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Potter Lake University of Kansas
Lawrence Campus, Douglas County, Kansas
Water Quality Evaluation 2011 - 2013
January 13, 2014
Prepared by the Department of Environment, Health & Safety
University of Kansas, Lawrence Campus
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Table of Contents
1. Introduction ............................................................................................................... 1
2. Description of the Potter Lake and its Watershed .................................................... 1
3. Remediation Measures Taken to Improve the Water Quality of Potter Lake .......... 6
4. Water Quality Monitoring Program for Potter Lake ................................................. 7
5. Results of the Monitoring Program ........................................................................... 7
A. Water Temperature .................................................................................................. 7
B. Secchi Disc Transparency ........................................................................................ 11
C. Turbidity .................................................................................................................. 11
D. Chlorophyll a ........................................................................................................... 16
E. Phosphorus .............................................................................................................. 16
F. Nitrogen ................................................................................................................... 19
G. pH ............................................................................................................................ 19
H. Alkalinity .................................................................................................................. 24
I. Dissolved Oxygen .................................................................................................... 24
6. Trophic State Index ................................................................................................. 27
7. Conclusions .............................................................................................................. 30
8. References ............................................................................................................... 31
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List of Tables
Table 1. Possible outcome predicted based on R. E. Carlson’s Trophic State Indices (from Carlson and Simpson, 1996). .......................................................................................... 28
Table 2. Potter Lake Trophic State Indices based on samples collected at 0.5 m between March and October, 2011 – 2013. ............................................................................................. 30
List of Figures
Figure 1. Bathymetric map of Potter Lake, Douglas County, Lawrence, Kansas (38.96° N Latitude; 95.24° W Longitude). This map was created by the Kansas Biological Survey (2010). ............................................................................................................................ 2
Figure 2. Watershed area (ca. 7.5 hectares or 19 acres) for Potter Lake (black-outlined area). . 3
Figure 3. Aerial view of the Potter Lake watershed. The black line delineates the approximate boundary of the watershed. .......................................................................................... 4
Figure 4. Stormwater conveyance system that provides the majority of the water input to Potter Lake. Secondary outflow is the dashed blue line. .............................................. 5
Figure 5. Water temperature profiles from Potter Lake between March, 2011, and December, 2013. ............................................................................................................................... 8
Figure 6. Water density profiles for Potter Lake between March, 2011 and December, 2013. .. 9
Figure 7. Linear regression analyses between the mean air temperature between sampling dates and the surface water (0.0, 0.5, and 1.0 meter depths) temperatures of Potter Lake between March, 2011, and December, 2013. ..................................................... 10
Figure 8. Secchi disc depth (centimeters) from Potter Lake between March, 2011 and December, 2013. .......................................................................................................... 12
Figure 9. Seasonal variations in the Secchi disc depth in Potter Lake between 2011 and 2013.13
Figure 10. Seasonal water column turbidity in Potter Lake between March, 2011, and December, 2013. ........................................................................................................ 14
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Figure 11. Linear regression relationship between the chlorophyll a concentration (0.5 m) and the Secchi disc depth (i.e., transparency)................................................................... 15
Figure 12. Non-algal turbidity (NAT) values for Potter Lake collected between March and October, 2011 through 2013. ..................................................................................... 17
Figure 13. Concentrations of chlorophyll a in Potter Lake at 0.5 m between March and October, 2011 through 2013. .................................................................................................... 18
Figure 14. Concentrations of (Nitrate-N + Nitrite-N), Kjeldahl Nitrogen, Total Nitrogen, and Total Phosphorus in Potter Lake at 0.5 meter between March and October, 2011 through 2013. ............................................................................................................. 20
Figure 15. Concentrations of (Nitrate-N + Nitrite-N), Kjeldahl Nitrogen, Total Nitrogen, and Total Phosphorus in Potter Lake at 3.0 meter between March and October, 2011 through 2013. ............................................................................................................. 21
Figure 16. Potter Lake pH values from March, 2011 through December, 2013. ........................ 22
Figure 17. Summertime mean pH (x ̄± 1 Standard Deviation) for Potter Lake (June through September, 2001 - 2013). ........................................................................................... 23
Figure 18. Dissolved oxygen concentrations (0.0 m – 3.0 m) for Potter Lake from March, 2011 through December, 2013............................................................................................ 25
Figure 19. Oxygen saturation (%) for Potter Lake (0.0 m – 3.0 m) from March, 2011 through December, 2013. ........................................................................................................ 26
Figure 20. Seasonal variations in the Trophic State Indices for Potter Lake between March and October, 2011 – 2013 (TSISD is graphed for the entire period March, 2011 to December, 2013). ....................................................................................................... 29
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1. Introduction Potter Lake was constructed in 1911 on the campus of the University of Kansas (KU), Lawrence Campus, in Douglas County, to provide fire protection for the campus. Kansas Department of Health and Environment (KDHE) added the lake to the state’s Clean Water Act, Section 303(d) list in 1996. In 2000, KDHE developed, and EPA approved, a Total Maximum Daily Load (TMDL) to address the primary impairment of the lake impacting its two designated uses, secondary contact recreation and aquatic life: eutrophication.
