For Minnesota we can draw on three studies to examine these patterns: 1) a study of 55 lakes distributed across Minnesota (Heiskary and Swain, 2002); 2) a study of nine southwest Minnesota lakes (Heiskary et al. 2003); and 3) the current study of nine west-central Minnesota lakes.
Ecoregion-based patterns in lake trophic status have long been recognized in Minnesota (e.g., Heiskary et al., 1987). Lakes in the forested Northern Lakes and Forests (NLF) ecoregion are moderately deep and exhibit relatively low TP while the shallow lakes in the highly agricultural Western Corn Belt Plains (WCP) and Northern Glaciated Plains (NGP) exhibit high TP concentrations. The transitional North Central Hardwood Forests (CHF), characterized by moderately deep lakes and a mosaic of land uses -- intermediate between these two extremes. We have also demonstrated differences in shallow versus deeper lakes within ecoregions – with differences being most pronounced in the CHF and WCP ecoregions (Fig. 1).
Of nine cores from the west-central lakes in 2004, diatoms were adequately preserved or in sufficient numbers to allow for estimates of pre-European TP in seven lakes (Fig. 24a). Of these, five are from the CHF ecoregion and two, Red Sand and Platte, are from the NLF ecoregion. Based on this comparison CHF pre-European TP ranged from a low of 27 µg/L in Fremont Lake to 51 µg/L in Quamba Lake (Fig. 24a). The average change (modern-day minus pre-European) was 19 µg/L (51 % increase) and ranged from 8 µg/L (21 %) in Johanna up to 53 µg/L (189 %) in Silver Lake. Red Sand, in the NLF, exhibited a 2 µg/L (10%) increase in TP.
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Distinct differences among regions were evident in comparisons of pre-European and modern- day diatom-inferred TP concentrations. The NLF lakes were significantly lower in TP as compared to the CHF, WCP and NGP lakes based on a comparison of group-means plus or minus standard error (SE) (Fig. 24b). Based on SE (typically 2-3 µg/L or less) and overall range of TP concentrations the NLF lakes were somewhat less variable as compared to lakes in the other ecoregions. Variability was slightly higher in the CHF lakes and no significant difference was evident in a comparison of rural and Metro-area CHF lakes (Fig. 24b). The shallow WCP and NGP lakes were much more variable by comparison, with a SE of 10 ug/L. However, if we express this as a percent off the mean the SE for all regions is about 10-20 percent of the corresponding means for each region.
For the NLF lakes, as a group, there was no significant difference in modern-day vs. pre- European TP (Fig. 24b). However, distinct increases in TP were noted for the CHF lakes and these increases exceeded the “natural variability” noted in comparisons of pre-European (1750 and 1800) TP concentrations (Ramstack et al., 2004). They also note that the degree of change in TP among the Metro CHF lakes is significantly correlated with the percent of the watershed in urbanized landuse, while those in the rural portion exhibit a significant correlation with the percent of landuse in agricultural uses or inversely the percent in forested uses. The five “deeper” WCP lakes also did not change significantly across the two time periods based on this analysis and no significant associations with landuse were noted by Ramstack et al. (2004) – presumably because of the small sample size (5 lakes) and the predominately agricultural landuse in these watersheds. It is important to note that the five “deep” WCP lakes were among the most eutrophic of the original 55 lakes and as such were on the fringe of the model development data set. Future models that include all diatom study lakes (71 lakes, Fig. 24b) could result in modified pre-European values for the deep WCP lakes. Also, while diatom- inferred values did not differ greatly between pre-European and modern-day samples in the deep WCP lakes, modern-day water quality samples indicate that three of five lakes have much higher TP as compared to pre-European diatom-inferred TP. The shallow WCP & NGP lakes added in the recent study of SW Minnesota lakes exhibited a significant increase when pre-European and modern-day TP are compared (Fig. 24b). Agricultural landuse predominates in all SW MN study lakes. The shallow CHF lakes exhibited higher pre-European TP as compared to the deeper CHF (metro and rural) lakes (Fig. 24b). In comparison to the deeper, rural CHF lakes (that were also from west-central MN) pre-European TP for the shallow lakes was on average 10 µg/L (37%) higher; however the relative change (on average) from pre-European to modern-day TP was similar at 51 %. However when diatom-inferred values are compared to modern-day measured (observed) TP the difference is much more pronounced.
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Figure 24. Comparison of diatom-inferred pre-European and modern-day TP for a) west- central shallow lakes and b) among ecoregions. a.
West Central Shallow Lakes: pre-E, modern dia-P & mod WQ
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90 80
70 60 50
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Diatom-inferred TP: Pre-European vs. Modern-day
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Pre-E Modern
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Discussion
Deriving nutrient criteria for shallow lakes
Deriving nutrient, chlorophyll-a and transparency thresholds relative to lake uses is an acceptable approach for developing eutrophication (nutrient) criteria according to USEPA (2000a). Several examples are presented in the guidance manual that consider a range of uses including coldwater fisheries, swimming, boating and other uses. Consideration of lake uses is also consistent with our original approach for setting ecoregion-based TP goals (Heiskary and Wilson, 1988). Thus, as we approach criteria development for shallow lakes (which have been defined as lakes with a maximum depth of 15 feet or less or where 80% or more of the lake is 15 feet or less) it is reasonable to consider the actual or potential uses of these lakes and how eutrophication criteria (TP, chlorophyll-a and Secchi) might be used to protect these uses. In the case of shallow lakes included in this current study a wide range of uses was evident including boating, fishing, hunting, and fish and waterfowl propagation and while swimming is a potential use it is most likely not the primary use of these and other lakes that share the characteristics of these lakes. It is evident that the value of shallow lakes for these activities declines when the lakes are dominated by phytoplankton and lack aquatic macrophytes.
As noted initially, Moss et al. (1996) and numerous other researchers who have studied shallow lake ecology help guide our understanding of how shallow lakes work. A well-accepted hypothesis is that shallow lakes exist in alternative stable states that range from plant dominance and clear water at low nutrient concentrations to algal-dominated, turbid conditions at high nutrient concentrations. The exact nutrient or chlorophyll-a concentration where this “switch” occurs is not explicitly stated but it is widely acknowledged that several factors including fish, zooplankton, lake depth and numerous other factors in addition to nutrients play a role.
