Boy Lake CASS COUNTY

Transcription

1 Boy Lake CASS COUNTY Summary Boy Lake is located near Remer, MN in Cass County. It covers 3,452 acres, which places it in the upper 1% of lakes in Minnesota in terms of size. Boy Lake has two main inlets and one outlet, which classify it as a drainage lake. Boy Lake is located near the end of the Boy River Watershed, and both the Boy and Swift Rivers inlet to Boy Lake. The Boy River then outlets to the north, flows through Leech Lake and eventually into the Leech Lake River. The Leech Lake River flows east from Leech Lake and joins the Mississippi River. Water quality data have been collected on Boy Lake since 25. These data show that the lake is mildly eutrophic (page 9). Mildly eutrophic lakes are characterized by moderately clear water with occasional algae blooms and "green" water toward the end of the summer. The Boy Lake Association has been involved in water quality monitoring, and is a member of the Association of Cass County Lakes. Vitals MN Lake ID: County: Cass Ecoregion: Northern Lakes and Forest Major Drainage Basin: Upper Mississippi River Latitude/Longitude: / Water Body Type: Public Monitored Sites (Primary): 21 Monitored Sites (Secondary): 22, 25 Invasive species: none documented Physical Characteristics Surface area (acres): 3,452 Littoral area (acres): 2,7 % Littoral area: 59% Max depth (ft): 45 (m): 13.7 Mean depth (ft): NA Lakeshed size (acres): 2,987 Lakeshed : lake area ratio 6:1 2 main inlets Inlets (Boy River and Swift River) 2 minor inlets (Unnamed) Outlets 1 (Boy River) Accesses 1 public Data Availability Transparency data Chemical data Inlet/Outlet data Secchi readings have been collected in 25, through the MPCA CLMP program. Total Phosphorus and Chlorophyll a data have been collected in 27 and 28. The location where the Boy River inlets to Boy Lake was monitored in 28. Recommendations For recommendations refer to page 11. Boy Lake Report generation: RMB Environmental Laboratories 1 of 11

2 Figure 1. Map of Boy Lake illustrating bathymetry, lake sample site locations, stream inlets and outlets and aerial land use. The pink shaded areas in the lake illustrate the littoral zone, where the sunlight can usually reach the lake bottom allowing aquatic plants to grow. Lake Site Depth (ft) Monitoring Programs 21* Primary Site 45 CLMP: 25, 27-28; RMB Labs: CLMP: 27-28; RMB Labs: ** at Boy River Inlet 26 CLMP: 27; RMB Labs 28 Boy Lake Report generation: RMB Environmental Laboratories 2 of 11

3 The information below describes available chemical data for the primary site (21) of Boy Lake through 28. The data set is limited, and all parameters are means for just 1992, 2 and data. Minnesota is divided into seven 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 1. Parameter Mean Ecoregion Range 1 Total phosphorus (ug/l) > 35 Chlorophyll a (ug/l) > 12 Chlorophyll a max (ug/l) 28 <15 Secchi depth (ft) < 4.5 Impaired Waters Standard 2 Interpretation Results are towards the high end of the expected range for the ecoregion. For more information about Impaired Waters Assessment, see page 12. Dissolved oxygen -- Data non-existent. Total Kieldahl Nitrogen (mg/l) Data non-existent. Alkalinity (mg/l) Indicates a low sensitivity to acid rain and a good buffering capacity. Color (Pt-Co Units) Data non-existent. ph Indicates a hardwater lake. Lake water ph less than 6.5 can affect fish spawning and the solubility of metals in the water. Chloride (mg/l) Data non-existent. Total Suspended Solids (mg/l) -- <1-2 Data non-existent. Conductivity (umhos/cm) Conductivity is slightly higher than the ecoregion average, which could be due to its geographical placement toward the end of the Boy River Watershed. High conductivity indicates high dissolved solids, which can come from watershed or human sources. Total Nitrogen :Total Phosphorus -- 25:1 35:1 Data non-existent. Data Source: RMB Environmental Laboratories, Inc.; 1992, 2 DNR Fisheries Surveys 1 The ecoregion range is the 25 th -75 th percentile of summer means from ecoregion reference lakes 2 For further information regarding the Impaired Waters Assessment program, refer to 3 Chlorophyll a measurements have been corrected for pheophytin Units: 1 mg/l (ppm) = 1, ug/l (ppb) Boy Lake Report generation: RMB Environmental Laboratories 3 of 11

