City of Plymouth Water Quality Monitoring 2003

Transcription

1 City of Plymouth Water Quality Monitoring 2003 Prepared For: City of Plymouth Prepared By Brian Vlach John Barten James Johnson Dean Almquist

2 City of Plymouth Water Quality Monitoring 2003 Table of Contents INTRODUCTION 1-2 METHODS 2-13 Lake Monitoring 2-3 Watershed Monitoring 4-11 Parkers Lake 5 Medicine Lake 7 Elm Creek 10 Shingle Creek-Bass Lake 11 Watershed and Lake Response Modeling RESULTS AND DISCUSSION Parkers Lake 14 Medicine Lake 22 Elm Creek 31 Shingle Creek 33 APPENDIX A: Watershed Nutrient Loading Estimates APPENDIX B: Lake Water Quality Goals and Monitoring Data APPENDIX C: Watershed Water Quality Monitoring Data APPENDIX D: Average Daily Flow and Hydrograph Data APPENDIX E: References i

3 LIST OF FIGURES Figure 1: Parkers Lake bathymetric map. 2 Figure 2: Medicine Lake bathymetric map. 2 Figure 3: Parkers Lake sub-watershed monitoring sites. 5 Figure 4: Medicine Lake sub-watershed monitoring sites. 7 Figure 5: Elm Creek watershed monitoring sites. 10 Figure 6: Shingle Creek sub-watershed monitoring site. 11 Figure 7: Parkers Lake annual changes in total phosphorus and secchi depth. Values are an average from May through September. 14 Figure 8: Parkers Lake annual Trophic Status Index values. 15 Figure 9: Parkers Lake total phosphorus and soluble reactive phosphorus concentrations from the surface to the bottom. 16 Figure 10: Seasonal changes in total phosphorus, soluble reactive phosphorus, total nitrogen, chlorophyll-a, and secchi depth for Parkers Lake. 17 Figure 11: Parkers Lake annual watershed phosphorus loading. 19 Figure 12: South Parkers Lake sub-watershed drainage area. 20 Figure 13: Medicine Lake annual changes in total phosphorus, total nitrogen, chlorophyll-a, and secchi depth (m). Values are an average from May through September. 22 Figure 14: Medicine Lake annual Trophic Status Index. 23 Figure 15: The percent phosphorus loading for each sub-watershed in Figure 16: Medicine Lake curlyleaf pondweed survey in Figure 17: Seasonal changes in total phosphorus, soluble reactive phosphorus, total nitrogen, chlorophyll-a concentrations and secchi depth for Medicine Lake. 26 Figure 18: Medicine Lake total phosphorus and soluble reactive phosphorus concentrations from the surface to the bottom. 27 Figure 19: The predicted change in Medicine Lake phosphorus concentration relative to a percent reduction in watershed phosphorus loading in 2002 and ii

4 LIST OF TABLES Table 1: Table 2: Table 3: Table 4: Table 5: Table 6: Table 7: Table 8: Parkers Lake sub-watershed total phosphorus loading (pounds/year). 19 Parkers Lake sub-watershed total run-off volume (x10 6 M 3 ). 19 Medicine Lake total phosphorus loading and run-off volume estimated from annual sub-watershed monitoring data. 24 Medicine Lake total phosphorus loading for each sub-watershed using WiLMS modeling simulations. 24 The Medicine Lake changes in watershed and internal phosphorus loading using WiLMS modeling to simulate variations in annual precipitation conditions. 28 The predicted change in Medicine Lake phosphorus concentration relative to reducing the amount of watershed loading in 2002 and Elm Creek estimated total phosphorus loading from 2000 through 2003 using FLUX modeling. 31 Shingle Creek estimated total phosphorus loading from 2000 through 2003 using FLUX modeling. 33 iii

5 INTRODUCTION The City of Plymouth has identified several lakes, streams, and wetlands that are considered high priority relative to water quality issues. With increased urbanization, there is a concern that further degradation will impact these high priority resources. A Water Resources Management Plan has been developed to address issues concerning the quantity and quality of water within the City of Plymouth (Short Elliot Hendrickson 2000). There have also been several sub-committees established to determine management objectives, and prioritize management strategies to improve the water quality for these resources. Monitoring efforts were necessary to identify degraded areas that would require the implementation of best management strategies. In addition, the monitoring data would also determine whether the implementation of best management strategies has improved the water quality of the resource. To successfully determine the potential factors that contribute to poor water quality, monitoring efforts need to occur from a watershed and in-lake perspective. A cooperative effort between the City of Plymouth and the Three Rivers Park District was developed to monitor the in-lake water quality of two high priority lakes, and determine the nutrient loading from their respective watersheds. In addition, we monitored the nutrient loading from a separate sub-watershed that ultimately provides drainage to a smaller lake; and monitored the nutrient loading from a creek that flows through a portion of the City of Plymouth that is to be considered for future development. Parkers and Medicine Lake are considered high priority lakes within the City of Plymouth. The respective watershed land use for each lake is a combination of industrial and residential; and the shoreline of each lake primarily consists of parkland and private residences. Consequently, the lakes have been impacted by considerable urban development. In addition, these lakes receive substantial water related recreational use, which further contributes to the increasing concerns about the in-lake water quality of these two lakes. We monitored the in-lake water quality for Medicine Lake from 2000 through 2003; and monitored the in-lake water quality for Parkers Lake from 2001 through We also monitored the watershed nutrient loading to determine potential sources of watershed degradation to these lakes. To determine the watershed nutrient loading for Medicine Lake, the number of sub-watersheds monitored was increased from two sites in 2001 to eight sites in There were two sub-watersheds monitored from 2000 through 2002 to determine watershed nutrient loading for Parkers Lake; and there were three sub-watersheds monitored for Parkers Lake in There are other resources within the City of Plymouth that are considered high priority and required monitoring efforts. The upper Shingle Creek sub-watershed is the largest sub-watershed that provides drainage to Bass Lake. The Bass Lake watershed provides a potential source of nutrient loading that may cause in-lake conditions that inhibits recreational use. In addition, the relatively poor lake water quality can further degrade the water quality downstream. Bass Lake has also been impacted by considerable urban development. Consequently, we monitored the nutrient loading for the upper Shingle Creek sub-watershed flowing into Bass Lake from 2000 through