A recent effort by the Upper Mississippi River Conservation Committee (UMRCC) to develop light-related water quality criteria necessary to sustain submerged aquatic vegetation (SAV) in the Upper Mississippi River provides some insights as well. While their work focused on the backwaters and pools on the Mississippi River some of the same concepts can be applied to shallow lakes elsewhere. As such we would like to draw upon their discussion as follows (UMRCC, 2003): “The negative impact of high turbidity or suspended particulate matter on SAV is well known and has been documented in many systems including Lake Chatauqua, Illinois (Jackson and Starret 1959), Rice Lake Wisconsin, and Chesapeake Bay (Dennison et al., 1993). These impacts are expressed through a reduction in light energy on leaf surfaces, which contribute to reduced growth and reproduction (Korschgen et al., 1997 and Kimber et al., 1995). The maximum depth of colonization of SAV has been directly linked to the transparency of water (Chambers and Kalff, 1985 and Canfield et al., 1985). Their regression plots of the maximum colonization depth versus Secchi disk depth are similar and suggests the relationship may have broad application to many freshwater systems. For example, this simple relationship could be used to establish the target depth for SAV establishment in the UMR navigation pools. Water quality management efforts would then be directed at controlling turbidity or suspended particulate matter to provide the necessary underwater light conditions to support SAV growth and reproduction. A similar approach has been suggested for Chesapeake Bay (Dennison et al., 1993).”
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Their work focused directly on the pools and fairly specifically on the production of wild celery (Vallisneria), a very important food source for migrating waterfowl. It took into account light regimes in the pools, depth of pools and extent of colonization (depth) of wild celery both in current times as well as historically. Their analysis led them to suggest the following values as criteria for the system as follows:
Recommended light-related water quality criteria necessary to support and sustain submersed aquatic vegetation in the Upper Mississippi River. Variable Value* Basis Light Extinction Coefficient 3.42 m-1 Average growing season light extinction necessary to promote Vallisneria growth and reproduction at 0.8 m depth Secchi Disk Depth 0.5 m Light extinction vs. Secchi depth regression, WDNR data for Pools 4-11 Total Suspended Solids 25 mg/L Light extinction vs. TSS regression - WDNR data for Lock & Dam 8 & 9 Turbidity 20 ntu Light extinction vs. turbidity regression - LTRMP data for Pools 8 & 13. * Values should be applied as a growing season average (May 15 to September 15) based on bi-weekly measurements. While in this case they focused principally on light they did acknowledge several other factors that also influence SAV growth and survival, e.g., “water level changes, waves, nutrients, floods, substrate composition and herbivore activity and other factors also play a role in governing the development and persistence of SAV communities on the river.” One point they make with respect to the negative impacts of excessive nutrient enrichment is on the enhancement of filamentous algae or epiphytic plant growth on SAV may be especially important since these attached plants have been implicated as a critical factor contributing to submersed aquatic macrophyte declines in freshwater systems (Phillips et al., 1978). Their work suggests excessive canopies of filamentous algae and other attached algae may lead to increased competition for light and nutrients and may promote the "switch" from a SAV dominated system to one dominated by algae. In the course of our study we did not specifically address this but it is just one more reason to minimize nutrient concentrations in these shallow systems. Based on the aforementioned analysis characteristics of lakes in each of the FQI classes were summarized (Table 6). The mean and median TP, chlorophyll-a Secchi and TSS are noted for each class. In general there is minimal difference among the lakes classified as medium to low FQI (Table 6). However those classified as high FQI had TP and chlorophyll-a values about three to four-fold lower than the medium class, while Secchi was about two-fold greater. TSS was extremely low as compared to the other two classes, with values that are fairly consistent with reference lake data for CHF lakes (Table 3). A similar pattern was evident as we compared lakes that support 15 or more plant species as compared to those that support less than 15. Trophic status characteristics were quite similar among the lakes supporting 15 or more species
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and the lakes with “high” FQI, which is not too surprising since the number of plant species (richness) is factored into the FQI calculation. Lake morphometry can be an important consideration as well since the “high” FQI lakes and lakes supporting 15 or more species tended to be deeper than the lakes in the other three groups (Table 6). In general lakes exhibiting high FQI had on average TP on the order of 50 µg/L or less, chlorophyll-a less than 20 µg/L and Secchi of 1.7 m as summer averages.
Table 6. Summary of morphometric and water quality characteristics for Floristic Quality Index “classes” and lakes supporting 15 or more species of plants. Based on 27 west- central Minnesota lakes.
Depth Area TP Chl-a Secchi TSS feet acres ppb ppb m ppm FQI mean / mean / mean / mean / Mean Max. Mean median median median median High 11 23 792 49 / 34 19 / 10 2.0 / 1.9 8 / 3 Medium 5 9 555 142 / 130 54 / 48 0.7 / 0.7 33 / 15 Low 5 8 502 194 / 210 74 / 45 0.5 / 0.4 43 / 38 Plant mean / mean / mean / mean / species Mean Max. Mean median median median median > 15 10 20 708 47 / 38 17 / 10 1.9 / 1.7 8 / 3 plant sp. < 15 5 8 548 171 / 155 65 / 48 0.6 / 0.5 39 / 32 plant sp.
Moss (1998) makes several points regarding the switching of lakes from one “state” to another. And while not stating a specific threshold he notes that at very high nutrient concentrations algal dominance may be the only possibility in a lake as macrophytes will be absent. This would seem to be the case in several of the west-central lakes with low FQI and extremely high chlorophyll-a (Fig. 23) and based on our study of shallow SW Minnesota lakes (Heiskary et al., 2003). He does note that a switch from plant-dominated to algal-dominated is easier at high nutrient concentrations – which again seems to be the case for several of the lakes in this study (Figs. 18, 19 & 22) and argues for trying to keep this shift from occurring in the first place (as most literature point to the difficulty in reversing this process once it occurs). Internal recycling of phosphorus, in shallow lakes that are in the turbid water state, often serves to keep lakes from switching back and contributes to the difficulty in restoring these lakes. This effectively maintains the lake in a “state” where it may have minimal values for fish, wildlife and aquatic recreational use. It is also quite likely that nutrient reduction alone may not be sufficient to switching to plant dominance from algal dominance as most cases where this has occurred tends to involve biomanipulation of the fish community and Moss (1998) notes further many instances exist where control of nutrient loading did not switch community back to plant dominance, but biomanipulation did.