4 Water Quality Characteristics - Historical Means Years monitored: 25, Parameters Primary Site 21 Site 22 Site 25 Total Phosphorus Mean (ug/l): Total Phosphorus Min: Total Phosphorus Max: Number of Observations: Chlorophyll a Mean (ug/l): Chlorophyll-a Min: Chlorophyll-a Max: Number of Observations: Secchi Depth Mean (ft): Secchi Depth Min: Secchi Depth Max: Number of Observations: Figure 2. Boy Lake total phosphorus, chlorophyll a and transparency historical ranges. The arrow represents the range and the black dot represents the historical mean (Primary Site 21). Figure adapted after Moore and Thornton, [Ed.] Lake and Reservoir Restoration Guidance Manual. (Doc. No. EPA 44/5-88-2) Boy Lake Report generation: RMB Environmental Laboratories 4 of 11

5 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. For all the sites that had more than two data points, the mean transparency ranges from 5.7 to 8.5 feet. Figure 3 shows the limited and disjointed nature of the transparency data set. Even though these means vary, the transparency throughout the lake appears to be relatively uniform in the same years. 14 Transparency: Annual Means Secchi Depth (ft) Site 21 Site 22 Site Figure 3. Annual mean transparency for sites 21, 22, and 25. Boy Lake transparency ranges from 3.8 to 17. ft at the primary site (21). Figure 4 shows the seasonal transparency dynamics. Boy Lake transparency is highest in May and then decreases steadily throughout the summer. In 25, data exist from October, and show that the transparency rebounds somewhat. Boy Lake is most likely not deep enough to recover to spring transparency in the fall. 25 had better transparency than 27 and 28. Transparency monitoring should be continued annually at sites 21, 22 and 25 in order to track water quality changes. Boy Lake Report generation: RMB Environmental Laboratories 5 of 11

6 Figure 4. Seasonal transparency dynamics and year-to-year comparison (Primary Site 21). 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. Boy Lake was rated as being "not quite crystal clear " 48% of the time between (Figure 5). 8% Physical Appearance Rating 8% Crystal clear water 48% Not quite crystal clear a little algae visible 44% 44% Definite algae green, yellow, or brown color apparent 48% % High algae levels with limited clarity and/or mild odor apparent % Severely high algae levels Figure 5. Physical appearance rating, as rated by the volunteer monitor (25-28). Boy Lake Report generation: RMB Environmental Laboratories 6 of 11

7 As the secchi depth decreases, the perception of recreational suitability of the lake decreases. Boy Lake was rated as being "beautiful" 79% of the time from (Figure 5). 18% % 3% Recreational Suitability Rating 79% Beautiful, could not be better 18% Very minor aesthetic problems; excellent for swimming, boating % Swimming and aesthetic enjoyment of the lake slightly impaired because of algae levels 3% Desire to swim and level of enjoyment of the lake substantially reduced because of algae levels 79% % Swimming and aesthetic enjoyment of the lake nearly impossible because of algae levels Figure 6. Recreational suitability rating, as rated by the volunteer monitor (25-28). Total Phosphorus Boy Lake is most likely phosphorus limited, which means that algae and aquatic plant growth is dependent upon available phosphorus. Total phosphorus was evaluated in Boy Lake in 27, and 28. The data indicate a slight increase in phosphorus from the beginning of the summer to the end (Figure 7). This pattern is typical for Minnesota lakes. In 28, site 25 had the lowest phosphorus concentrations and site 22 had the highest. Site 25 is located where the Boy River Inlet enters the lake. These results show that the water entering Boy Lake from the Total Phosphorus (ug/l) Jun Total Phosphorus Figure 7. Historical total phosphorus concentrations (ug/l) for Boy Lake (data sets from 27-28). Boy River has relatively low phosphorus concentration. Phosphorus should continue to be monitored to track any future changes in water quality. 18-Jun 2-Jul 16-Jul 3-Jul 13-Aug 27-Aug Site 21, 27 Site 21, 28 Site 22, 28 Site 25, 28 1-Sep Boy Lake Report generation: RMB Environmental Laboratories 7 of 11

