Otter Tail Lake 56-0242-00 OTTER TAIL COUNTY Lake Water Quality Summary Otter Tail Lake is located at the city of Ottertail, MN in Otter Tail County. It is a long and oval lake covering 14,074 acres (Table 1), and is the seventh deepest and tenth largest lake in Minnesota. Otter Tail Lake has five inlets and one outlet, which classify it as a drainage lake. The main inlets are the Otter Tail River, which enters on the northeast corner of the lake, and the Dead River, which enters on the northwest side of the lake. The Otter Tail River outlets at the southwest corner of the lake and eventually joins the Red River. Water quality data have been collected on Otter Tail Lake since 1969 (Tables 2 & 3). These data show that the lake is mesotrophic (TSI = 46) with moderately clear water conditions most of the summer and excellent recreational opportunities. Otter Tail Lake is a part of the Otter Tail Lake Property Owners Association. OTLPOA was incorporated on January 5, 1967. Since its beginning, the association has been focused on the preservation of fish and wildlife, water quality, and the prevention of invasive species. OTLPOA is also a member of the Otter Tail Coalition of Lake Associations (COLA). Table 1. Otter Tail Lake location and key physical characteristics. Location Data Physical Characteristics MN Lake ID: 56-0242-00 Surface area (acres): 14,074 County: Otter Tail Littoral area (acres): 6,717 North Central Hardwood % Littoral area: 48 Ecoregion: Forests Max depth (ft), (m): 112, 34.1 Major Drainage Basin: Red River Inlets: 5 Latitude/Longitude: 46.3982/-95.6626 Outlets: 1 Invasive Species: None as of 2013 Public Accesses: 4 Table 2. Availability of primary data types for Otter Tail. Data Availability Transparency data Excellent data source from 1998-2012. Chemical data Excellent data source from 1998-2012. No inlet or outlet data exist. See page 20 for monitoring Inlet/Outlet data recommendations. Recommendations For recommendations refer to page 20. RMB Environmental Laboratories, Inc. 1 of 22 2013 Otter Tail Lake Map Figure 1. Map of Otter Tail Lake with 2010 aerial imagery and illustrations of lake depth contour lines, sample site locations, inlets and outlets, and public access points. The light green areas in the lake illustrate the littoral zone, where the sunlight can usually reach the lake bottom, allowing aquatic plants to grow. Table 3. Monitoring programs and associated monitoring sites. Monitoring programs include the Minnesota Pollution Control Agency Lake Monitoring Program (MPCA), Citizen Lake Monitoring Program (CLMP) and RMB Environmental Laboratories Lakes Program (RMBEL). Lake Site(s) Depth (ft) Monitoring Programs 101-102 NA MPCA: 1980 103-131 NA MPCA: 1969 201 45 CLMP: 2004-2011 202 8 MPCA: 1979-1980 203 10 CLMP: 1980 204 12 CLMP: 1989 205 45 CLMP: 1996-2006 206*Primary site 110 RMBEL: 1998-2012 207 70 RMBEL: 1998-2010 208 60 RMBEL: 2004-2012 RMB Environmental Laboratories, Inc. 2 of 22 2013 Otter Tail Average Water Quality Statistics The information below describes available chemical data for Otter Tail Lake through 2012 (Table 4). Data for total phosphorus, chlorophyll a, and Secchi depth are from the primary site 206. All additional chemical data is from sites 101 and 102 and reflects mean values from 1980. Minnesota is divided into 7 ecoregions based on land use, vegetation, precipitation and geology. The MPCA has developed a way to determine the "average range" of water quality expected for lakes in each ecoregion. For more information on ecoregions and expected water quality ranges, see page 11. Table 4. Water quality means compared to ecoregion ranges and impaired waters standard. Impaired Ecoregion Waters 1 2 Parameter Mean Range Standard Interpretation Total phosphorus (ug/L) 17.7 23 – 50 > 40 3 Chlorophyll a (ug/L) 6.4 5 – 22 > 14 Results are better than the expected range for the Chlorophyll a max (ug/L) 14 7 – 37 ecoregion. Secchi depth (ft) 10.8 4.9 – 10.5 < 4.6 Dissolved oxygen Dimictic Dissolved oxygen depth profiles see page 8 show that the deep areas of the lake are anoxic in late summer. Total Kjeldahl Nitrogen 0.88 <0.60 – 1.2 Indicates insufficient nitrogen to (mg/L) support summer nitrogen- induced algae blooms. Alkalinity (mg/L) 177 75 – 150 Indicates a low sensitivity to acid rain and a good buffering capacity. Color (Pt-Co Units) 10.8 10 – 20 Indicates clear water with little to no tannins (brown stain). pH 8.3 8.6 – 8.8 Indicates a hard water lake. Lake water pH less than 6.5 can affect fish spawning and the solubility of metals in the water. Chloride (mg/L) 9.7 4 – 10 Within the expected range for the ecoregion. Total Suspended 3 2 – 6 Within the expected range for Solids (mg/L) the