6 A portion of the City of Plymouth that has not been significantly impacted by urbanization is Elm Creek. The Elm Creek watershed drains the undeveloped northwestern corner of Plymouth. The existing area is comprised of low-density residential and agricultural land use with wetlands/flood plain covering a major portion of the area. These upland areas adjacent to wetlands are considered for future development. Therefore, it is anticipated that this area will also be impacted by urbanization, and subsequently effecting the water quality within the watershed. To monitor the potential changes in drainage, we monitored the water quality from two sites located on Elm Creek from 2000 through We determined the nutrient loading coming from the City of Medina prior to entering the City of Plymouth; and determined the amount of nutrient loading leaving the City of Plymouth prior to entering Maple Grove. Lake Monitoring METHODS Three Rivers Park District sampled the water quality for two lakes (Medicine and Parkers Lake) in the City of Plymouth to determine in-lake trophic status. We sampled Parkers Lake from 2001 through 2003 (Figure 1); and Medicine Lake was sampled from 2000 through 2003 (Figure 2). Figure 1: Parkers Lake bathymetric map. bathymetric map. Figure 2: Medicine Lake 2

7 The lakes were sampled bi-weekly from the spring (prior to lake-stratification) through the fall (after fall lake turn-over) to determine seasonal changes in water quality. Samples were collected at the deepest location within each lake. At each sampling location, a water quality profile was measured at each meter with a YSI 600 XL water quality probe. The water quality parameters collected included temperature, dissolved oxygen, specific conductivity, and ph. Water clarity was also measured at each sampling site with a 20-cm black and white secchi disk. We collected a composite surface water sample with a 2-m PVC tube. A water sample was also collected at the thermocline and 1-m from the bottom with a Kemmerer bottle sampler. The samples were placed on ice and transported to the laboratory for nutrient analysis. We used Standard Methods for the Examination of Water and Wastewater 1995 to determine nutrient concentrations of the water samples. The surface samples were analyzed in the laboratory for total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN), and chlorophyll-a. The thermocline and bottom samples were analyzed for TP and SRP. Sample analysis was prioritized by analyte holding time to ensure that analyses are completed within the recommended time interval. Samples were stored at 4 C in a refrigerator until all analysis was completed. A quality assurance and quality control protocol was followed to ensure the precision and accuracy of laboratory data analysis. We compared the nutrient concentrations for Medicine and Parkers Lake to historic data to determine whether there are significant changes in water quality. We determined whether the data was normally distributed and performed Leven s tests for homogeneity of variances among groups. If the data followed a normal distribution and variances were found to be homogenous, we used analysis of variance (ANOVA) procedures to determine whether there are significant differences in water quality among years. A non-parametric Kruskal-Wallis analysis of variance by ranks was used when the data was not normally distributed and variances were not homogenous. Several multiple comparison procedures were used to determine which groups among years were significantly different from each other. All statistical analysis was performed with SPSS software, and the level of significance for all statistical analysis was at the 0.05 probability level. Nutrient lake concentration information was also used to determine the lake trophic status. We calculated lake trophic status by using Carlson s Trophic Status Index (Carlson 1977). This index was developed from the interrelationships of secchi depth transparency, chlorophyll-a concentrations, and total phosphorus concentrations. The resulting index values generally range between 0 and 100 with the increasing values indicating more eutrophic conditions. The TSI was calculated by using the annual average for each water quality parameter. We averaged the TSI values for each of the three water quality parameters (Chlorophyll-a, Total Phosphorus, Secchi Depth) to obtain a single TSI value for each lake. We compared the TSI values among years to determine whether there has been a change in lake trophic status. 3

8 Watershed Monitoring Three Rivers Park District installed automatic Isco sampling stations to determine the extent of nutrient loading throughout various locations within the City of Plymouth. At each particular sampling site, a flow meter measured and recorded the level, velocity, and flow using an area velocity meter. The flow meter was programmed to trigger an automatic sampler to collect a water sample based on a pre-determined increase in water level due to a particular storm event. After sampling was initiated, flow weighted composite water samples were sequentially collected to encompass the entire storm distribution. Flow meter data (level, velocity, and flow) was downloaded via laptop computer within the field; and samples were retrieved within 24 hours of collection. We also periodically collected grab samples during base flow conditions to determine nutrient loading during non-storm events. All samples were labeled immediately after collection, stored in a cooler with ice, and delivered to the Three Rivers Park District for laboratory analysis. Similar standardized laboratory procedures that were followed for the lake samples were also used for the run-off samples (Standard Methods for the Examination of Water and Wastewater 1995). However, the water quality parameters that were analyzed included total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN), and total suspended solids (TSS). Sample analyses holding time as well as quality assurance and control protocol was followed to ensure the precision and accuracy of laboratory analysis. Watershed nutrient loading for each particular site was estimated using the U.S. Army Corps of Engineer s FLUX model Version 5.1 (Walker 1996). The input data requirements for FLUX includes the nutrient concentration (from the laboratory analysis) and daily mean flow (collected from the flow meters) to calculate annual nutrient loading. FLUX automatically pairs sample concentrations with corresponding daily mean flows and determines nutrient loading based on six algorithms. The particular algorithm that is applicable becomes dependent upon the relationship between concentration and flow as well as the coefficient of variation (CV) estimate. The CV reflects sampling error in the flow-weighted mean concentration. Typically, CV values between 0.1 and 0.2 were considered adequate for modeling purposes. The calculation method with the smallest amount of bias and variance was used to estimate the nutrient loading during the sampling period for each of the following monitored sites. To determine annual nutrient loading, estimates for the sampling period were adjusted for precipitation received beyond the sampling interval. The method of estimating annual nutrient loading assumes that the volume of run-off is dependent upon the amount of precipitation received. The percentage of the annual precipitation monitored during the sampling interval was used to calculate an annual nutrient loading estimate. 4

9 Parkers Lake The Parkers Lake watershed area that contributes significant nutrient loading into the lake consists of 1,153 acres. The watershed area consists of four sub-watersheds. To determine the total annual nutrient loading into Parkers Lake, we monitored the nutrient loading for three of the sub-watersheds prior to flowing into the lake (Figure 3). We installed automatic samplers and flow meters connected to area velocity probes at each of the three sampling sites. The fourth sub-watershed was not monitored because approximately 215 acres contributed direct runoff from the landscape surrounding the lake. Figure 3: Parkers Lake sub-watershed monitoring sites. 5