The eutrophication of Potter Lake was believed to be associated with fertilizer usage within the watershed (KDHE TMDL, 2000). Background inputs of phosphorus were suspected to come from nutrient recycling from the sediments, geological sources and wildlife waste. There were no identified point sources of phosphorus input within the watershed. The lake has a very small watershed [approximately (ca.) 0.03 square miles] and the lake has no natural stream inflow; rather, most of the inflow waters are derived from the stormwater conveyance system in the areas to the south and west of the lake.
Ongoing efforts by the University of Kansas have been undertaken to improve the water quality of Potter Lake. These efforts have significantly improved the lake’s water quality and the results of a three-year (2011 – 2013) monitoring program are presented in this report. Based on the findings of the water quality monitoring program, it is believed that Potter Lake can now be recommended for delisting from the Kansas list of impaired waters 303(d) list.
2. Description of the Potter Lake and its Watershed Potter Lake [referred to as “Potter’s Lake” by the KDHE and the U.S. Environmental Protection Agency (EPA); U.S. Geological Survey Hydrologic Unit Code (HUC) 8: 10270104, HUC 11: 020; Station: LM073401] is located in northwestern Kansas in Lawrence, Kansas, and lies within the Lower Kansas River Basin. Situated on the campus of the University of Kansas, Lawrence Campus, the lake has a surface area of ca. 0.4 hectare (1.0 acre), a maximum depth of ca. 3.6 meters (11.8 feet) and a mean depth of ca. 1.7 meters (5.4 feet) (Figure 1). The lake holds ca. 6,752 m3 of water (1,783,690 gallons or 5.4 acre feet). The land use within the 7.7 hectare watershed has been described as “100% urban (campus)” (KDHE TMDL, 2000); however, over 50% of the drainage area directly surrounding the lake is covered with grass and trees (Figure 2, 3). There are no natural stream inflows to the lake. The primary source of water to Potter Lake is from stormwater runoff via its conveyance system in the area (Figure 4).
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Figure 1. Bathymetric map of Potter Lake, Douglas County, Lawrence, Kansas (38.96° N Latitude; 95.24° W Longitude). This map was created by the Kansas Biological Survey (2010).
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Figure 2. Watershed area (ca. 7.5 hectares or 19 acres) for Potter Lake (black-outlined area).
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Figure 3. Aerial view of the Potter Lake watershed. The black line delineates the approximate boundary of the watershed.
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Figure 4. Stormwater conveyance system that provides the majority of the water input to Potter Lake. The secondary outflow is indicated by the dashed blue line.
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There are small amounts of water input to Potter Lake from direct precipitation onto the surface of the lake, sheet runoff, and possibly of subsurface groundwater inflow. Surface water outflow from the lake is not continuous. Two water outflow pathways are present, both of which flow into the regional stormwater drainage system which eventually flows into the Kansas River. The primary outflow pathway is via an overflow standpipe within the lake. The secondary outflow is an overflow spillway located on the northwestern end of the lake.
The soils of the Potter Lake watershed have been mapped as Martin-Sogn-Vinland association, although the Vinland-Martin complex makes up the major part of the drainage area (7 – 15% slopes) (U.S. Department of Agriculture, Soil Conservation Service, 1977). This complex is on the slide slopes below limestone and sandstone formations. The presence of limestone within the watershed has an influence on the chemical characteristics (e.g., pH and alkalinity) of Potter Lake.