While we have not established any precise empirical relationships upon which to base our criteria for shallow lakes we do have several pieces of information from this study, our previous work (e.g., Heiskary and Wilson, 1988), and review of the literature that allow for a weight-of evidence approach to setting these criteria that includes:
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• Interrelationships among TP, chlorophyll-a, Secchi and nuisance bloom frequency • Patterns in FQI and macrophyte richness and abundance relative to TP, chlorophyll-a and Secchi; • Ecoregion-specific distributions for TP, chlorophyll-a and Secchi for all assessed lakes and TP for shallow lakes more specifically; • Lake user perception data; • Pre-European diatom-inferred TP concentrations for both deep and shallow CHF lakes; and • Extensive literature on shallow lakes and factors that contribute to “alternate states” and the overall value of shallow lakes.
A comparison of distributions for TP, chlorophyll-a and Secchi, drawn from different populations of CHF lakes, (including USEPA criteria development manuals; USEPA 2000b) can help provide some perspective as we explore potential criteria values and what these values may mean relative to the various populations. For TP we have comparisons for pre-European (diatom-inferred) TP and modern-day distributions for all assessed CHF lakes and a subset of that “shallow lakes” as bases for comparison (Fig. 25). For this exercise the shallow lakes distribution represents all assessed lakes with a maximum depth of less than 20 feet, which should include most of the lakes that meet our definition of shallow lakes – “maximum depth of 15 feet or less or 80% or more of the lake is littoral.”
As noted previously the shallow, polymictic lakes of the CHF ecoregion tend to be more nutrient-rich as compared to the overall population (Fig. 25) and in particular when compared to deeper, dimictic lakes of the same ecoregion (Fig. 1). For example the 25th percentile for the shallow lakes is about 60 ppb, which is slightly above the median for the MPCA assessed lakes (Fig. 25). The IQ range for the 2003 study lakes is fairly similar to the TP range for shallow lakes and would appear to be a reasonable representation of the range in TP found in shallow CHF lakes. It is also evident that pre-European TP was higher in the shallow lakes as compared to deeper CHF lakes (Fig. 24b).
Similar distributions were prepared for chlorophyll-a and Secchi to provide perspective on these trophic state parameters as well (Figs. 26 & 27). As with TP the shallow lakes exhibit higher chlorophyll-a as compared to the overall MPCA assessed lakes. In this case the 25th percentile for the shallow lakes – 22 µg/L is near the median for the MPCA assessed lakes. Again the IQ range for the 2003 study lakes is quite similar to the assessed shallow lakes (Fig. 26). As anticipated, Secchi transparency for the assessed shallow lakes is less than the overall assessed lakes with a median value of 1.1 m corresponding to the 75th percentile for the assessed population (Fig. 27), i.e., 75 % of the assessed lakes have transparencies greater than 1.1 m. The 2003 study lakes exhibited a similar but slightly lower range of Secchi as compared to the assessed shallow lakes.
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Figure 25. Summer-mean total phosphorus distributions (IQ range) for CHF ecoregion.
CHF Ecoregion Lakes TP Distributions
200 180 160 140 120 100
TP ppb 80 60 40 20 0 Pre-E (deep, Pre-E MPCA-Ref MPCA- EPA-Assess PCA-Assess 2003 study N=30) (shallow, (N=43) Assess (N=469) (shallow, lakes (N=27) N=5) (N=691) N=192)
25th 50th 75th
Figure 26. Summer-mean chlorophyll-a distributions (IQ range) for CHF ecoregion
CHF Ecoregion Chlorophyll-a Distributions
80 70 60 50 40
Chl-a ppb 30 20 10 0 MPCA-Ref MPCA-Assess EPA-Assess PCA-Assess 2003 study (N=43) (N=691) (N=273) (shallow, lakes (N=27) N=176)
50th 25th 75th
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Figure 27. Summer-mean Secchi distributions (IQ range) for CHF ecoregion.
Secchi Interquartile range for CHF ecoregion
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A primary focus in setting eutrophication criteria for shallow lakes is to allow for a balanced population of macrophytes that helps support a broad array of aquatic life uses and aquatic recreation (Class 2b & 2c water quality standards; Minn. Rule Ch. 7050, 2002). As such maintaining adequate transparency to allow native plants to establish themselves over much of the basin, minimizing the chance that non-native species (e.g., curly-leaf) become dominant, minimizing the occurrence of nuisance algal blooms, and keeping TP concentrations below a range that promotes excessive algal growth are all important considerations upon which to base eutrophication criteria. And of these three variables (parameters), transparency may be the most important. In turn, transparency can be directly related to TP and chlorophyll-a, though several biotic factors, such as dominance of benthivorous (e.g., carp and bullhead) or planktivorous fish (fatheads, sunfish and crappies), and abiotic factors such as suspended sediments, lake depth, wind erosion and resuspension may also influence transparency and the ability of the lake to support macrophytes.
Based on Table 6, and the figures that precede it, transparency should remain above about 0.7 m and ideally 1.0 m or more to minimize the likelihood of low FQI and a reduced number of species of rooted plants. Relative to the assessed shallow CHF lakes 0.7 m and 1.0 m correspond to about the 25th and 50th percentiles respectively (Fig. 24). A summer average transparency of 0.7 – 1.0 m should allow for SAV colonization to a depth of about 1.5 – 2.0 m (~5 - 6 ft.) based on Fig. 21b and equations developed by Canfield et al. (1985) and Chambers and Kalff (1985). This would represent an appreciable portion of the lake-basins included in this study (Table 1), which collectively had an average mean depth of 1.9 m (6.3 ft.).