8 Chlorophyll a Chlorophyll a is the pigment that makes plants and algae green. Chlorophyll a is tested in lakes to determine the algae concentration or how "green" the water is. Chlorophyll a concentrations greater than 1 ug/l are perceived as a mild algae bloom, while concentrations greater than 2 ug/l are perceived as a nuisance. Chlorophyll a (ug/l) Chlorophyll a Site 21, 27 Site 21, 28 Site 22, 28 Site 25, 28 Chlorophyll a was evaluated in Boy Lake in (Figure 5). Chlorophyll a concentrations for all dates at all sites increased steadily toward the end of the summer. All sites reached concentrations greater than 1 ug/l, indicating mild algae 5 4-Jun 18-Jun Figure 8. Chlorophyll a concentrations (ug/l) for Boy Lake (data sets from 27-28). blooms. Site 21 exceeded 2 ug/l in 27, indicating a nuisance algae bloom. Chlorophyll a should continue to be monitored to track changes in water quality. 2-Jul 16-Jul 3-Jul 13-Aug 27-Aug 1-Sep Dissolved Oxygen There is no dissolved oxygen data for Boy Lake. Monitoring dissolved oxygen in future years would describe dynamics occurring in the water column and the habitat quality for game fish. Trend Analysis For detecting trends, a minimum of 8-1 years of data with 4 or more readings per season are recommended. Minimum confidence accepted by the MPCA is 9%. This means that there is a 9% chance that the data are showing a true trend and a 1% chance that the trend is a random result of the data. Only short-term trends can be determined with just a few years of data, because there can be different wet years and dry years, water levels, weather, etc., that affect the water quality naturally. There is not enough historical data to perform trend analysis for transparency, total phosphorus or chlorophyll a on Boy Lake. In another 2 years, short-term trend analysis will be possible. Boy Lake Report generation: RMB Environmental Laboratories 8 of 11

9 Trophic State Index Phosphorus (nutrients), chlorophyll a (algae concentration) and Secchi depth (transparency) are related. As phosphorus increases, there is more food available for algae, resulting in increased algal concentrations. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases. Trophic State Index Site 21 Site 22 TSI Total Phosphorus TSI Chlorophyll-a TSI Secchi TSI Mean 5 5 Trophic State: Mildly Eutrophic Mildly Eutrophic Numbers represent the mean TSI for each parameter. The results from these three measurements cover different units and ranges and thus cannot be directly compared to each other or averaged. In order to standardize these three measurements to make them directly comparable, we convert them to a trophic state index (TSI). The mean TSI indicates that Boy Lake is mildly eutrophic (Figure 9). There is good Boy Lake agreement between the TSI for phosphorus, chlorophyll a and transparency, indicating that these variables are strongly related. In addition, all sites are relatively similar in TSI. Boy Lake is toward the end of the Boy River Watershed, which could contribute to its higher phosphorus concentrations. Hypereutrophic Eutrophic Mesotrophic Oligotrophic Eutrophic lakes (TSI 5-7) are characterized by "green" water toward the end of the summer. Boy Lake chlorophyll a results indicated that there were algae blooms in August. "Eu" means true and the root "trophy" means nutrients therefore, eutrophic literally means true nutrients or truly nutrient rich. Figure 9. Trophic state index chart with corresponding trophic status. TSI Attributes Fisheries & Recreation <3 Oligotrophy: Clear water, oxygen throughout Trout fisheries dominate. the year at the bottom of the lake, very deep cold water. 3-4 Bottom of shallower lakes may become anoxic (no oxygen). Trout fisheries in deep lakes only. Walleye, Tullibee present. 4-5 Mesotrophy: Water moderately clear most of the summer. May be "greener" in late summer. No oxygen at the bottom of the lake results in loss of trout. Walleye may predominate. 5-6 Eutrophy: Algae and aquatic plant problems possible. "Green" water most of the year. Warm-water fisheries only. Bass may dominate. 6-7 Blue-green algae dominate, algal scums and aquatic plant problems. Dense algae and aquatic plants. Low water clarity may discourage swimming and boating. 7-8 Hypereutrophy: Dense algae and aquatic Water is not suitable for recreation. plants. >8 Algal scums, few aquatic plants. Rough fish (carp) dominate; summer fish kills possible. Source: Carlson, R.E A trophic state index for lakes. Limnology and Oceanography. 22: Boy Lake Report generation: RMB Environmental Laboratories 9 of 11