3. Remediation Measures Taken to Improve the Water Quality of Potter Lake
The watershed area directly surrounding Potter Lake is totally covered with sod grass and trees (see Figure 3). Since at least 2000, that area has received limited fertilizer application, an activity which was identified in the TMDL as a potential source of phosphorus to the lake. In the spring of 2008, the Potter Lake Project, a student-led effort, was established to coordinate with efforts being made by the KU Departments of Design and Construction Management and Facilities Services, Landscaping Division, to improve and enhance the lake’s water quality. In March 2009, Asian Grass Carp were added to the lake to help control the growth of aquatic weeds. Also in 2009, 2011, and 2012, student/staff volunteers were organized by the Potter Lake Project to manually remove some of the aquatic vegetation (coon’s tail, Ceratophyllum demersum, and water lilies, Nymphaea sp.) from the lake. In September, 2010, a $125,000 dredging project was completed to remove approximately 5,000 cubic yards (3,823 cubic meters) of sediments containing decaying vegetation from the lake bottom. Nutrients from these sediments and decaying vegetation were being recycled back into the lake water feeding the growth of a green surface “scum.” That green material, which covered the lake surface in 2010, was actually made up of a very small (ca. 1 millimeter across), rootless, seed-bearing flowing plant called watermeal (Wolffia sp.). The growth of this plant is usually indicative of an abundant availability of nutrients (e.g., phosphorus and nitrogen) in the lake water. Also in the fall of 2010, a $200,000 stormwater runoff project was undertaken to reduce the amount of runoff from Jayhawk Boulevard.
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4. Water Quality Monitoring Program for Potter Lake The water quality monitoring program was begun in March, 2011, and has continued through December, 2013. The monitoring program involved monthly sampling of physical and chemical characteristics of the water quality. Sampling time was midday between 11:00 and 13:00 hours. Sampling was conducted over the deepest part of the lake (Latitude: 38.96 N; Longitude: 95.25 W). The following parameters were measured: Secchi disc transparency and depth profiles, collected every 0.5 meter (1.64 feet) from the surface to 3.0 meters (9.84 feet), of temperature, pH, turbidity, and dissolved oxygen concentration using an Horiba U-10 Water Quality Meter. Between March and October each year, water chemical analyses were conducted in surface waters at 0.5 meter and near-bottom waters at 3.0 meters for concentrations of total phosphorus, total nitrogen, Kjeldahl nitrogen, nitrate, nitrite (conducted by Pace Analytical Services, Inc., Lenexa, Kansas), and chlorophyll a (conducted by the Kansas Department of Health and Environment, Bureau of Environmental Field Services, Topeka, Kansas). On two occasions (June, 2011, and May, 2013), water samples were analyzed for alkalinity (as CaC03 mg/liter; by Pace Analytical Services, Inc.). These water samples were kept cold and in the dark and delivered to the analytical laboratories within 24 hours of collection.
5. Results of the Monitoring Program
A. Water Temperature The depth and seasonal changes in water temperature observed in the water column of Potter Lake were similar to other aquatic water bodies in the Midwest. Given the lake’s latitude (38.96°N) and elevation (289 meters or 949 feet), the lake lies at the boundary between a dimictic and a warm monomictic thermal lake type (Hutchinson and LÖffler 1956). The lake does not always freeze over during the winter and when it does, the period of ice cover is not throughout the winter. Therefore, Potter Lake behaves like a warm monomictic lake with a fall, winter, spring holomictic period and a summer thermal stratification period. Potter Lake usually begins to thermally stratify between March and May each year (Figure 5) when the warm, less dense waters begin to form at the surface (0 to 1 meter) (Figure 6). The lake remains thermally stratified usually until between August and October each year when it becomes isothermic throughout the water column. The ambient air temperature is directly related to the surface water temperatures as is demonstrated by the strong positive dependent relationship between the two variables (0.0 meter r2 = 0.9147; 0.5 meter r2 = 0.9080; 1.0 meter r2 = 0.9113) (weather station KLWC data from Lawrence Municipal Airport; Weather Underground web page) (Figure 7).
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Figure 5. Water temperature profiles from Potter Lake between March, 2011, and December, 2013.
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Figure 6. Water density profiles for Potter Lake between March, 2011 and December, 2013.