10 The South Parkers Lake sub-watershed drains approximately 258 acres to the south portion of the Lake generally between Vicksburg Lane and I-494. Approximately, 80 acres of the drainage area (31%) is considered impervious acreage (Short Elliot Hendrickson 2000). To monitor and sample runoff from the south subwatershed drainage area, Parkers Lake Sampling Site 1 (PL1) was located at a 48-inch storm sewer culvert draining directly to the lake off of the north side of the Luce Line State Trail. The sampling site was monitored from the time period of spring snow-melt to freezing conditions from 2000 through The general characteristics of the subwatershed include predominately residential urban development that consists of sandy soils with relatively little landscape slope. The North Parkers Lake sub-watershed drains approximately 189 acres to the northwest portion of the lake that is located west of and adjacent to Fernbrook Lane between 26 th Avenue and County Road 6. The sub-watershed western boundary is Vicksburg Lane. The Parkers Lake Sampling Site 2 (PL2) was located at a 48-inch storm sewer culvert on the northwest portion of the lake west of the public boat access. The PL2 site was monitored from 2000 through 2003 during similar conditions that were monitored for PL1. Approximately, 98 acres of the watershed (52%) is considered impervious surface that is predominately residential and commercial use (Short Elliot Hendrickson 2000). The soils tend to be sandy, but the relatively steep to moderate slope of the watershed contributes to substantial runoff. The Fernbrook Lane sub-watershed drains approximately 491 acres to the north portion of the Lake. The sub-watershed is bordered on the east by I-494, to the north by Highway 55, to the south by County Road 6, and to the west by Fernbrook Lane. The Parkers Lake Sampling Site 3 (PL3) was located at a 98-inch culvert on the north side of Parkers Lake adjacent to County Road 6. The PL3 site was monitored during the 2000 sampling period. However, the sampling site was removed in September of 2000 due to a lake effect that resulted in inaccurate flow readings and water sample concentrations. The sampling station was not monitored in 2001 and 2002 due to the above mentioned logistic problems. The PL3 sampling site was relocated on 21 st Avenue North (east of Niagra Lane North and west of Fernbrook Lane North) in The PL3 sampling site has similar sandy soil characteristics as PL1 and PL2. However, approximately 305 acres of the sub-watershed (62%) is considered imperious acreage that covers a large industrial area which ultimately drains to Parkers Lake (Short Elliot Hendrickson 2000). 6

11 Medicine Lake The Medicine Lake watershed consists of a total area of 11,613 acres. There are several sub-watersheds that contribute to significant nutrient loading from creeks and storm sewer outlets at various locations along the lakeshore (Figure 4). We monitored the nutrient loading from seven primary sub-watersheds that flows to Medicine Lake. Eight sampling sites were installed with automatic samplers and flow meters using area velocity probes that measured level, velocity, and flow conditions. Two sites could be accessed via cellular phone modem capabilities to provide near real-time data. Figure 4: Medicine Lake sub-watershed monitoring sites. 7

12 The Plymouth Creek sub-watershed drains 6,380 acres to the west portion of Medicine Lake. The drainage area discharges into the southwest bay of Medicine Lake. This particular sub-watershed consists of several smaller sub-watersheds that ultimately drain to Plymouth Creek at various locations. Due to the relatively large size of the subwatershed, we monitored two sampling sites on Plymouth Creek from 2001 through 2003 to determine the nutrient loading to Medicine Lake from the upper and lower portions of the Plymouth Creek sub-watershed. The furthest Plymouth Creek upstream sampling site (Fernbrook) was located below the weir in Plymouth Park west of Fernbrook Lane between County Road 9 and Hwy 55. This particular area receives water from the Upper Plymouth Creek sub-watershed (1,710 acres), Turtle Lake sub-watershed (409 acres), and a portion of Middle Plymouth Creek sub-watershed (approximately 792 acres) before discharging to the Lower Plymouth Creek sub-watershed. The downstream sampling site (Plymouth Creek) was located on the northwest side of where Plymouth Creek crosses Medicine Lake Drive West. This particular site receives nutrient loading from the Fernbrook upstream sampling site as well as the drainage from the remaining portion of the Middle Plymouth Creek sub-watershed (approximately 792 acres) and the Lower Plymouth Creek sub-watershed (518 acres). Approximately 70% of the Plymouth Creek sub-watershed area discharges to Medicine Lake through Plymouth Creek. The existing land use of the Plymouth Creek sub-watershed includes approximately 28% commercial/industrial; 40% single-family residential; 4% multi-family residential; 7% highway; 7% parks and undeveloped land; and 14% water surface area (Short Elliott Hendrickson, Inc. 2000). This particular sub-watershed provides significant nutrient loading to Medicine Lake. The Ridgedale Creek sub-watershed drains 1,579 acres between Evergreen Lane on the east and Pineview Lane to the west. The runoff from the sub-watershed discharges through an open channel and a series of culverts into the eastern portion of the southwest bay of Medicine Lake. The sampling site was located north of Hwy 55 where Ridgedale Creek crosses Medicine Lake Drive West. We monitored the nutrient loading at the Ridgedale Creek site from 2001 through This drainage area provides significant nutrients to Medicine Lake as a result of exiting land use. The existing land use consists of approximately 21% commercial/industrial; 44% single-family residential; 7% highway; 5% institutional; 3% parks and undeveloped land; 2% multi-family residential; and 18% water surface area (Short Elliott Hendrickson, Inc. 2000). The North Medicine Lake sub-watershed drains 487 acres between 48 th Avenue and 39 th Avenue directly north of Medicine Lake to Schmidt Lake. The sampling site (ML1) is located on the Three Rivers Park District hiking trail south of County Road 9. We monitored the nutrient loading at ML1 from 2002 to This particular site receives a significant amount of sediment/nutrient pollutant loading from stream bank erosion located upstream of the County Road 9 underpass. 8

13 The Northeast Medicine Lake sub-watershed drains 663 acres south of Schmidt Lake to the south of 36 th Avenue where it discharges to Medicine Lake. The subwatershed western boundary is located at Larch Lane, and the eastern boundary extends ¼ mile east of Zachary Lane. The sampling site (ML2) is located within a storm sewer culvert that flows under 36 th Avenue. The existing drainage of the sub-watershed is primarily through open channel flow that provides potential sources of nutrient loading. However, there is approximately 148 acres of wetlands and storm water ponds that provides significant treatment potential throughout the sub-watershed. The nutrient loading at ML2 was monitored from 2002 to The North Bassett Creek sub-watershed drains 192 acres south and southwest of 36 th Avenue prior to discharging into Medicine Lake. The sub-watershed boundary extends approximately ¾ mile northeast of East Medicine Lake Boulevard, and just north of 36 th Avenue North. The sampling site (ML3) is located within the open channel that crosses East Medicine Lake Boulevard. The nutrient loading at ML3 was monitored from 2002 to The outlet above and below East Medicine Lake Boulevard has significant erosion prior to discharging to Medicine Lake. The South Bassett Creek sub-watershed drains 531 acres extending south of 36 th Avenue and approximately ¾ mile east of Highway 169. The sub-watershed drains portions of the City of Plymouth, New Hope, and Golden Valley. The sampling site (ML4) is located at the intersection of Medicine Ridge Road and Nathan Lane. The nutrient loading at ML4 was monitored from 2002 to The sub-watershed has potential drainage issues because some of the storm sewer pipes are undersized resulting in localized flooding. In addition to the drainage problems, the sub-watershed does not have adequate water quality treatment prior to discharging to Medicine Lake. The East Medicine Lake Park sub-watershed drains 157 acres extending approximately ⅓ mile east of Highway 169. The northern portion of the sub-watershed boundary is located southwest of Duluth Street, and the southern portion of the subwatershed boundary is located north of Plymouth Avenue North. The sub-watershed drains portions of the City of Plymouth, City of Medicine Lake, and Golden Valley. The sampling site (ML5) is located southeast of Medicine Lake Park. This particular site receives nutrient loading from a storm sewer system that drains a large industrial area. The nutrient loading for ML5 was monitored in

