Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1997.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." Fish consumption uses remain assessed as "fully supported." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1997. Further monitoring and investigation are needed to better characterize water quality conditions at this lake.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal and non-algal turbidity threaten the Class A and Class B(LW) uses of Easter Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 70, 60, and 64, respectively. According to Carlson (1977), the index value for total phosphorus places this lake at the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and moderately poor water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Easter Lake is 12. This TN:TP ratio suggests that nitrogen limitation on algal production exists at this lake. Data from Downing et al. (2002), however, show relatively small populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 5% of the dry mass of the zooplankton community in the late July sample and 30% of the mid-August sample. According to Carlson (1991), the occurrence of a high TSI value for secchi depth relative to that for chlorophyll-a suggests that non-algal turbidity limits algal production. The levels of inorganic suspended solids at this lake are typical for Iowa lakes and suggest at least some potential for contributing to in-lake turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Easter Lake was 6.5 mg/l. This level of ISS may lead to slight increases in-lake turbidity that may limit algal production. The very high TSI values for total phosphorus and moderately high TSI for chlorophyll-a suggest potential threats to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. Also, data from Downing et al. (2002) suggest that the presence of nuisance algal species (i.e., bluegreen algae) may present a threat to these uses. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 5% in the late June sampling but increased to approximately 85% in the late July sampling, and remained high at approximately 75% in the late August sampling.
The eutrophic conditions at this lake, including high nutrient levels and moderate levels of algal turbidity, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened." Information from the IDNR Fisheries Bureau, however, suggests that siltation remains a significant problem at this lake and that the Class B(LW) uses should be assessed as "partially supported." Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The January 22, 2003 "Des Moines Register" described the phased construction of a series of ten ponds in the watershed of this lake to control stormwater runoff and to reduce delivery of sediment to the lake that is eroded from construction sites and agricultural portions of the watershed. All ten ponds are to be constructed by summer 2003. Snyder and Associates and the city of Des Moines are involved in the design and implementation of these ponds. Fish consumption uses remain assessed as "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring conducted in 1997.