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Lake user perception can provide an additional perspective for determining an appropriate level of transparency – even though swimming may not be the primary use of shallow lakes. Based on user perception for CHF lakes a transparency of 0.7 m represents the average transparency associated with “severe nuisance blooms” and/or “swimming and aesthetic enjoyment nearly impossible,” whereas 1.0 m was the average transparency associated with “high algal levels” and/or “desire to swim reduced because of algae levels (Smeltzer and Heiskary, 1990). This would suggest a transparency closer to 1.0 m may be more desirable based on potential lake users.
Chlorophyll-a is the next consideration and based on Carlson’s TSI (Fig. 9) and interrelationships developed in this study corresponding chlorophyll-a concentrations would be on the order of 20 µg/L (1 m Secchi) to 30 µg/L (0.7 m Secchi). However, based on a desire to minimize nuisance blooms a concentration closer to 20 µg/L would be more desirable since the frequency of nuisance blooms (chlorophyll-a > 30 µg/L) increases from about 15% at 20 µg/L up to about 45% at a chlorophyll-a of 30 µg/L (Heiskary and Wilson, 1988) and would likely lead to an algal-dominated system. Also the average chlorophyll-a associated with high FQI was 19 µg/L (Table 6). As a frame of reference, a chlorophyll-a concentration of about 20 µg/L ranks near the 25th percentile for shallow CHF lakes (Fig. 26).
A corresponding range of TP concentrations to yield a transparency of 0.7-1.0 m would be on the order of 48-68 µg/L based on Carlson’s TSI (Fig. 9) and about 60-80 µg/L based on Fig. 10. TP concentrations greater than about 60-80 µg/L would be undesirable since the frequency of nuisance blooms increases substantially (Fig. 12) and the number of species of rooted plants declines (Fig. 18) and with perhaps a few notable exceptions signals a shift to algal-dominated systems. As noted earlier the average TP associated with high FQI or lakes supporting 15 or more species, was 47- 49 µg/L (Table 6). Lake response to increased TP over the range from 60- 90 µg/L is rather variable for these shallow lakes in terms of Secchi, chlorophyll-a and the number of plant species (Fig. 10, 11, & 18); however the general pattern is toward increased chlorophyll-a, declining transparency, and declining numbers of species of rooted plants. Also the potential for curly-leaf dominance appears to increase as mean in-lake TP increases above about 50 µg/L (John Barten, personal communication, 2005). As further frames of reference, a TP of 60 µg/L ranks near the 25th percentile based on assessed shallow lakes (Fig. 25) and based on MINLEAP model runs for lakes in this study, 60 µg/L was the average predicted TP for the 19 CHF lakes (Table 5).
While we did not address nitrogen in detail, there appeared to be some relationship between TKN and the number of plant species and FQI. As with TP, as concentrations increased, there was a tendency toward reduced number of species and FQI (Fig. 19). And, consistent with some recent literature (Sagrario et al., 2005), there appeared to be a “threshold-type” response at 2.0 mg/L; whereby as TKN exceeded this concentration range, a marked decline in number of plant species and FQI were noted (Fig. 19).
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In summary, based on the various interrelationships among trophic status variables, rooted plant metrics and other considerations it appears that appropriate ranges for selecting eutrophication criteria values for shallow lakes in the CHF ecoregion are: • Secchi transparency - 0.7 to 1.0 meters; • Chlorophyll-a - 20 – 30 µg/L; • Total phosphorus – 60 – 80 µg/L;
Given this range of values, and acknowledging that other biotic and abiotic factors can be very significant in determining whether a lake can support a healthy and diverse population of rooted macrophytes, we are inclined to recommend criteria be set at the lower end of each range of the aforementioned values, i.e. maintain summer average Secchi of 1.0 m or greater, summer average chlorophyll-a of 20 µg/L or lower, and summer average total phosphorus of 60 µg/L or lower. While we are not offering nitrogen criteria at this time, it would appear to be beneficial to keep TKN below 2.0 mg/L when possible. Based on the relationship between TP and TKN, maintaining TP below 60-80 µg/L should yield TKN <2.0 mg/L.
Maintaining values in these ranges will not absolutely ensure that a shallow lake will remain in a macrophyte-dominated state and support the various uses described for 2b & 2c waters (Minn. Rule Ch. 7050) but should reduce the likelihood that the lake will switch to an algal-dominated state, which as repeatedly noted in the literature can be rather hard to reverse once the change has occurred. Also, maintaining trophic status values at or below these ranges should decrease the likelihood that curly-leaf, a non-native species, will become dominant in the lake.
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REFERENCES
Attayde, J.L. and L.A. Hanson L.A. 2001. “Fish-mediated Nutrient Recycling and the Trophic Cascade in Lakes. Canadian Journal of Fish and Aquatic Science, 58:1924-1931.
Barten, J. 2005. Personal Communication. Three Rivers Park District, Hennepin County.
Birks, H. J. B., Line, J. M., Juggins, S., Stevenson, A. C. and ter Braak, C. J. F. 1990. “Diatoms and pH reconstruction.” Phil. Trans. R. Soc. Lond. Biological Sciences. 327: 263-278.
Canfield, D.E., K.A. Langeland, S.B. Linda, and W.T. Haller. 1985. “Relations Between Water Transparency and Maximum Depth of Macrophyte Colonization In Lakes.” J. Aquat. Plant Mgt. 23:25-28.
Canfield, D. and R. Bachman. 1981. “Prediction of Total Phosphorus Concentrations, Chlorophyll-a and Secchi Depth in Natural and Artificial lakes.” Can. J. Fish. Aquat. Sci. 38:414-423
Carlson, R. E.1977. “A Trophic State Index for Lakes.” Limnol. Oceangr. 22:361-369.
Chambers, P.A. and J. Kalff. 1985. “Depth Distribution and Biomass of Submersed Aquatic Macrophyte Communities in Relation to Secchi Depth.” Can. J. Fish. Aquat. Sci. 42:701- 709.