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y = 0.8523x + 5.1575R² = 0.9147
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y = 0.8095x + 4.7295R² = 0.9113
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Figure 7. Linear regression analyses between the mean air temperature between sampling dates and the surface water (0.0, 0.5, and 1.0 meter depths) temperatures of Potter Lake between March, 2011, and December, 2013.
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B. Secchi Disc Transparency The Secchi disc depth is a measure of the transparency of the lake water. In Potter Lake, the Secchi depth ranged from 75 - 327 cm between March, 2011, and December, 2013 (Figure 8). The mean (x)̄ Secchi disc depth for this study period was 196±76 cm (median = 195 cm). During the period from June – September each of the three years, the range was 114 – 195 cm (x ̄ summer ± 1 Std. Dev. = 156 ±48 cm; median = 153 cm). These Secchi disc means (full study period and the summer months) would place the lake in the lower boundary of a eutrophic lake according to Carlson’s Trophic State Indices (Carlson and Simpson 1996; see Section 6 of this report further discussion). Seasonally, the Secchi disc depth was greatest (i.e., most transparent) in the winter and was the smallest (i.e., least transparent) in the spring or summer of each year (Figure 9).
C. Turbidity The waters of Potter Lake were fairly clear throughout the sampling period, especially from the surface to 2.0 m (Figure 10). The overall average turbidity for the 0.0 – 2.0 m portion of the lake’s water column was 1.8±2.7 NTU. It should be noted that ca. 10% of the turbidity reading were removed due to equipment error. The Horiba U-10 Water Quality Meter turbidity sensor malfunctioned on these occasions giving anomalous high reading. Attempts to clean/clear the sensor with clean lake water or deionized water were unsuccessful. Also, some of the turbidity readings from water deeper than 2.0 m were very high due to disturbance of epiphytic periphyton (attached algae) growing on benthic macrophytes (e.g., Ceratophyllum demersum) and contaminating the water sample at those depths.
The turbidity observed in Potter Lake appeared to be due to algal material based on an evaluation of the Non-Algal Turbidity (NAT) (Walker 1987). That analysis estimates whether the turbidity, as it affects the water clarity (i.e., Secchi disc depth), is due to algal material (i.e., chlorophyll a) or is due to non-algal material (e.g., suspended clay or other inorganic material). The relationship is as follows:
NAT = 1/Secchi (m) – 0.025*Chl-a (ug/L) (resulting units of m-1).
The -0.025 term associated with the chlorophyll concentration is a default value of the slope of the chlorophyll versus Secchi linear relationship (units of m2/mg) calibrated from 65 Corps of Engineer impoundments data set (Walker 1984). The data for Potter Lake also follows a similar linear relationship (Figure 11). If the NAT value is less than 0.4, the turbidity is all due to algal cells. NAT values above 1.0 indicate increasing importance of clay or other inorganic (i.e., non-algal) material. Values that calculate to negative numbers should be given zero values as there is little guidance on that result.
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Figure 8. Secchi disc depth (centimeters) from Potter Lake between March, 2011 and December, 2013.
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Figure 9. Seasonal variations in the Secchi disc depth in Potter Lake between 2011 and 2013.
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Figure 10. Seasonal water column (0.0 – 2.0 meters) turbidity in Potter Lake between March, 2011, and December, 2013.
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y = -6.2689x + 226.35R² = 0.2802
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Figure 11. Linear regression relationship between the chlorophyll a concentration (0.5 m) and the Secchi disc depth (i.e., transparency).
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In Potter Lake, the mean NAT value for 0.5 m samples was 0.43 ± 0.26 (range: 0 – 1.20; median: 0.43) (Figure 12). These NAT values indicate that the turbidity of Potter Lake is primarily due to algal material. On the three occasions when the NAT values were highest, there had been a significant rain event just prior to that sampling date (March, 2012: 0.59” of rain two day before sampling; March, 2013: 0.97” 5 days prior; April, 2013: 0.95” one day prior). Because the primary water input source to the lake is from stormwater runoff (See Figure 4), it appears likely an increased amount of sediment material (i.e., non-algal inorganic material) would have entered the lake just prior to sampling on these dates resulting in these high NAT values.