14 Elm Creek Elm Creek drains 3,072 acres of the undeveloped northwestern corner of Plymouth (Figure 5). The existing area is comprised of low-density residential (1,352 acres) and agricultural land use (1,405 acres) with wetlands/floodplain covering up to 25% of the area (Short Elliot Hendrickson 2000). This particular area is to be considered for future development; therefore, it is anticipated that this area will have increased peak flow conditions and runoff volume. To provide for conveyance of the anticipated changes in runoff, it becomes necessary to determine the nutrient loading relative to the current flow conditions of Elm Creek prior to development. We installed two automatic sampling stations and flow meters with area velocity probes to monitor the potential changes in drainage from 2000 to 2003 (Figure 5). A level and flow relationship curve was determined manually for each sampling site using a Marsh McBirney portable flow meter. Figure 5: Elm Creek watershed monitoring sites. Elm Creek flows northeasterly into Plymouth from Medina under Highway 55 just east of Sioux Drive (Highway 101). Approximately 4,000 acres from Medina provides runoff to Elm Creek before it enters Plymouth. We installed a sampling site to determine the total nutrient loading of Elm Creek prior to entering the City of Plymouth. The first sampling site (Elm Creek at Hamel Rd.) was located on the north side of where Elm Creek crosses Hamel Road prior to flowing under Highway 55. The stream continues in a northeasterly direction, crossing under Peony Lane/Troy Lane before turning north through a wide flood plain, and crossing under County Road 47 into Maple Grove. To determine the total nutrient loading leaving the City of Plymouth, the second sampling site (Elm Creek at Elm Road) was located on the south side of where Elm Creek crosses Elm Road in Maple Grove. This particular sampling site does measure a small amount of runoff from Maple Grove. However, the loading appears to be insignificant because the sampling site is only ½ mile downstream from where Elm Creek leaves the City of Plymouth. 10

15 Shingle Creek-Bass Lake Shingle Creek drains the urbanized north central and northeastern part of Plymouth (Figure 6). We were specifically interested in a portion of Shingle Creek that discharges to the southern inlet of Bass Lake. The Bass Lake drainage area receives water from seven subwatersheds that consists of a total area of 2,626 acres in which 24% is considered impervious surface (652 acres) (Short Elliot Hendrickson 2000). However, Shingle Creek only receives drainage from four of these sub-watersheds (a total of 1,946 acres) prior to discharging to Bass Lake. To determine the amount of nutrient loading that Shingle Creek may contribute to Bass Lake, we installed an automatic sampler and flow meter with area velocity probe to collect water samples and monitor changes in water level, velocity, and flow. The sampling site (Shingle Creek) was located near the open channel of Shingle Creek and west of Pineview Lane (north of Schmidt Lake Road and south of the railroad crossing). This particular sampling site was monitored from 2000 through A level and flow relationship for this portion of Shingle Creek was determined manually with a Marsh McBirney portable flow meter to compare to flow meter measurements. Figure 6: Shingle Creek sub-watershed monitoring site. 11

16 Watershed and Lake Response Modeling We used watershed and in-lake response modeling to further understand the relationship between the potential sources of pollutant nutrient loading and in-lake water quality for Parkers and Medicine Lake. The modeling efforts are intended to identify significant sources of nutrient loading that may have dramatic effects on in-lake water quality. Consequently, it becomes critical that the modeled relationship between nutrient loading and in-lake water quality accurately reflect observed conditions. If modeling efforts are accurate, then the models should predict similar in-lake trophic conditions that were observed for the particular year monitored. The model simulations can be used to determine the changes in water quality relative to corresponding changes in nutrient loading. We used two in-lake trophic response models (LTROPHIC and WiLMS) to assess the phosphorus loading impacts on in-lake water quality conditions. The LTROPHIC model estimated in-lake total phosphorus concentrations based on several parameters. The model uses several hydraulic and phosphorus loading coefficients that are derived from monitoring data to predict in-lake trophic conditions. The lake morphometric (lake volume and surface area) and hydraulic (outflow volume) characteristics are input into the model to calculate hydraulic residence time and flushing rates. Based on the calculated hydraulic conditions, the annual watershed phosphorus loading (derived from FLUX modeling) is used to calculate the phosphorus loading components that are necessary to predict the in-lake conditions. Typically, the model under estimates the in-lake trophic conditions because the internal loading component of the lake is not taken into consideration. However, the internal loading can be estimated by adjusting the annual phosphorus loading parameter until the predicted in-lake conditions are similar to observed monitored conditions. Consequently, the increase in phosphorus loading that is necessary to obtain the observed lake conditions was used as an estimate of internal loading. The above mentioned parameters are used to calculate in-lake phosphorus conditions based on five trophic equations. The trophic response model that produces a value similar to observed conditions is usually selected as the model that best represents the lake conditions. The WiLMS model estimated in-lake trophic conditions based on five complex module components (Panuska and Kreider 2003). The Hydrologic and Morphometric Module calculates the annual water budget for the lake by incorporating annual precipitation conditions, annual watershed run-off volume to the lake, and morphological lake characteristics (volume and surface area). The Non-point Source Module uses phosphorus export coefficients (derived from FLUX modeling) from each sub-watershed drainage area to calculate the total non-point source watershed loading to the lake. The Point Source Module estimates the phosphorus loading from specific sources (i.e. septic tanks or waste water treatment facilities). The Internal Load Module uses four methods for estimating the internal loading of lakes with anoxic conditions. The WiLMS model will automatically determine the total phosphorous loading range (from a low, most likely, and high nutrient loading perspective) for each nutrient loading component (nonpoint source, point source, and internal loading). The total phosphorus loading estimates are used within the Prediction and Uncertainty Analysis Module to estimate the in-lake trophic conditions from thirteen lake response models. The module provides statistical analysis of the predicted lake trophic conditions to assist with selecting a lake response model that is similar to observed conditions. The trophic response model that produces a value similar to observed conditions and fits the model parameters is selected as the model that best represents the lake conditions. 12

17 After the appropriate watershed and in-lake response models have been selected, several different model simulations were performed to estimate the phosphorus reduction necessary to achieve water quality goals. To simulate a reduction in phosphorus loading, the LTROPHIC model manually requires reducing the total phosphorus loading parameter by a certain percentage to observe the corresponding change in lake trophic conditions. However, the WiLMS model automatically allows the user to reduce the phosphorus loading by a certain percentage to determine the corresponding change in trophic conditions. The change in-lake trophic conditions were evaluated for each percent reduction in phosphorus loading. The in-lake trophic conditions that were calculated from the reduced phosphorus loading simulations were compared to in-lake water quality goals. These simulations provided the percent phosphorus removal that is necessary to achieve water quality goals, and identified potential sources of nutrient loading that impact in-lake water quality. 13

