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 "partially supporting." 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 phytoplankton 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 Thayer 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 77, 56, and 68, 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 upper range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a (suspended algae), and moderately poor water transparency. The TSI value for chlorophyll-a is low relative to the TSI for total phosphorus and suggests some type of limitation to algal production. Factors limiting production of algae include nitrogen limitation, grazing by zooplankton, and/or high levels of non-algal turbidity. Based on median values of ISU lake sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus (TN/TP) at Thayer Lake is 9; this TN/TP ratio suggests that nitrogen limitation on algal production may occur at this lake. Data from Downing et al. (2002) show relatively large populations of zooplankton species at Thayer Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 90% of the dry mass of the zooplankton community in the mid-July sample and 60% in the early August sample. These data suggest that zooplankton populations may tend to suppress expression of chlorophyll as suspended algae. High levels of inorganic suspended solids also likely tend to suppress algal production. 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. Of 130 lakes sampled, Thayer Lake had the 21st highest median level of inorganic suspended solids (16.9 mg/l), thus suggesting that non-algal turbidity may limit the production of algae as well as contribute to impairments of both the primary contact recreation and aquatic life uses. These conditions indicate potential impairments to the Class A (primary contact) uses through presence high levels of non-algal turbidity. Also, data from Downing et al. (2002) suggest the presence of nuisance algal species (i.e., bluegreen algae) at this lake. Sampling in 2000 showed that 100% of the wet mass of the phytoplankton community at Thayer Lake was in bluegreen algae in the mid-July and early August samples. The moderately low levels of chlorophyll-a at this lake, however, suggest that impacts from bluegreen algae should be minor. The eutrophic to hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "partially supported" due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. 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.
