Lakes: Ann, Gilchrist, Grove, Leven, Reno, Villard, Smith)
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Status and Trend Monitoring Summary for Selected Pope and Douglas County, Minnesota Lakes 2000 (Lakes: Ann, Gilchrist, Grove, Leven, Reno, Villard, Smith) Minnesota Pollution Control Agency Environmental Outcomes Division Environmental Monitoring and Analysis Section Andrea Plevan and Steve Heiskary September 2001 Printed on recycled paper containing at least 10 percent fibers from paper recycled by consumers. This material may be made available in other formats, including Braille, large format and audiotape. MPCA Status and Trend Monitoring Summary for 2000 Pope County Lakes Part 1: Purpose of study and background information on MN lakes The Minnesota Pollution Control Agency’s (MPCA) core lake-monitoring programs include the Citizen Lake Monitoring Program (CLMP), the Lake Assessment Program (LAP), and the Clean Water Partnership (CWP) Program. In addition to these programs, the MPCA annually monitors numerous lakes to provide baseline water quality data, provide data for potential LAP and CWP lakes, characterize lake conditions in different regions of the state, examine year-to-year variability in ecoregion reference lakes, and provide additional trophic status data for lakes exhibiting trends in Secchi transparency. In the latter case, we attempt to determine if the trends in Secchi transparency are “real,” i.e., if supporting trophic status data substantiate whether a change in trophic status has occurred. The lake sampling efforts also provide a means to respond to citizen concerns about protecting or improving the lake in cases where no data exists to evaluate the quality of the lake. For efficient sampling, we tend to select geographic clusters of lakes (e.g., focus on a specific county) whenever possible. In 2000, the MPCA monitored the following six lakes: Ann, Gilchrist, Grove, Leven, Reno, and Villard Lakes in Pope County; and Smith Lake in adjacent Douglas County. Water quality samples were collected monthly from June through September at most lakes. These lakes represent a cross section of the lakes found in this area in terms of water quality, lake morphometry and watershed characteristics. They also represent a range in terms of the amount of data available; ranging from a completed LAP study on Gilchrist Lake, completed Clean Water Partnership Phases I and II work on Grove, to Ann Lake which only had Secchi data. Grove and Smith Lakes were a part of MPCA’s ecoregion reference lake monitoring effort in the mid 1980s. More recently, several of the lakes have been monitored as part of a Pope County water plan and efforts of a coalition of lake associations. MPCA monitoring in 2000 was conducted by Mike Vavricka, Steve Heiskary, and student interns Stephanie Johnson and Dan Barringer. The state of Minnesota is divided into seven ecoregions (Figure 1), based on soils, landform, potential natural vegetation, and land use. All of the lakes monitored for this study are located in the North Central Hardwood Forest (NCHF) ecoregion. By comparing a lake’s water quality to that of reference lakes in the same ecoregion, one can gain a clearer picture of where the lake falls in the spectrum of water quality parameters relative to other lakes in that ecoregion. Lake phosphorus criteria have been established for Minnesota lakes located in several of the ecoregions represented in Minnesota (Table 1; Heiskary and Wilson 1988). These criteria vary depending on the intended use of the water resource. For example, phosphorus criteria for a drinking water supply are more stringent than the criteria for recreation and aesthetics. Phosphorus concentrations in the lakes in the present study can be compared to the lake phosphorus criteria in Table 1. In general, lakes that are at or below the specific criterion for primary contact recreation and aesthetics will have adequate transparency and sufficiently low amounts of algae to support swimmable use throughout most of the summer. Whenever possible these lakes should be protected from increases in nutrient concentrations that would tend to stimulate algal and plant growth and 1 reduce transparency. For lakes above the phosphorus criteria, the suggested levels may serve as a restoration goal for the lake; however, restoration goals should be determined by individual examination of the lake and its watershed characteristics. Thus criteria, which were originally developed in 1988, may be modified in the future, as Minnesota is in the process of formally developing (through rule-making) nutrient criteria as a part of a national effort. Lake depth can have a significant influence on lake processes and water quality. One such process is thermal stratification (formation of distinct temperature layers), in which deep lakes (maximum depths of 30 - 40 feet or more) often stratify (form layers) during the summer months and are referred to as dimictic. Shallow lakes (maximum depths of 20 feet or less) in contrast, typically do not stratify and are often referred to as polymictic. Some lakes, intermediate between these two (e.g., lakes with moderate depth and large surface area) may stratify intermittently during calm periods. The combined effect of depth and stratification can influence overall water quality. The epilimnia of deeper, stratified lakes often have lower phosphorus concentrations as compared to shallow well-mixed lakes in the same ecoregion (Table 2). The percentiles in Table 2 can provide an additional basis for comparing observed summer mean TP and may further serve as a guide for deriving an appropriate TP goal for the lake. Note that the percentile values (median, for example) can be quite different within an ecoregion for different types of lakes. Stream water quality was not monitored as a part of this study. However, we have provided typical concentrations of total phosphorus and total suspended solids for streams in six of MN’s ecoregions (Table 3). If data are available from other sources, local stream water quality parameters can be compared to these values. The values in Table 3 represent the “central tendency” (25th to 75th percentiles) of concentrations from representative, minimally-impacted river sites in each ecoregion. These data were derived from Minnesota’s Milestone monitoring program and should not be considered as “reference” streams nor does this represent the most pristine streams in each ecoregion. The data do, however provide useful yardsticks for evaluating data obtained from streams in the respective ecoregions. 2 Figure 1. Minnesota’s seven ecoregions as mapped by U.S. EPA. Northern Minnesota Wetlands Red River Valley Northern Lakes and Forests Pope County North Central Hardwood Forests Northern Glaciated Plains Driftless Area Western CornBelt Plains Table 1. Minnesota lake total phosphorus criteria, by ecoregion. (Heiskary and Wilson 1988) Ecoregion Most Sensitive Use TP Criteria Northern Lakes and Forests drinking water supply < 15 µg/L cold water fishery < 15 µg/L primary contact recreation and aesthetics < 30 µg/L North Central Hardwood drinking water supply < 30 µg/L Forests primary contact recreation and aesthetics < 40 µg/L Western Corn Belt Plains drinking water supply < 40 µg/L primary contact recreation (full support) < 40 µg/L (partial support) < 90 µg/L Northern Glaciated Plains primary contact recreation and aesthetics < 90 µg/L (partial support) 3 Table 2. Total phosphorus concentrations(µg/L), by mixing status and ecoregion. Based on all assessed lakes for each ecoregion. D = Dimictic, I = Intermittent, P = Polymictic. Northern Lakes and North Central Western Corn Belt Forests Hardwood Forest Plains Percentile D I P D I P D I P 90 % 37 53 57 104 263 344 -- -- 284 75 % 29 35 39 58 100 161 101 195 211 50 % (median) 20 26 29 39 62 89 69 135 141 25 % 13 19 19 25 38 50 39 58 97 10 % 9 131219213225--69 # of obs. 257 87 199 152 71 145 4 3 38 Table 3. Interquartile range of annual mean concentrations for minimally impacted streams in Minnesota, by ecoregion. Data from 1970-1992. TP = total phosphorus, TSS = total suspended solids. (McCollor and Heiskary 1993) TP (mg/l) TSS (mg/l) Region 25% 50% 75% 25% 50% 75% NLF 0.02 0.04 0.05 1.8 3.3 6.0 NMW 0.04 0.06 0.09 4.8 8.6 16 NCHF 0.06 0.09 0.15 4.8 8.8 16 NGP 0.09 0.16 0.25 11 34 63 RRV 0.11 0.19 0.30 11 28 59 WCBP 0.16 0.24 0.33 10 27 61 4 Part 2: Lake survey Methods This report includes data from 2000 as well as previously collected data available in STORET, U.S. EPA’s national water quality data bank (Appendix). The following discussion assumes familiarity with basic limnologic terms as used in a “Citizens Guide to Lake Protection” and as commonly used in LAP reports. A glossary is included in the Appendix as well. For the lake survey, one to two sites in each lake were monitored monthly, from June through September. If more than one site was monitored, one of the sites was the “primary site,” and the other site a “secondary site.” If only one site in a lake was monitored, it was treated as a primary site. Lake size and morphometry determined the number of sites monitored per lake. At each primary site, the following parameters were analyzed from a surface sample of water: chlorophyll a, total phosphorus (TP), total Kjeldahl nitrogen (TKN), total suspended solids (TSS), total suspended volatiles (TSV), alkalinity, color, pH, and conductivity. Additionally, a temperature and dissolved oxygen depth profile was taken. At sites that were stratified, total phosphorus was analyzed from a hypolimnetic (bottom) water sample as well. At each secondary site, only surface chlorophyll a and total phosphorus were analyzed. Secchi disk transparency was recorded at all sites. Additional information, such as bathymetric and location maps, was obtained from the DNR’s lakefinder website (http://www.dnr.state.mn.us/lakefind/index.html) and the MPCA website (http://www.pca.state.mn.us).