Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, 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 (monitored) as "partially supported" due to moderately poor water transparency caused primarily by inorganic turbidity that indicates an aesthetically objectionable condition. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to siltation impacts at this lake. Fish consumption uses remain assessed (evaluated) as "fully supported" based on U.S. EPA/IDNR fish contaminant monitoring in 1997. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1997.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately poor water transparency impairs the Class A uses of Easter Lake; the somewhat elevated levels of chlorophyll-a are also of concern at this lake but do not appear to impair the Class A uses. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 70, 61, and 66, 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that nitrogen-availability and moderately high levels of inorganic turbidity seem to limit algal production. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Easter Lake is 11. This TN:TP ratio suggests that nitrogen limitation on algal production may exist at this lake.
Data from Downing et al. (2002), however, show relatively small populations of zooplankton species at this lake that graze on algae, thus suggesting that zooplankton grazing does not limit algal production. 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. The 2000 average summer mass of Cladoceran grazers (5.8 mg/l) was the 38th lowest of the 131 lakes sampled, thus suggesting that zooplankton grazers do not limit algae production at this lake. 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Easter Lake was 5.2 mg/l. This level of ISS, the 76ths highest of the 131 lakes sampled, is moderately high and may lead to increases in-lake turbidity that may limit algal production and that contribute to reductions in water transparency. Thus, the moderately high TSI for Secchi depth suggests an impairment of the Class A (primary contact) uses through reductions in water transparency caused primarily by inorganic suspended solids and secondarily by suspended algae.
Data from Downing et al. (2002) suggest the presence of nuisance algal species (i.e., bluegreen algae) at this lake but not at levels that suggest nuisance conditions or impairments to designated uses: 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 2000 summer average mass of bluegreen algae at this lake (3.9 mg/l) was the 37th lowest of the 131 lakes and thus does not suggest the potential for bluegreen algae to threaten or impair the designated uses of Easter Lake.
The eutrophic conditions at this lake, including high nutrient levels and moderate levels of inorganic turbidity, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) 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 thus be assessed (evaluated) as "partially supported." 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 (evaluated) as "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring conducted in 1997 (see assessment results for the 1998 report for more information).