D. Chlorophyll a The distribution of chlorophyll a in lake water is an indicator of the distribution of phytoplankton (i.e., algal) biomass in aquatic systems. On four occasions between March and October, 2011 through 2013, the samples collected at 0.5 meter were contaminated with attached algal material, epiphytic periphyton, which had broken off the submerged macrophytes. Eight of the 3.0 meter samples were contaminated with epiphytic periphyton. Chlorophyll a concentrations in the 0.5 m samples ranged from 1.99 – 18.92 µg/liter with a mean of 7.69 ± 5.40 µg/liter (median = 5.98 µg/liter) (analytical method: AWWA, APHA, WEF 10200-H) (Figure 13). KDHE has set a 12 µg/liter target as the limit for primary contact recreation and a 20 µg/liter target for secondary contact. Potter Lake only exceeded the primary target limit four times over the three-year monitoring program and never exceeded the secondary contact target limit. For the summer months (June through September), the mean chlorophyll concentration in 0.5 m samples was 10.16 ± 5.50 µg/liter (range: 3.20 – 18.92 µg/liter; median = 8.73 µg/liter). Therefore, Potter Lake now meets the endpoint set in the TMDL of the summer chlorophyll a concentration being at or below 20 µg/liter.
E. Phosphorus Phosphorus concentrations in lake water were measured as Total Phosphorus, that is, the combination of both organic and inorganic forms of phosphorus. The analytical method used to analyze for total phosphorus (EPA 365.4: Total Phosphorous, Colorimetric, Automated, Block Digester AA II) has a detection limit of 5 µg/liter. The total phosphorus concentrations in water samples collected at 0.5 m in Potter Lake were all below the level of detection throughout the three-year monitoring program except for one date (April, 2012) when 13 µg/liter was detected (Figure 14). The total phosphorus concentrations in water samples collected at 3.0 m were also all below the level of detection except on five dates (April, May, June 2012 and July, September 2013), when total phosphorus concentrations ranged from 15 to 62 µg/liter. During almost all of these months, the 3.0 meter waters were anoxic; therefore,
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Figure 12. Non-algal turbidity (NAT) values for Potter Lake collected between March and October, 2011 through 2013.
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Figure 13. Concentrations of chlorophyll a in Potter Lake at 0.5 m between March and October, 2011 through 2013.
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inorganic phosphorus would be soluble and it would likely be released from the sediments into the bottom (hypolimnetic) lake waters.
F. Nitrogen The amount of nitrogen was measured as dissolved inorganic nitrogen (Nitrate-N + Nitrite-N), Kjeldahl Nitrogen and Total Nitrogen. Forms of dissolved inorganic nitrogen are related to nutrient availability for algal growth. Kjeldahl nitrogen is a measure of the organic nitrogen plus inorganic ammonium-N present in the water (it does not measure the amount of nitrate or nitrite present). Total nitrogen is a measure of all forms of nitrogen present in the water column, both inorganic and organic. Because both Kjeldahl and Total Nitrogen are reflective of the amount of biomass in the water sample, those analyses for samples that were contaminated with plant material were eliminated (i.e., not reported).
For samples collected from 0.5 m in Potter Lake, concentration of nitrite was never above the level of detection (0.10 mg/liter) throughout the study period (Figure 14). Nitrate at 0.5 m was above the level of detection (0.015 mg/liter) six occasions (March, April 2011, May, June 2012, March, June 2013) with concentrations ranging from 0.024 to 0.330 mg/liter. Samples from 3.0 m followed a similar pattern as 0.5 m with nitrite below the level of detection throughout the study period and nitrate only detectable once (March, 2012: 0.450 mg/liter) (Figure 15).
The trend in Kjeldahl nitrogen and total nitrogen were nearly identical at both depths throughout the study period. The amount of total nitrogen was almost always equal to or slightly greater than the amount of Kjeldahl nitrogen. For samples collected from 0.5 m, the total nitrogen concentration ranged from 0.00 – 1.30 mg/liter (x ̄= 0.46 ± 0.35 mg/liter; median = 0.40) (Figure 14). For those samples collected from 3.0 m, the total nitrogen concentration was slightly higher than at 0.5 m ranging from 0.30 – 1.90 mg/liter (x ̄ = 0.88 ± 0.51 mg/liter) (Figure 15).