10 Ecoregion Comparisons Minnesota is divided into seven 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. From , the MPCA evaluated the lake water quality for reference lakes. These reference lakes are not considered pristine, but are considered to have little human impact and therefore are representative of the typical lakes within the ecoregion. The "average range" refers to the 25 th - 75 th percentile range for data within each ecoregion. For the purpose of this graphical representation, the means of the reference lake data sets were used. Boy Lake is in the Northern Lakes and Forests Ecoregion. The mean total phosphorus, chlorophyll a and transparency (secchi depth) for Boy are all slightly higher than the expected ecoregion ranges (Figures 1a-c). Boy Lake is toward the end of the Boy River Watershed, which could contribute to why it is higher than the ecoregion average range. 6 3 Total Phosphorus (ug/l, ppb) Chlorophyll-a (ug/l, ppb) Secchi depth (ft) increased algae NLF Ecoregion Boy NLF Ecoregion a b c Boy 25 NLF Ecoregion Boy crystal clear Figures 1a-c. Boy Lake ranges compared to Northern Lakes and Forest Ecoregion ranges. The Boy Lake total phosphorus and chlorophyll a ranges are from 16 data points collected in May-September of 1989, 24, and 28. The Boy Lake secchi depth range is from 118 data points collected in May-September from Boy Lake Report generation: RMB Environmental Laboratories 1 of 11

11 Inlet/Outlet Data Assessment Site 25 is located where the Boy River Inlet drains into Boy Lake. Phosphorus concentrations at that location were similar to in-lake concentrations. A biological assessment was conducted on July 19, 2 by the MPCA in the Boy River at the site in Figure 11. Water quality was found to be very good (Table 1). Figure 11. Boy River sample site, marked with a red dot. Assessment/Findings Recommendations Table 1. Results from July 19, 2 MPCA Biological Assessment on the Boy River. Category Result Fish IBI 79 Fish Rating Good Invertebrate IBI 79 Invertebrate Rating Good Water Temperature C 21.1 Conductivity (mhos/cm) 264. Field Turbidity (NTU) 2. Dissolved Oxygen (mg/l) 8.6 ph 8.5 Flow (m 3 /sec) 1.9 Nitrogen (mg/l) <.5 Total Phosphorus (ug/l) 31 Total Suspended Solids (mg/l) 14 Transparency Transparency monitoring at sites 21 and 22 should be continued annually. It is important to continue transparency monitoring weekly or at least bimonthly every year to enable year-to-year comparisons and trend analyses. Impaired Waters Assessment 33(d) List There are two main types of Impaired Waters Assessment for lakes: eutrophication (excess phosphorus) for aquatic recreation and mercury in fish tissue for aquatic consumption. Boy Lake was listed as impaired for mercury in fish tissue in the 28 Impaired Waters List. It is not part of the statewide mercury TMDL. Boy Lake is not considered impaired for eutrophication. Aquatic Recreational Use Assessment 35(b) In the 28 MPCA Aquatic Use Assessment (35(b)) Boy Lake was classified as having insufficient data. The data collected in 28 will count toward the next assessment, and Boy Lake will be assessed then. Inlet/Outlet Assessment Because of the limited inlet/outlet data, a mass balance project should be considered. This study answers questions about nutrient loading into the lake and nutrient budget within the lake. Organizational contacts and reference sites Boy Lake Association Cass County Environmental Services Department DNR Fisheries Office Regional Minnesota Pollution Control Agency Office Regional Board of Soil and Water Resources Office 33 Minnesota Avenue W, P.O. Box 3, Walker, MN (218) State Hwy 371 NW, Walker, MN (218) College Road, Suite 15, Baxter, MN (218) Minnesota Drive, Brainerd, MN 5641 (218) Boy Lake Report generation: RMB Environmental Laboratories 11 of 11