18 RESULTS AND DISCUSSION Parkers Lake Parkers Lake water quality conditions currently meet the established in-lake water quality goals. Parkers Lake had been considered a eutrophic lake with relatively poor water quality since the early 1970 s. The in-lake water quality conditions would potentially inhibit waterbased recreational activities. As a consequence, Parkers Lake water quality goals (Appendix B) and implementation plan were developed to establish priorities and guidelines for improving the in-lake water quality conditions (Barr Engineering 1993; Bonestroo Associates and Blue Water Science 2001; Short Elliot Hendrickson 2000). The Parkers Lake water quality has improved substantially since 1990, and has achieved the established water quality goals since Several in-lake water quality parameters that are used to determine the lake s trophic status have improved. Phosphorus concentrations gradually declined from 45 µg/l in 1991 to 20 µg/l in 2001 (Figure 7). Water clarity (measured by secchi disk depth) also improved from 2.2 m in 1990 to 5.2 m in 2001 (Figure 7). Using these parameters, an average trophic status index (TSI) was calculated to determine the eutrophication status of the lake. Typically, a lake becomes more eutrophic as the TSI value increases. Plotting the TSI for each year and calculating a trend analysis on the data further suggests that there have been significant improvements in water quality (Figure 8). The TSI significantly decreased from 56 in 1995 to 40 in Comparing these TSI values to similar lakes indicates that values from 1991 through 1995 were within the average range for the ecoregion. As water quality conditions improved from 1995 through 2001, the below average TSI values suggest that water quality conditions were considerably better than other lakes within the ecoregion. Total Phosphorus at the Surface 50 Secchi Depth 45 Total Phosphorus (µg/l) Water Quality Goal Secchi Depth (m) Water Quality Goal Year Figure 7: Parkers Lake annual changes in total phosphorus and secchi depth. Values are an averge from May through September. 14

19 Although there have been water quality improvements, there is concern that these conditions may only be temporary. The water quality conditions from 2001 to 2003 are better in comparison to conditions observed from 1991 through However, the water quality has degraded slightly from 2001 through 2003 (Figure 7 & 8). Total phosphorus concentrations increased to 30 µg/l in 2002, and further increased to 34 µg/l in 2003 (Figure 7). In addition, secchi depth decreased to 3.8 m in 2002, and decreased to 2.8 m in 2003 (Figure 7). The corresponding TSI values increased to 48 in 2002, and increased to 54 in 2003 (Figure 8). The Parkers Lake watershed received above average precipitation conditions that contributed to the change in water quality in However, the annual precipitation in 2003 was considerably lower in comparisons to 2002, which suggests that other factors are significantly impacting Parkers Lake water quality. Average TSI values R 2 = 0.35 Parkers Lake Trophic Status Index R 2 = Year Figure 8: Parkers Lake annual Trophic Status Index values. The in-lake water quality for Parkers Lake is highly dependent upon the nutrient loading that the lake receives. There are two types of nutrient sources that contribute to the total annual loading of a particular lake (internal loading and watershed loading). A primary source of nutrient loading is internal loading, which is the re-suspension or recycling of nutrients within the lake. Internal loading can occur when nutrients are released from the sediments during anoxic conditions, or nutrients become available after the senescence of aquatic plants. The other primary source of nutrient loading is watershed loading. Watershed loading is the source of nutrients that enters the lake from the surrounding land areas that drain to the lake. Watershed monitoring allows identification of potential areas that provide significant sources of nutrient loading to the lake. After these nutrient loading components have been determined, management efforts can then focus on those areas that contribute significant amounts of nutrients. 15

20 Currently, internal loading of nutrients released from the sediments during anoxic conditions does not appear to be a significant component within Parkers Lake. Parkers Lake remained stratified throughout the summer (2001 through 2003) until the breakdown of lake stratification occurred during fall lake turnover. Parkers Lake stratified in May with anoxic conditions gradually developing below 8 m (Appendix B). As a consequence of anoxia, phosphorus concentrations in the hypolimnion of the lake ranged between 100 µg/l to 700 µg/l in 2003 (Figure 9). Parkers Lake water quality has the potential to become significantly influenced by internal loading if complete mixing of the water column were to occur. Although in-lake monitoring of stratification conditions was not continuous, complete mixing of Parkers Lake was not observed during bi-weekly sampling from 2001 through However, partial mixing of the water column due to minor changes in stratification appears to occur frequently (Appendix B). Diurnal changes in stratification and wind action entrained some of these nutrients from the hypolimnion. These partial mixing events did not account for significant amounts of nutrient loading, but did result in slightly higher nutrient concentrations that were typically observed during the summer and fall (August through October). Surface TP & SRP 800 Phosphorus (ug/l) TP Surface SRP Surface 04/15/03 05/06/03 05/21/03 06/03/03 06/17/03 07/01/03 Date 07/14/03 07/28/03 08/11/03 08/25/03 09/08/03 09/29/03 Phosphorus (ug/l) Thermocline TP & SRP TP Thermocline SRP Thermocline 04/15/03 05/06/03 05/21/03 06/03/03 06/17/03 07/01/03 07/14/03 07/28/03 08/11/03 08/25/03 09/08/03 09/29/03 Date Phosphorus (ug/l) TP Bottom SRP Bottom Bottom TP & SRP 04/15/03 05/06/03 05/21/03 06/03/03 06/17/03 07/01/03 07/14/03 07/28/03 08/11/03 08/25/03 09/08/03 09/29/03 Date Figure 9: Parkers Lake total phosphorus and soluble reactive phosphorus concentrations from the surface to the bottom. 16