G. pH
The relative acidity (low pH) or basicity (high pH) of lake waters is a function of the buffering capacity or acid-neutralizing capacity (alkalinity) of the water, the rate of photosynthesis by algae and macrophytes, and the rate of respiration by plants and bacteria. The overall trend of the pH was decreasing pH with increasing depth. The range in pH in Potter Lake (0.0 – 3.0 m) during summer (June through September, 2011 – 2013) was 5.18 (at 3.0 meters) – 8.56 (at 0.0 meter) (Figure 16). For the surface waters (0.0 – 1.5 meters) pH for the
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Figure 14. Concentrations of (Nitrate-N + Nitrite-N), Kjeldahl Nitrogen, Total Nitrogen, and Total Phosphorus in Potter Lake at 0.5 meter between March and October, 2011 through 2013.
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Figure 15. Concentrations of (Nitrate-N + Nitrite-N), Kjeldahl Nitrogen, Total Nitrogen, and Total Phosphorus in Potter Lake at 3.0 meter between March and October, 2011 through 2013.
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Figure 16. Potter Lake pH values from March, 2011 through December, 2013.
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Figure 17. Summertime mean pH (x̄ ± 1 Standard Deviation) for Potter Lake (June through September, 2001 - 2013).
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entire monitoring period ranged from 6.19 – 8.56 with the mean a mean (±1 std. dev.) of 6.91 ± 0.35. It should be noted that all calculation of the mean values and standard deviation for pH were derived from the hydrogen ion concentration values then converted back to a pH value because the pH units are logarithmic numbers (pH = -log[H+] or, X = -(10pH) where X = the hydrogen ion concentration; pH ranges from 0 - 14). The lower pH readings in the deeper (hypolimnetic) waters are due to the increased amount of organic material decomposition during the thermally stratified summer period (Figure 17). Furthermore, the waters below 1.5 meters during the summer are anoxic (Figure 18). The mean summertime pH (±1 std. dev.) at 0.5 m during the three-year monitoring program was 7.2 ± 0.28. In the TMDL, KDHE has set an optimal pH range of 6.5 – 8.5 for the surface waters of aquatic water bodies. Therefore, the surface waters of Potter Lake are well within that range.
H. Alkalinity The buffering capacity or acid neutralizing capacity of lake water is referred to as alkalinity. This parameter is a measure of the inorganic carbon equilibrium within a system imparted by the presence and concentrations of carbon dioxide, bicarbonate and carbonate in the water. In Kansas and within the watershed of Potter Lake, there is a significant amount of limestone (i.e., carbonate) present that directly influences the alkalinity of Potter Lake.
Water samples from 0.5 and 3.0 m were analyzed for alkalinity in June, 2011, and in May, 2013. The alkalinity at both depths in 2011 was the same, 110 mg/liter as calcium carbonate (2,198 µeq/liter as calcium carbonate) while the alkalinity in 2013 was 101 (2,018 µeq/liter) and 108 (2,158 µeq/liter) at 0.5 m and 3.0 m, respectively. At these values, Potter Lake is very well buffered and a non-sensitive aquatic system. EPA has set a value of <25 mg/liter (500 µeq/liter) as calcium carbonate as the level below which a lake would be sensitive to acidic inputs.
I. Dissolved Oxygen The amount of dissolved oxygen in lakes reflects the balance between the rates of supply of oxygen from the atmosphere and photosynthesis versus the consumption of oxygen by organism (e.g., respiration) and nonbiotic chemical reactions. During the spring months in Potter Lake, the supply of oxygen to the surface waters was very high in contrast to consumptive uses of oxygen (Figure 18). The percent oxygen saturation in the surface waters during the spring was over 100% in the surface waters (Figure 19). Usually by May or June each year, the bottom waters become anoxic following thermal stratification. Depending on the weather conditions, especially the occurrence of strong northerly winds, Potter Lake would
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Figure 18. Dissolved oxygen concentrations (0.0 m – 3.0 m) for Potter Lake from March, 2011 through December, 2013.
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0
50
100
150
200
250
J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D
Oxy
gen
Satu
ratio
n (%
)
2011 2012 2013
Potter Lake - Oxygen Saturation (%)March, 2011 - December, 2013
0.0 meter
0.5 meter
1.0 meter
1.5 meters
2.0 meters
2.5 meters
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Figure 19. Oxygen saturation (%) for Potter Lake (0.0 m – 3.0 m) from March, 2011 through December, 2013.