ecoregion. Indicates low suspended solids and clear water. Specific Conductance 320 300 – 400 Within the expected range for (umhos/cm) the ecoregion. Total Nitrogen : Total 50:1 25:1 – 35:1 Indicates the lake is Phosphorus phosphorus limited, which means that algae growth is limited by the amount of phosphorus in the lake. 1The ecoregion range is the 25th-75th percentile of summer means from ecoregion reference lakes 2For further information regarding the Impaired Waters Assessment program, refer to http://www.pca.state.mn.us/water/tmdl/index.html 3Chlorophyll a measurements have been corrected for pheophytin Units: 1 mg/L (ppm) = 1,000 ug/L (ppb) RMB Environmental Laboratories, Inc. 3 of 22 2013 Otter Tail Water Quality Characteristics - Historical Means and Ranges Table 5. Water quality means and ranges for primary sites. Primary Parameters Site Site Site 206 207 208 Total Phosphorus Mean (ug/L): 17.7 16.7 17.6 Total Phosphorus Min: <5 7 <5 Total Phosphorus Max: 28 27 36 Number of Observations: 64 62 40 Chlorophyll a Mean (ug/L): 6.4 6.8 6.9 Chlorophyll-a Min: <1 <1 3 Chlorophyll-a Max: 14 12 13 Number of Observations: 63 60 40 Secchi Depth Mean (ft): 10.8 11.3 11.4 Secchi Depth Min: 6.5 7.0 7.5 Secchi Depth Max: 18.3 17.0 16.0 Number of Observations: 66 62 53 FigureFigure 2. Otter 2. Lake Tail “insert” total phosphorus, total phosphorus, chlorophyll chlorophyll a and transparencya and transparency historical historical ranges. ranges. The arrow The arrow representsrepresents the therange range and and the theblack black dot dotrepresents represents the thehistorical historical mean mean (Primary (Primary Site Site 206). xxx). Figure Figure adapted adapted after Moore and Thornton, [Ed.]. 1988. Lake and Reservoir Restoration Guidance Manual. (Doc. No. EPA 440/5-88-002) after Moore and Thornton, [Ed.]. 1988. Lake and Reservoir Restoration Guidance Manual. (Doc. No. EPA 440/5-88-002) RMB Environmental Laboratories, Inc. 4 of 22 2013 Otter Tail Transparency (Secchi Depth) Transparency is how easily light can pass through a substance. In lakes it is how deep sunlight penetrates through the water. Plants and algae need sunlight to grow, so they are only able to grow in areas of lakes where the sun penetrates. Water transparency depends on the amount of particles in the water. An increase in particulates results in a decrease in transparency. The transparency varies year to year due to changes in weather, precipitation, lake use, flooding, temperature, lake levels, etc. The annual mean transparency in Otter Tail Lake ranges from 7.2 to 14.0 feet. The transparency throughout the lake appears to be relatively uniform at all three main monitoring sites. The transparency was low in 2011 after a very wet year in 2010 (Figure 3). Transparency monitoring should be continued annually at sites 206, 207 and 208 in order to track water quality changes. 20 Precipitation Transparency and Precipitation 100 Primary Site 206 18 90 Site 207 16 80 Site 208 14 70 Mean (in) 12 60 (ft) 10 50 8 40 Depth Precipitation 6 30 Secchi 4 20 2 10 0 0 Date Figure 3. Annual mean transparency compared to long-term mean transparency. Otter Tail Lake transparency ranges from 6.5 to 18.3 feet at the primary site (206). Figure 4 shows the seasonal transparency dynamics. The maximum Secchi reading is usually obtained in early summer. Otter Tail Lake transparency is high in May and June, and then declines through August. The transparency then rebounds in October after fall turnover. This transparency dynamic is typical of a Minnesota lake. The dynamics have to do with algae and zooplankton population dynamics, and lake turnover. It is important for lake residents to understand the seasonal transparency dynamics in their lake so that they are not worried about why their transparency is lower in August than it is in June. It is typical for a lake to vary in transparency throughout the summer. RMB Environmental Laboratories, Inc. 5 of 22 2013 Otter Tail Seasonal Transparency Trends 1997 20 1998 18 1999 2000 16 2001 14 2002 2003 (ft) 12 2004 2005 10 Depth 2006 8 2007 Secchi 2008 6 2009 4 2010 2011 2 2012 Poly. (Pattern) 0 Date Figure 4. Seasonal transparency dynamics and year to year comparison (Primary Site 206). The black line represents the pattern in the data. User Perceptions When volunteers collect Secchi depth readings, they record their perceptions of the water based on the physical appearance and the recreational suitability. These perceptions can be compared to water quality parameters to see how the lake "user" would experience the lake at that time. Looking at transparency data, as the Secchi depth decreases the perception of the lake's physical appearance rating decreases.