Dennison, W.C., Orth, R.J., Moore, K.A., Stevenson, J.C., Carter, V., Kollar, S., Bergstrom, P.W., and Batiuk, R.A. 1993.“Assessing Water Quality with Submersed Aquatic Vegetation. Habitat Requirements as a Barometer of Chesapeake Bay Health.” BioScience 43:86-94.
Hall, D. 2004. Personal communication. MPCA.
Hanson, M. A. and M. G. Butler. 1994. “Responses to food web manipulation in a shallow waterfowl lake.” Hydrobiologia 279-280: 457-466.
Hanson, M.A. and Butler, M.G. 1994. “Responses of Plankton, Turbidity, and Macrophytes to Biomanipulation in a Shallow Prairie Lake.” Canadian Journal of Fish and Aquatic Science, 51:1180-1188.
Heiskary, S. and W.W. Walker Jr. 1988. Developing Phosphorus Criteria for Minnesota Lakes. Lake and Reservoir Management. 4(1): 1-9.
Heiskary, S., C. B. Wilson, and D.P. Larsen.1987. “Analysis of regional patterns in lake water quality: Using ecoregions for lake management in Minnesota.” Lake and Reserv. Manage. 3: 337-344.
123
Heiskary, S. and Wilson, B. 1988. “Minnesota Lake Water Quality Assessment Report.” MPCA. St. Paul MN 95 pp
Heiskary, S. and Wilson, B. 1989. “The Regional Nature of Lake Water Quality Across Minnesota: An Analysis for Improving Resource Management.” Journal of the Minnesota Academy of Science, 55:71-77.
Heiskary, S. and Wilson, B. 1990. “Minnesota Lake Water Quality Assessment Report. 2nd Ed.” MPCA. St. Paul MN 95 pp
Heiskary, S. and Wilson, B. 2005. “Minnesota Lake Water Quality Assessment Report: Developing Nutrient Criteria.” 3rd Ed.” MPCA. St. Paul MN 150 pp (in prep.)
Heiskary, S. A. and Swain, E. B. 2002. “Water Quality Reconstruction from Fossil Diatoms: Applications for Trend Assessment, Model Verification, and Development of Nutrient Criteria for Lakes in Minnesota, USA.” Minnesota Pollution Control Agency, Environmental Outcomes Division, St. Paul, Minnesota. 103 pp.
Heiskary, S. and H. Markus. 2003. Establishing Relationships Among In-stream Nutrient Concentrations, Phytoplankton Abundance and Composition, fish and macroinvertebrate Indices, and Biochemical Oxygen Demand in Minnesota USA Rivers. MPCA, St. Paul MN 100 pp
Heiskary, S., M. Lindon, and H. Markus. 2003. “Shallow lakes of southwestern Minnesota: status and trend summary for selected lakes.” Part of a series on Minnesota Lake Water Quality Assessment. St. Paul MN 80 pp.
Helstrom, T. 1991. “The Effect of Resuspension on Algal Production in a Shallow Lake.” Hydrobiologia 213:183-190.
Hembre, L.K. and R.O. Megard. 2003. “Seasonal and Diel Patchiness of a Daphnids Population: An Acoustic Analysis.” Limnology and Oceanography, 48(6): 2221-2233.
Herb, W. and H. Stefan. 2003. Integral growth of submersed macrophytes in varying light regimes. Ecological Modeling 168: 77-100
Jackson, H.O. and W. C. Starret. 1959. “Turbidity and Sedimentation at Lake Chautauqua, Illinois.” J. Wild. Mgt., 23:157-168.
James, M.R., and D. J. Forsyth.1990 “Zooplankton-Phytoplankton Interactions in a Eutrophic Lake.” Journal of Plankton Research, 1990, 12:455-472
Jensen, J., et al. 1994. “Impact of Nutrients and Physical Factors on the Shift from Cyanobacterial to Chlorophyte Dominance in Shallow Danish Lakes.” Canadian Journal of Fish and Aquatic Science 51:1692-1698.
124
Jones, J. and R. Bachman. 1976. “Prediction of Phosphorus and Chlorophyll-a Levels in Lakes.” J. Wat. Pollut. Cont. Fedr. 48: 2176-2182
Juggins S. 2003. C2 version 1.3, Software for ecological and paleoecological data analysis and visualization. University of Newcastle, Newcastle upon Tyne, NE17RU, United Kingdom
Kimber A., J.L. Owens, and W.G. Crumpton, 1995. “Light Availability and Growth of Wildcelery (Vallinseria americana) in the Upper Mississippi River Backwaters.” Regulated Rivers: Research. & Management. 11:167-174.
Korschgen, C.E., W.L. Green, and K.P. Kenow, 1997. “Effects of Irradiance on Growth and Winter Bud Production by Vallisneria americana and Consequences to Its Abundance and Distribution.” Aquat. Bot,. 58:1-9.
Kristensen. P., et al. 1990. “Resuspension in a Shallow Eutrophic Lake.” Hydrobiologia 228:101-109.
Luettich, R. et al. “Dynamic Behavior of Suspended Sediment Concentrations in a Shallow Lake Perturbed By Episodic Wind Events.” Limnology and Oceanography 35:1050-1067.
Madsen, J. 1999. Point Intercept and Line Intercept Methods for Aquatic Plant Management. Aquatic Plant Control Technical Note M1-02. U.S. Army Engineer Research and Development Center, Vicksburg, MS.
Madsen, J.D., et al. 2001. “The Interaction Between Water Movement, Sediment Dynamics, and Submerged Macrophytes.” Hydrobiologia 444:71-84.
Meijer, M. et al. 1990. “Is Reduction of the Benthivorous Fish an Important Cause Of High Transparency Following Biomanipulation in Shallow Lakes?” Hydrobiologia 200/201:303- 315.