21 Internal loading of nutrients from the senescence of aquatic plants has the potential to have a significant influence on Parkers Lake water quality. The senescence of aquatic plants can provide a nutrient pulse that contributes to water quality degradation. Aquatic plant surveys for Parkers Lake indicated that there were two plant species that had frequent periods of senescence (McComas and Stuckert 2003). Curlyleaf pondweed (Potamogeton crispus) was the predominant exotic species in the lake. This aquatic plant dies-off at the end of June or early July when water temperatures increase. The senescence of curlyleaf pondweed has contributed to increased nutrient levels and water quality degradation in similar lakes within the ecoregion. The other aquatic plant that has had periods of senescence was a narrow-leaf pondweed (Potamogeton foliosus/pusillus). Narrow-leaf pondweeds are predominant native plant specie in Parkers Lake that occasionally die-off during the summer (McComas and Stuckert 2003). Curlyleaf pondweed has been present in Parkers Lake since 1992 (Bonestroo Associates and Blue Water Science 2001). Curlyleaf pondweed becomes established during the winter, which allows the exotic plant to have a competitive advantage over native plant species. Within Parkers Lake, curlyleaf pondweed coverage was approximately 11 acres in 2002 and 18 acres in The increased curlyleaf pondweed acreage in 2003 may have contributed to poorer water quality in comparison to Seasonal changes in water quality further suggest that increased phosphorus concentrations from the end of June (21 µg/l) to early July (48 µg/l) in 2003 may have been attributed to a nutrient release from the senescence of curlyleaf pondweed (Figure 10). The released nutrients are in a soluble form that is readily available for algae uptake. Based on secchi depth measurements, water clarity conditions decreased from 5.3 m to 1.8 m due to the development of algae blooms (Figure 10). Total Phosphorus and SRP (µg/l) Total Phosphorus, Soluble Reactive Phosphorus, and Total Nitrogen at the Surface in 2003 Total Phosphorus Soluble Reactive Phosphorus Total Nitrogen Total Nitrogen (mg/l) /15/03 05/06/03 05/21/03 06/03/03 06/17/03 07/01/03 07/14/03 07/28/03 08/11/03 08/25/03 09/08/03 09/29/03 Date Secchi Depth and Chlorophyll a at the Surface in Secchi Depth (m) Chlorophyll a (µg/l) Secchi Depth Chlorophyll a /15/03 05/06/03 05/21/03 06/03/03 06/17/03 07/01/03 07/14/03 07/28/03 08/11/03 08/25/03 09/08/03 09/29/03 Date Figure 10: Seasonal changes in total phosphorus, soluble reactive phosphorus, total nitrogen, chlorophyll-a and secchi depth for Parkers Lake. 17

22 Despite the change in water quality conditions, the impact does not appear to be as significant in comparison to other lakes within the ecoregion. Curlyleaf pondweed acreage consisted of approximately 11% to 19% of the surface area of Parkers Lake in 2002 and 2003 (McComas and Stuckert 2003). The level of growth was not considered nuisance because there was very little surface matting (McComas and Stuckert 2003). Analysis of sediment samples in 2002 and 2003 further suggested that there is a low potential for nuisance curlyleaf growth for most of Parkers Lake (McComas and Stuckert 2003). Based on non-nuisance growth conditions, curlyleaf pondweed does not affect water quality conditions throughout the remaining portion of the year. The impact curlyleaf pondweed may have on water quality conditions may change if nuisance conditions became more predominate in Parkers Lake. Although curlyleaf pondweed typically competes with other native species, narrow-leaf pondweed has become the most common submersed aquatic plant in Parkers Lake. Aquatic plant surveys indicated that narrow-leaf pondweed species became significantly established in Parkers Lake by The frequency of transect occurrence for narrow-leaf pondweed in 2000 was documented as 80% (McComas and Stuckert 2003). Unfortunately, there were no aquatic plant surveys in However, narrow-leaf pondweed growth covered approximately 37 acres in 2002, in which 13 acres was considered nuisance coverage (McComas and Stuckert 2003). In 2003, narrow-leaf pondweed was the most common submerged plant in Parkers Lake covering 29 acres. The amount of nuisance growth was approximately 7 acres in 2003 (McComas and Stuckert 2004). These nuisance growth conditions in 2002 and 2003 often inhibited potential recreational use, but did not persist throughout the remaining portion of the year. Aquatic plant surveys in the fall of 2002 and 2003 indicated that narrow-leaf pondweed died-off in the summer. The period of senescence coincided with the degradation of water quality conditions in Parkers Lake for 2002 and 2003 (Figure 7). This was unexpected because narrow-leaf pondweed did not senescence in 2000 (McComas and Stuckert 2003) when water quality conditions were considered excellent (Vlach and Barten 2000; Vlach and Barten 2002). Although senescence does not occur every year, it appears that narrow-leaf pondweed may have significant impacts on water quality in Parkers Lake during years when senescence occurs. Unfortunately, the amount of internal nutrient loading and the conditions that are conducive for the senescence of narrowleaf pondweed are relatively unknown (personal communication with Steve McComas). Watershed nutrient loading is another potential factor that influences Parkers Lake water quality. The nutrient loading for three sub-watersheds that discharge into Parkers Lake were monitored (Appendix A). Based on monitoring efforts, the three sub-watersheds in 2003 contributed 516 pounds of annual phosphorus loading to Parkers Lake (Table 1). The Fernbrook Lane (PL3) sub-watershed contributes approximately 70% of the total annual nutrient loading (Table 1; Figure 11). The PL3 sampling site provides a significant proportion of the total nutrient loading because it accounts for 52% of the total drainage area to Parkers Lake. The other two sub-watersheds contribute smaller amounts of nutrient loading due to their smaller drainage areas (Table 1; Figure 11). The North sub-watershed (PL2) drainage area accounts for 20% of the total drainage area to Parkers Lake, and provides approximately 16% of the total nutrient loading to the lake. The South sub-watershed (PL1) accounts for 28% of the total drainage to Parkers Lake, and contributes 14% of the total nutrient loading to Parkers Lake. 18

23 Table 1: Parkers Lake sub-watershed total phosphorus loading (pounds/year). Parkers Lake Total Phosphorus Loading (pounds/year) Site PL PL PL3 397 Note: PL3 was not sampled from 2000 to Total Phosphorus Loading 500 TP (lbs/yr) PL1 PL2 PL3 Figure 11: Parkers Lake annual watershed phosphorus loading. Variations in annual watershed nutrient loading can cause changes in Parkers Lake water quality. The amount of watershed nutrient loading becomes dependent upon the run-off volume from precipitation that the area receives. The above average precipitation conditions in 2001 (36 inches) and 2002 (41 inches) resulted in an increase in run-off volume to Parkers Lake (Table 2). Consequently, there was an increase in watershed nutrient loading in 2001 and 2002 (Appendix A). However, total watershed nutrient loading did not correspond to the change in precipitation conditions for Despite the below average precipitation (26 inches) for 2003, the amount of watershed nutrient loading did not significantly decline in comparison to nutrient loading in 2001 and Most of the precipitation in 2003 was high intensity rainfall events occurring over a short period of time. These conditions produced volumes of water draining to Parkers Lake similar to those observed in 2001 and 2002 (Table 2). Consequently, the precipitation conditions from 2001 to 2003 resulted in an increase in watershed nutrient loading. Typically, a significant increase in watershed nutrient loading can cause degradation to in-lake water quality conditions. This may have contributed to the partial degradation in water quality for Parkers Lake from 2001 through Table 2: Parkers Lake sub-watershed total run-off volume (x 10 6 M 3 ). Parkers Lake Total Run-off Volume (x 10 6 M 3 ) Site PL PL PL Note: PL3 was not sampled from 2000 to