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experience a breakdown in the thermal stratification in the late summer (August, 2011) or fall (October or November, 2012 and 2013) (Figure 6) after which the lake waters mix from top to bottom and oxygen becomes more uniformly distributed throughout the water column.
6. Trophic State Index The natural aging process of lakes is called eutrophication. Generally, as inputs of nutrients (e.g., nitrogen and phosphorus) increase in lake water, the more plants and animals can be sustained by the lake. Using the Secchi disc transparency, chlorophyll a, total phosphorus and total nitrogen, the degree of eutrophication, or trophic status, of a lake can be defined. Based on three of these parameters, R. E. Carlson (Carlson1977, 1981, 1983; Carlson and Simpson 1996) developed the following Trophic Status Indices (TSI) and C. R. Kratzer and P. L. Brezonik (1981) developed a TSI based on the total nitrogen data from lake water.
TSISD = 60 - 14.41 [ln Secchi Disk (meters)]
TSITP = 14.42 [ln Total Phosphorus (µg/L)] + 4.15
TSICHL = 9.81 [ln Chlorophyll a (µg/L)] + 30.6
TSITN = 14.43 [ln Total Nitrogen (mg/L)] + 54.45
If the TSI value is above 70 the water body is considered Hypereutrophic; between 50 and 70, Eutrophic; between 40 and 50, Mesotrophic; less than 40, Oligotrophic (Table 1).
Using the data collected from 0.5 m in Potter Lake between March through October, 2011 to 2013, the Trophic State Indices for Secchi disc depth, chlorophyll and total nitrogen ranged from Oligotrophic to Eutrophic (26 – 64) although the mean TSI was 48 ± 8 and the median TSI was 48, both in the upper Mesotrophic range for TSI (Figure 20; Table 2).
The TSITP was always in the Oligotrophic range with a mean of 37 ± 1 (median = 37). It should be noted that a value of total phosphorus = 10 µg/liter was used to calculate the TSITP because the 0.5 m concentrations of total phosphorus were all below the level of detection (5 µg/liter) throughout the sampling period except for one sampling date (April, 2012 TP = 13 µg/liter; TSITP = 41) . A value of 10 µg/liter for total phosphorus, two times the detection limit, was deemed the best estimate to use to calculate the TSITP.
The TSICHL data are believed to best reflect the trophic status of Potter Lake. The mean TSICHL was 47 ± 7 (median = 47), also well within the Mesotrophic range. At no time during the three-year monitoring did the TSICHL ever exceed a value of 60.
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Table 1. Possible outcome predicted based on R. E. Carlson’s Trophic State Indices (from Carlson and Simpson, 1996).
Chl SD TP(µ/L) (m) (µg/L)
<30 <0.95 >8 <6Oligotrophy: Clear water, oxygen throughout the year in the hypolimnion
Water may be suitable for an unfiltered water supply.
Salmonid fisheries dominate
30-40 0.95-2.6 8 - 4 6 - 12 Hypolimnia of shallower lakes may become anoxic
Salmonid fisheries in deep lakes only
40-50 2.6-7.3 4 - 2 12 - 24
Mesotrophy: Water moderately clear; increasing probability of hypolimnetic anoxia during summer
Iron, manganese, taste, and odor problems worsen. Raw water turbidity requires filtration.
Hypolimnetic anoxia results in loss of salmonids. Walleye may predominate
50-60 7.3-20 2 - 1 24-48Eutrophy: Anoxic hypolimnia, macrophyte problems possible
Warm-water fisheries only. Bass may dominate.
60-70 20-56 0.5-1 48-96Blue-green algae dominate, algal scums and macrophyte problems
Episodes of severe taste and odor possible.
Nuisance macrophytes, algal scums, and low transparency may discourage swimming and boating.
>80 >155 <0.25 192-384 Algal scums, few macrophytes Rough fish dominate; summer fish kills possible
TSI Attributes Water Supply Fisheries & Recreation
70-80 56-155 0.25- 0.5 96-192Hypereutrophy: (light limited productivity). Dense algae and macrophytes
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20
30
40
50
60
70
J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J
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2011 2012 2013
Potter Lake - Trophic Status Indices (TSI)2011 - 2013
Secchi Disc TSI
Chlorophyll TSI
Total Phosphorus TSI
Total Nitrogen TSI
Eutrophic
Mesotrophic
Oligotrophic
Figure 20. Seasonal variations in the Trophic State Indices for Potter Lake between March and October, 2011 – 2013 (TSISD is graphed for the entire period March, 2011 to December, 2013).