Meyer, B. 2003. Wildlife lake habitat survey report: West Twin. MDNR. Division of Wildlife
Minnesota Rules Chapter 7050. 2002. “Standards for the Protection of the Quality and purity of the Waters of the State. Revisor of Statutes and Minnesota Pollution Control Agency. St. Paul MN
Mitchell, S. F. and M. R. Perrow. 1998. “Interactions Between Grazing Birds and Macrophytes” in The Structuring Role of Submerged Macrophytes in Lakes. E. Jeppesen, Martin Sondergaard, Morten Sondergaard, and K. Christofferson, Eds. Springer, New York. Pp 175-196.
MPCA. 2003. Guidance Manual for Assessing the Quality of Minnesota Surface Waters for the Determination of Impairment. 305(b) and 303(d) list. Minnesota Pollution Control Agency. St. Paul MN. http://www.pca.state.mn.us/water/tmdl/index.html#publications
125
MPCA, 2004. Minnesota Lake Water Quality Assessment: 2004. Minnesota Pollution Control Agency. St. Paul MN.
Moss, B. 1998 “Shallow Lakes Biomanipulation and Eutrophication.” Scope Newsletter 29:2-45. Produced by Centre European d’Etudes des Polyphosphates, Belgium.
Moss, B., et al. 1996. “Progressive Restoration of a Shallow Lake: A 12 Year Experiment in Isolation, Sediment Removal and Biomanipulation.” Journal of Applied Ecology 33:71-86.
Newman, R.M. 1998. “Assessing Macrophytes in Minnesota’s Game Lakes.” Final report submitted to the Minnesota Department of Natural Resources.
Nichols, S. A. 1999. “Floristic Quality Assessment of Wisconsin Lake Plant Communities with Example Applications.” J. of Lake and Reserv. Mgmt. 15(2): 122-144.
Pace, M.L. 1984. “Zooplankton Community Structure, but Not Biomass, Influences the Total Phosphorus-Chlorophyll A Relationship.” Canadian Journal of Fish and Aquatic Science, 41:1089-1096.
Perleberg, D. 2003. “Floristic Quality Assessment of Minnesota Aquatic Macrophyte Community” Draft report. MDNR Brainerd MN
Perleberg, D. 2004. Personal communication. MDNR
Phillips, G.L., D. Eminson, and B. Moss.1978. “A Mechanism to Account for Macrophyte Decline in Progressively Eutrophicated Freshwaters.” Aquat. Bot,. 4:103-126.
Piepgras, S. and A. Giesen. 2003. “Wildlife lake habitat survey report: East Twin Lake:2002.” MDNR . Division of Wildlife.
Portielje, R., and T. Van der Molen. 1999 “Relationships Between Eutrophication Variables: from Nutrient Loading to Transparency.” Hydrobiologia. 408/409:375-387.
Radomski, P and T. Goeman. 2001. “Consequences of Human Lakeshore Development on Emergent and Floating-leaf Vegetation Abundance.” N. Amer. J. Fish. Manage. 21: 46-61.
Ramstack, J.M. 1999. “Pollution Trends in Minnesota Lakes. The use of Fossil Diatoms as Paleoindicators of Human Impact” MS Thesis Lehigh University. PA
Ramstack, J. M., S.C. Fritz, D.R. Engstrom, and S.A. Heiskary. 2003. “The Application of a Diatom-based Transfer Function to Evaluate Regional Water-quality Trends in Minnesota since 1970.” J. Paleolimnol. 29: 79-94.
126
Ramstack, J. M., Fritz, S. C. and Engstrom, D. R. 2004. “Twentieth-century water-quality trends in Minnesota lakes compared with pre-settlement variability.” Canadian Journal of Fisheries and Aquatic Sciences 61:561-576.
Ryding, S. and C. Forsberg. 1980. “Short-term Load-response relationships in Shallow Polluted Lakes.” In Developments of Hydrobiology. Vol. 2 Hypereutrophic Ecosystems. J. Baricia and L. Mur (Eds) Dr. W. Junk, Publisher – The Hague, Netherlands
Sagario, M.A. et al. 2005. “Does High Nitrogen Loading Prevent Clean-water Conditions in Shallow Lakes at Moderately High Phosphorus Concentrations?” Freshwater Biol. 50(1):27 -___.
Scheffer, M. 1998. “Ecology of Shallow Lakes.” Chapman and Hall, London UK
Scheffer, M. et al. 2001. “Catastrophic Shifts in Ecosystems.” Nature 413:591-596.
Shapiro, J. and D. I. Wright. 1984. “Lake Restoration by Biomanipulation: Round Lake, MN, the First Two Years.” Freshwater Biology 14:371-383
Smeltzer, E. and S. Heiskary. 1990. Analysis and Application of Lake User Survey Data. Lake and Reserv. Manage. 6(1): 109-118
Smith, V. 1983. “Low Nitrogen to Phosphorus Ratios Favor Dominance by Blue-green Algae in Lake Phytoplankton.” Science 221: 669-671.
Sondergaard, M. Kristensen, P. and Jeppesen, E. 1993. “Eight Years of Internal Phosphorus Loading and Changes in the Sediment Phosphorus Profiles of Lake Sobygaard Denmark.” Hydrobiologia, 253: 345-356.
Sondergaard, M., et al. 1990. “Phytoplankton Biomass Reduction After Planktivorous Fish Reduction in a Shallow, Eutrophic Lake: A Combined Effect of Reduced Internal P-Loading and Increased Zooplankton Grazing.” Hydrobiologia 200/201:229-240. ter Braak, C. and I. Prentice. 1988. “A Theory of Gradient Analysis.” Adv. Ecol. Res.18: 271- 317.
Turner, A.M. and G.G. Mittelbach, 1992 “Effects of Grazer Community Composition and Fish on Algal Dynamics.” Canadian Journal of Fish and Aquatic Science 49:1908-1915.
UMRCC. 2003. “Proposed Light-Related Water Quality Criteria Necessary to Sustain Submersed Aquatic Vegetation in the Upper Mississippi River.” Upper Mississippi River Conservation Committee. Water Quality Technical Section. 6 pp.
USEPA. 2000a. “Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs.” First Ed. Office of Water. EPA-822-B00-001. Washington D.C.
