Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain "not assessed." 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). 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 non-algal turbidity may adversely affect the Class A and Class B(LW) uses of Meyers 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 72, 56, and 66, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) excessive levels of phosphorus, (2) relatively low (and less than expected) levels of chlorophyll-a (suspended algae), and (3) 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) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Meyers Lake is 12. This TN:TP ratio suggests that nitrogen limitation may affect algal production at this lake. Data from Downing et al. (2002) show a relatively small populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 5% of the dry mass of the zooplankton community in the samples from early August and late August. 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, however, are relatively low and do not suggest a strong potential for impairing designated uses or 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 Meyers Lake was 4.3 mg/l. Regardless of the source, the relatively high TSI values for secchi depth suggests potential impairments to the Class A (primary contact) uses through presence of non-algal turbidity. Also, the presence of nuisance algal species (i.e., bluegreen algae) may contribute to impairment of these uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 80% in the early July sampling to approximately 95% in the late August sampling. Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