24 Currently, the amount of watershed nutrient loading does not appear to significantly alter the trophic conditions of Parkers Lake. However, there is a potential concern that the watershed nutrient loading may increase due to the proposed installation of conventional curb and gutter storm-sewer system in the South Parkers Lake (PL1) sub-watershed drainage area. Currently, water drains from impervious surfaces to grass swales that have a moderate elevation change (Figure 12). The sub-watershed area also has moderately rapid permeable soils. These conditions were more conducive for infiltration, which reduces the volume of run-off and nutrient loading from the watershed area. Figure 12: South Parkers Lake sub-watershed drainage area. The installation of conventional curb and gutter storm-sewer may increase total run-off volume and nutrient loading. The City of Plymouth has proposed to build a nutrient detention basin on the south side of Parkers Lake to offset any additional run-off volume and nutrient loading that would be expected from the installation of the conventional curb and gutter storm sewer system. The effectiveness of the proposed detention basin in reducing nutrient loading is currently unknown. However, there have been several detention basins constructed within the Parkers Lake watershed that have successfully reduced the amount of nutrient loading. Monitoring efforts should be considered after the completion of the proposed storm-sewer project for the South Parkers Lake (PL1) sub-watershed to determine the nutrient detention basin s effectiveness in reducing nutrient loading to Parkers Lake. There have been additional proactive best management strategies within the watershed that have been implemented by the City of Plymouth to improve the water quality. The City of Plymouth implemented an ordinance that restricted the use of phosphorus fertilizer as an effort to reduce the amount of watershed nutrient loading to area lakes. The ordinance for the phosphorus restriction on commercial fertilizer use was implemented in 1995, and the restriction for residential use was implemented in The trophic status of Parkers Lake significantly improved after the phosphorus ban on commercial fertilizer use was implemented (Figure 8). 20

25 A further improvement in Parkers Lake trophic status from 2000 to 2002 was observed following the phosphorus ban on residential fertilizer use (Figure 8). Lake modeling simulations as part of the Parkers Lake Implementation Plan by Bonestroo Associates and Blue Water Science in 2001 estimated a 15% phosphorus loading reduction due to the phosphorus free fertilizer ordinance. The modeled total loading estimates were similar to the total loading estimates from monitoring data (Bonestroo Associates and Blue Water Science 2001). Preliminary analysis from previous studies (Barten and Jahnke 1997; Garn 2002) suggests that the ordinance has improved the water quality by reducing the amount of phosphorus run-off to Parkers Lake. However, the significance the ordinance has had in improving Parkers Lake water quality requires additional monitoring efforts on a sub-watershed basis. The efforts to reduce nutrient loading have significantly improved Parkers Lake water quality. The in-lake water quality for Parkers Lake currently meets the criteria to support full recreational use. Consequently, it is essential to continue efforts to reduce nutrient loading in order to maintain the current water quality conditions in Parkers Lake. Achievement of the water quality goals would suggest that Parkers Lake does not have any water quality problems. However, the water quality improvements have been conducive for other potential in-lake problems. Parkers Lake has had fewer algae blooms during the summer due to reduced nutrient concentrations. As a result, Parkers Lake has had remarkable water clarity in comparison to other sub-urban lakes. The improved water clarity allows more light penetration through the water column, which contributes to an abundance of aquatic plants in Parkers Lake that can frequently grow to the surface. These aquatic plants are considered a nuisance to those that use the lake for recreational purposes. Currently, Parkers Lake has a diverse aquatic plant community. There are two exotic species, curlyleaf pondweed and Eurasian watermilfoil, in Parkers Lake that have achieved nuisance growth conditions in similar lakes within the ecoregion. These particular exotic species often compete with native species. A change in the plant community from native species to predominately exotic species and the development of nuisance growth conditions could have potential water quality implications. Consequently, the specific conditions that contribute to nuisance growth of these species should be thoroughly investigated. It is recommended to continue aquatic plant surveys in the spring and fall to monitor potential changes in the plant community. The City of Plymouth currently harvests aquatic plants during the summer to increase the useable surface area of the lake. However, harvesting efforts should be coordinated in the spring and fall to try and reduce the spread of exotics. Future lake management efforts should also consider an aquatic plant management plan to encourage the development of native species. 21

26 RESULTS AND DISCUSSION Medicine Lake Medicine Lake is an important resource within the City of Plymouth that receives a considerable amount of recreational use. The lake has had a history of degraded water quality conditions that potentially inhibit recreational use. Due to the poor water quality conditions, the Environmental Protection Agency (EPA) has proposed to place Medicine Lake onto the list of Impaired Waters for excess nutrients in In addition, the City of Plymouth developed a Water Resources Management Plan in 2000 that identified Medicine Lake as a high priority resource that requires water quality improvements (Short Elliot Hendrickson 2000). A Medicine Lake subcommittee was established to develop and facilitate a comprehensive management plan to pro-actively address the water quality issues. Water quality goals (Appendix B) were developed for Medicine Lake to provide guidelines in making management decisions that would improve in-lake water quality conditions (Barr Engineering 2000; Barr Engineering 2001). Currently, Medicine Lake water quality conditions do not meet the established water quality goals. Although the water quality conditions vary within a particular year, we found that there were no significant differences (p<0.05 level) in water quality from 1990 to The average total phosphorus concentrations ranged between 50 and 83 µg/l (Figure 13). These high phosphorus concentrations were conducive for the development of algae blooms. The amount of algae in the water column is determined by measuring chlorophyll-a concentrations. In Medicine Lake, chlorophyll-a concentrations ranged between 20 and 40 µg/l (Figure 13). In addition, water clarity most often did not exceed 2.0 m with an average secchi depth transparency ranging between 1.2 to 2.0 m (Figure 13). Total Phosphorus (µg/l) Total Phosphorus and Total Nitrogen at the Surface Total Phosphorus Total Nitrogen Year Total Nitrogen (mg/l) Secchi Depth (m) Secchi Depth and Chlorophyll a Secchi Depth Chlorophyll a Year Chlorophyll a (µg/l) Figure 13: Medicine Lake annual changes in total phosphorus, total nitrogen, chlorophyll-a, and secchi depth. 22