127
USEPA. 2000b. “Ambient Water Quality Criteria Recommendations: Lakes and Reservoirs in Nutrient Ecoregion VII.”” EPA 822-B00-009. Office of Water.Washington D.C.
Wetzel, R.G. 2001. Limnology: Lake and River Ecosystems. 3rd Ed. Academic Press. San Diego
Wilson, C.B. and W.W. Walker, Jr. 1989. “Development of Lake Assessment Methods Based Upon the Aquatic Ecoregion Approach” Lake and Reserv. Manage. 5(2): 11-22.
Wright. H. 1991. “Coring Tips.” J. Paleolimnology 6:37-49
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Appendix: Plant and related survey data summary
Native Max Depth % Plant of Native Stations Mean Max. Max max Plant Survey with Lake Name DOW# Eco Depth Depth1 Depth2 Depth Colonization Year Veg. Lake Name ft ft % of Max % Veg. Tiger 10-0108 CHF 3.0 8.0 3.0 2002 NA Tiger Diamond 27-0125 CHF 6.0 8.0 7.5 6.0 75% 2001 75% Diamond French 27-0127 CHF 2.0 3.0 3.0 3.0 100% 2001 100% French Prairie 27-0177 CHF 3.0 6.0 Prairie Quamba 33-0015 CHF 6.0 11.0 11.0 10.0 91% 2003 44% Quamba Ringo 34-0172 CHF 5.0 10.0 10.0 7.0 70% 1995 Ringo Florida Slough 34-0204 CHF 2.5 5.9 5.9 5.9 100% 2002 94% Florida Slough Johanna 61-0006 CHF 7.0 12.0 10.0 10.0 83% 2000 Johanna Nelson 61-0101 CHF 6.0 9.0 15.0 5.0 56% 2003 52% Nelson Fremont 71-0016 CHF 7.0 10.0 10.0 10.0 100% 2003 99% Fremont Silver 72-0013 CHF 4.5 9.0 5.5 5.5 61% 2001 100% Silver Cedar 73-0226 CHF 20.0 36.0 36.0 16.0 44% 2000 Cedar Cedar 73-0255 CHF 5.0 8.0 6.0 6.0 75% 2003 98% Cedar McCormic 73-0273 CHF 7.0 12.0 12.0 11.0 92% 2003 93% McCormic Monson 76-0033 CHF 12.0 21.0 21.5 13.3 63% 2002 68% Monson Trace 77-0009 CHF 6.0 9.0 8.5 8.5 94% 2002 88% Trace Pelican Pelican (South) 86-0031 CHF 5.0 9.0 11.3 6.5 72% 2001 52% (South) Cedar 86-0073 CHF 15.0 47.0 Cedar Smith 86-0250 CHF 3.0 5.0 4.5 4.5 90% 2001 100% Smith Jennie 21-0323 NGP 3.0 7.0 7.0 7.0 100% 2003 100% Jennie Shaokatan 41-0089 NGP 7.0 12.0 2002 2% Shaokatan West Twin 41-0102 NGP 2.3 4.4 4.4 4.4 100% 2002 95% West Twin East Twin 41-0108 NGP 2.4 5.0 5.0 5.0 100% 2002 38% East Twin Hattie 75-0200 NGP 6.0 9.0 9.0 4.0 44% 1996 n/a Hattie Hollerberg 76-0057 NGP 3.5 5.5 5.5 5.5 100% 2003 97% Hollerberg Hassel 76-0086 NGP 4.0 5.0 4.5 4.5 90% 2002 82% Hassel Platte 18-0088 NLF 10.0 23.0 23.0 15.0 65% 2003 90% Platte Clark 18-0374 NLF 15.0 31.0 31.0 20.0 65% 2002 89% Clark Red Sand 18-0386 NLF 7.0 23.0 20.0 18.0 78% 2001 97% Red Sand East Solomon 34-0246 WCP 9.5 13.0 13.0 12.6 97% 2002 52% East Solomon Titlow 72-0042 WCP 2.0 4.8 4.8 4.8 100% 2002 50% Titlow
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Plant and survey data continued
% Max # Stations depth. % w/ # floating- Floristic w/ Curly of Native Submerged leaf # plant Quality leaf curly- Bull- Lake Name DOW# Vegetation Native native species Index pondweed leaf3 Carp4 head4 # % Native # Sub #FL plants FQI %Curly feet Tiger 10-0108 NA unk* ? ? Diamond 27-0125 75% 4 0 4 8 0% unk* 1 3 French 27-0127 100% 3 0 3 9 0% unk* ? ? Prairie 27-0177 0 0 Quamba 33-0015 35% 8 2 10 16 30% 6 1 1 Ringo 34-0172 5 2 7 12 unk* 2 2 Florida Slough 34-0204 94% 7 0 7 16 0% unk* ? ? Johanna 61-0006 14 3 17 22 n/a unk* 0 3 Nelson 61-0101 52% 5 0 5 13 0% unk* 0 2 Fremont 71-0016 53% 15 4 19 23 87% 10 0 2 Silver 72-0013 100% 9 0 9 17 0% unk* 0 3 Cedar 73-0226 15 2 17 22 unk* 0 1 Cedar 73-0255 98% 9 9 18 16 0% unk* 0 3 McCormic 73-0273 93% 10 0 10 20 18% 11 0 0 Monson 76-0033 38% 6 0 6 16 65% 15 1 2 Trace 77-0009 88% 7 0 7 15 0% unk* 0 3 Pelican (South) 86-0031 48% 8 1 9 17 17% 4.5 ? ? Cedar 86-0073 0 2 Smith 86-0250 100% 9 0 9 17 0% unk* 3 3 Jennie 21-0323 100% 4 0 4 10 0% unk* Shaokatan 41-0089 2% 3 3 1 2 West Twin 41-0102 95% 9 0 9 18 0% unk* ? ? East Twin 41-0108 38% 3 0 3 10 0% unk* ? ? Hattie 75-0200 n/a 2 0 2 6 n/a unk* 2 3 Hollerberg 76-0057 97% 7 0 7 16 0% unk* ? ? Hassel 76-0086 82% 10 0 10 19 0% unk* ? ? Platte 18-0088 71% 19 5 24 27 39% 19 1 3 Clark 18-0374 89% 18 4 22 26 0% unk* 1 2 Red Sand 18-0386 97% 14 4 18 23 0% unk* 1 2 East Solomon 34-0246 49% 4 0 4 12 15% 10.5 2 3 Titlow 72-0042 50% 2 1 3 6 0% unk* 3 3
1 Maximum depth as determined by MPCA staff based on MDNR bathymetric maps or field observation during water quality sampling; 2 Maximum depth as determined during course of MDNR plant surveys. 3 plant survey conducted late in summer so distribution of curly leaf unknown 4 relative scale of significance or dominance of carp or black bullhead in lake based on web-based fishery survey data and/or survey of MDNR area fishery manager, whereby: 0 = “not found in survey”, 1 = “low, uncommon or not perceived as a problem”, 2 = “moderately abundant” and 3 = “very abundant, problem levels”;
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Appendix: Methods
I. ZOOPLANKTON COLLECTION PROCEDURE
1. Record the lake ID, Date, on the 125-mL wide-mouth glass bottle 2. Lower the zooplankton net so that the mouth of the net is ~1.0 m from the bottom and note the depth (slowly so the net does not become inverted but the weight of the bucket keeps it vertical and upright). 3. Slowly (1 m per sec) and steadily haul the net to the surface; splash lake water around outside of net to wash zooplankton into collection bucket.