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Table 2. Potter Lake Trophic State Indices based on samples collected at 0.5 m between March and October, 2011 – 2013. Trophic status takes into account the range of the TSI values associated with the standard deviation of the mean.
TSI Parameter Mean TSI Std. Dev. Median Trophic StatusSD 54 ± 6 54 Eutrophic to Mesotrophic
Chl a 48 ± 6 47 Mesotrophic to EutrophicTP 38 ± 1 37 OligotrophicTN 42 ± 8 41 Mesotrophic to Oligotrophic
7. Conclusions The TMDL for Potter Lake approved by EPA in 2000 identified two parameters that were associated with the lake’s impairment: pH and eutrophication. The pH levels cited in the TMDL for samples collected during the summer of 1994 ranged from 8.82 – 9.01 (x ̄= 8.92). Also cited as an impairment in the TMDL was the TSICHL greater than 70 (chlorophyll a concentration ≥ 53 µg/L), a value that put the lake’s trophic status in the Hypereutrophic category.
During the summer months (June – September, 2011 - 2013), the highest pH measured was 8.56 in one surface water sample (0.0 m) collected in August, 2013. The overall three-year mean pH for all depths in Potter Lake was 7.20 ± 0.31 while the overall mean for all depths during the summer months was 6.90 ± 0.69. KDHE has set an optimal pH range of 6.5 – 8.5 for aquatic water bodies; therefore, Potter Lake is well within that range.
The TSICHL over the three-year monitoring program ranged from 37 – 59 (x ̄ = 48 ± 6). During the summer months, the TSICHL ranged from 42 – 59 (x ̄ = 52 ± 6). These data would characterize the lake as generally Mesotrophic to slightly Eutrophic. At no time was the TSICHL value near the Hypereutrophic category value of 70 referred to in the 2000 TMDL.
Those efforts that have been undertaken by the University of Kansas since 2000 appear to have remediated the water quality problems referred to in the 2000 TMDL. The University of Kansas will continue its ongoing efforts to maintain and improve Potter Lake to ensure the water quality of Potter Lake is maintained and enhanced. Therefore, these recent data collected during this three-year monitoring program (2011 - 2013) of Potter Lake indicate that the pollutants or conditions (pH and chlorophyll) listed in the TMDL are no longer a potential cause of water quality impairment. These findings would support the position that Potter Lake can now be recommended for delisting from the State of Kansas 303(d) list of impaired water bodies under the Clean Water Act.
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8. References
Carlson, R.E. 1977. A trophic state index for lakes. Limnology and Oceanography. 22:361-369.
Carlson, R.E. 1981. Using trophic state indices to examine the dynamics of eutrophication. p. 218-221. In: Proceedings of the International Symposium on Inland Waters and Lake Restoration. U.S. Environmental Protection Agency. EPA 440/5-81-010.
Carlson, R.E. 1983. Discussion on “Using differences among Carlson’s trophic state index values in regional water quality assessment”, by Richard A. Osgood. Water Resources Bulletin. 19:307-309.
Carlson, R.E. and J. Simpson. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp.
Hutchinson, G. E. and H. LÖffler. 1956. The thermal classification of lakes. Proc. Nat. Acad. Sci. USA 42:84-86.
Kratzer, C.R. and P.L. Brezonik. 1981. A Carlson‑type trophic state index for nitrogen in Florida lakes. Water Res. Bull. 17: 713-715.
U.S. Department of Agriculture, Soil Conservation Service, 1977. Soil survey of Douglas County, Kansas. 73pp.
Walker, W. W., Jr. 1984. Trophic Indices for Reservoirs. Lake and Reservoir Management, North American Lake Management Society. U.S. Environmental Protection Agency. EPA 440/5/84-001. pp. 435 – 440.
Walker, W. W., Jr., 1987. “Empirical methods for predicting eutrophication in impoundments; Report 4, Phase III: Applications Manual”. Vicksburg, MS: US Army Engineer Waterways Experiment Station. Technical Report E–81–9.