27 Using these parameters, an average annual trophic status index (TSI) was calculated to determine the trophic conditions of the lake. The TSI values suggest that Medicine Lake has eutrophic conditions that often exceed the average range for similar lakes within the ecoregion. Analysis of TSI values from 1990 to 2003 further suggested that the water quality conditions do not appear to be improving in Medicine Lake (Figure 14). These water quality conditions were above critical levels that potentially inhibit recreational use. Consequently, it becomes necessary to assess the watershed and internal nutrient loading components that contribute to the water quality conditions of Medicine Lake Medicine Lake Trophic Status Index R 2 = Average TSI values Year Figure 14: Medicine Lake annual Trophic Status Index. Watershed nutrient loading is a significant factor affecting the water quality of Medicine Lake. Medicine Lake received approximately 5,523 pounds of phosphorus loading from six subwatersheds in 2002 (Table 3; Appendix A); and received approximately 3,432 pounds of phosphorus loading from seven sub-watersheds in 2003 (Table 3; Appendix A). The volume of run-off from precipitation was the primary factor that determined variations in annual nutrient loading. The Medicine Lake watershed area received near record annual precipitation in 2002 (40.6 inches) in comparison to the average precipitation received in 2003 (26.3 inches). These conditions were used to model changes in watershed nutrient loading relative to variations in precipitation. Based on model simulations, the amount of watershed phosphorus loading for low precipitation (19.9 inches) conditions was estimated at 3,178 pounds (Table 4); and the amount of watershed phosphorus loading during high precipitation (40.63 inches) conditions was estimated at 6,801 pounds (Table 4). The observed watershed nutrient loading in 2002 and 2003 were similar to these modeled conditions (Table 3 & 4). The amount of precipitation in 2002 resulted in 26% more run-off volume in comparison to 2003 conditions. Consequently, the higher total flow volumes in 2002 delivered more watershed nutrient loading to Medicine Lake relative to 2003 (Table 3). Despite these variations in annual watershed loading, the amount of nutrients delivered to Medicine Lake in 2002 and 2003 was sufficient to cause eutrophic conditions. 23

28 Table 3: Medicine Lake total phosphorus loading and run-off volume estimated from annual sub-watershed monitoring data. Total Phosphorus Loading (lbs/yr) Total Flow Volume (x10 6 M 3 ) Site Plymouth Creek Fernbrook Ridgedale Creek ML ML ML ML ML Table 4: Medicine Lake total phosphorus loading for each sub-watershed using WiLMS modeling simulations. Watershed Loading (lbs/yr) Precipitation Conditions Site Low Moderate High Plymouth Creek Ridgedale Creek N Medicine Lake (ML1) NE Medicine Lake (ML2) N Bassett Creek (ML3) S Bassett Creek (ML4) E Medicine Lake Park (ML5) Total Low Total Phosphorus loading estimates using 19.9 inches of annual precipitation with 2003 data. Moderate Total Phosphorus loading estimates using 26.4 inches of annual precipitation with 2003 data. High Total Phosphorus loading estimates using inches of annual precipitation with 2002 data. The watershed source of nutrient loading was not evenly distributed throughout the Medicine Lake drainage area. Plymouth Creek is the largest sub-watershed providing approximately 68% of the total phosphorus watershed loading to Medicine Lake (Figure 15). The Plymouth Creek sub-watershed consists of several smaller watersheds that collectively delivered 3,931 and 2,274 pounds of total phosphorus to Medicine Lake in 2002 and Within the Plymouth Creek sub-watershed, the drainage area upstream of the Fernbrook sampling site contributes approximately 50% to 70% of the nutrient loading for Plymouth Creek. There are several other sub-watersheds that also contribute significant amounts of nutrient loading to Medicine Lake. Ridgedale Creek sub-watershed is the secondary major contributor that provided 12% of the total phosphorus loading to Medicine Lake (Figure 15). The South and North Bassett Creek sub-watersheds provided approximately 9% and 5% of the total phosphorus loading (Figure 15). The other sub-watersheds were smaller contributors of the total watershed phosphorus loading (Figure 15). 24

29 N Bassett Creek 5% NE Medicine Lake 2% N Medicine Lake 1% Ridgedale Creek 12% Medicine Lake Watershed Phosphorus Loading S Bassett Creek 9% 2003 E Medicine Lake Park 3% Plymouth Creek 68% Figure 15: The percent phosphorus loading for each sub-watershed. Internal nutrient loading is another significant factor that affects the water quality of Medicine Lake. Internal nutrient loading is the re-suspension or recycling of the nutrients within the lake. The re-suspended or recycled in-lake nutrients often become available for algae uptake. Consequently, internal loading of nutrients is conducive for the development of algae blooms. There are two different sources of internal loading of nutrients that need to be considered in Medicine Lake. A primary source of internal loading occurs when nutrients are released and become available after the senescence of aquatic plants. Another primary source of internal loading can occur when nutrients are released from the sediments during the development of anoxic conditions. Although these two sources can alter water quality conditions, the significance each internal loading source has on water quality conditions becomes dependent upon the time period and frequency of occurrence. Medicine Lake has very dense curlyleaf pondweed growth that provide an internal source of nutrients for the lake. Curlyleaf pondweed is an exotic species that typically competes with other native species. According to aquatic plant surveys, Medicine Lake had approximately acres of curlyleaf pondweed in 2003 (Figure 16). Curlyleaf pondweed frequently grew to the surface resulting in nuisance conditions. Approximately, 80% of the total acreage was considered nuisance growth conditions. An increase in summer temperatures stimulated the senescence of curlyleaf pondweed. A nutrient pulse from curlyleaf pondweed die-off developed at the end of June or early July. In 2003, phosphorus concentrations increased from 25 µg/l to 46 µg/l (Figure 17). A significant decline in water quality was observed after the time period of senescence. The increased nutrients were in a soluble form that became available for algae uptake. Chlorophyll-a concentrations gradually increased from 14 µg/l to 28 µg/l shortly after curlyleaf pondweed senescence (Figure 17). Consequently, a decrease in water clarity conditions developed in response to the pulse of increased nutrients that are subsequently followed by algae blooms. The senescence of curlyleaf pondweed exacerbates the eutrophication process by causing poor water quality conditions earlier in the season. 25

30 Figure 16: Medicine Lake curlyleaf pondweed survey in Total Phosphorus and SRP (µg/l) Total Phosphorus, Soluble Reactive Phosphorus, and Total Nitrogen at the Surface in 2003 Total Phosphorus Soluble Reactive Phosphorus Total Nitrogen Total Nitrogen (mg/l) Secchi Depth (m) Secchi Depth and Chlorophyll a at the Surface in 2003 Secchi Depth Chlorophyll a Chlorophyll a (µg/l) 0 04/18/03 05/07/03 05/20/03 06/03/03 06/17/03 07/01/03 Date 07/15/03 07/29/03 08/12/03 08/26/03 09/09/03 09/30/ /18/03 05/07/03 05/20/03 06/03/03 06/17/03 07/01/03 Date 07/15/03 07/29/03 08/12/03 08/26/03 09/09/03 09/30/03 0 Figure 17: Seasonal changes in total phosphorus, soluble reactive phosphorus, total nitrogen, chlorophyll-a and secchi depth for Medicine Lake. 26