IF THE NET TOUCHES BOTTOM AND MUD ENTERS THE NETS, COMPLETELY RINSE THE NET AND REPEAT THE PROCEDURE.
4. Carefully remove the collection bucket from the net and place over bottle; if necessary, swirl bucket to remove water through mesh to reduce sample volume to less than 100 mL; after removing plug and allowing sample to enter sample bottle, rinse bucket with squirt bottle until all zooplankton are removed from the bucket, but room for the preservative (next step). 5. Dispense 5 mL formalin-sucrose solution (40 g/L sucrose + formalin1) into the bottle.2 6. Record the number of tows and length of each tow on the bottle and the field sheet 7. Verify that all information on the labels and the form is complete and correctly recorded.
MODIFICATION FOR CLEAR, SHALLOW LAKES ONLY: If the depth <2 m and the Secchi disk could be seen on the bottom, then conduct a second tow of the same length (i.e., repeat steps above twice) and combine the contents of both tows into one bottle. Record "2 tows" with the depth of the tow on the bottle and field sheet.
ZOOPLANKTON IDENTIFICATION PROCEDURE
1. A 2-mL subsample is collected from sample bottle using a modified 2-mL volumetric pipette and placed in a counting chamber and covered with cover slip. 2. Pour remaining sample volume into 100-mL graduated cylinder and record volume + 2 mL. Return water from graduated cylinder to sample bottle and rinse cylinder. If there is not sufficient room in bottle for rinse water, decant sufficient water from the sample to allow for 2-mL subsample and rinse water from the graduated cylinder and the counting chamber.
1 Alcohol has been found to not work well for zooplankton. The 40 g/L sucrose in formalin (37% formaldehyde) prevents “ballooning” of the Cladocera, making them easier to identify and reducing egg loss (Haney, JF and DJ Hall. 1973. Sugar-coated Daphnids: A preservation technique for Cladocera. Limnol & Oceanogr. 18: 331-333.) Haney and Hall also used carbonated water to first anesthetize the zooplankton before preserving them. Egg loss is further reduced by keeping the formalin-sucrose solution chilled (Prepas, E. 1978. Sugar-frosted Daphnids: An improved fixation technique. Limnol. & Oceanogr. 23: 557-559. 2 Formalin-sugar solution is made up ahead of time in gallon container and smaller amount transferred to another container for field use.
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3. At 40x power on a compound microscope, zooplankton are identified by taxa. Cladocera are identified by genus and the remaining zooplankton are identified as copepods or rotifers, as well as estimating percentages of dominant genera. Based on this analysis, the percent biomass is estimated for large cladocera, small cladocera, copepods, and rotifers. 4. Once the sample is analyzed, the subsample in the counting cell is rinsed back into the sample bottle. 5. The sample will be kept for one year following collection Minnesota Zooplankton Rapid Assessment ver. 1.0 BAM 3FEB04
Lake Name: Assessment Date: Site ID: Subsample Volume: 2 mL Lake ID: Number & Depth of Tows: Assessed by: Total Sample Volume (mL) B. Monson
DIVISION TAXON % Abundance Notes CLADOCERA Large Cladocera Small Cladocera Daphnia pulicaria Daphnia galeata mendotae Daphnia ambigua/parvula Daphnia retrocurva Bosmina sp. Chydorus sp. Diaphanosoma sp. Ceriodaphnia sp. Leptodora kindtii COPEPODA Copepoda Nauplii Cyclops sp. Mesocyclops sp. Diaptomus sp. ROTIFERA Rotifera Kellicottia sp. Conochilus sp. Keratella cochlearis Keratella quadrata Polyarthra vulgaris Brachionus sp. Filinia longiseta Aplanchna sp. Trichocerca sp. Ascomorpha sp. OTHER
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II. Phytoplankton
Minnesota Phytoplankton Rapid Assessment Method
1. Pour preserved sample into settling chamber. Allow to settle (often overnight). 2. Scan sample using an inverted microscope and identify genera (and species where easily identified) of algae present in sample. 3. Under lower power, scan a large enough proportion of sample to estimate percent abundance by volume, for each genera identified. Estimate should consider size and density of algal types. Typically do not count anything less than 5% based on biovolume. 4. Record estimated percent abundance for each taxon. 5. Optional: Calculate estimated chlorophyll-a value for each taxon based on measured chlorophyll-a concentration for the sample.
[Method as originally described by Dr. Ed Swain and Carolyn Dindorf, Minnesota Pollution Control Agency, 6/16/1989. Comments or questions on methodology can be directed to Dr. Howard Markus at howard.markus.pca.state.mn.us, or (651) 296-7295.]
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