Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.
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 "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). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class A uses of Hannen 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 74, 52, and 49, 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, however, places this lake in the lower range of eutrophic lakes. The TSI value for secchi depth falls in the upper range of mesotrophic lakes. These index values suggest (1) very high levels of phosphorus, (2) low, and much less than expected, production of suspended algae, and (3) exceptional, and much better than expected, 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 Hannen Lake is 12. This ratio suggests that algal production at this lake is, in part, limited by availability of nitrogen. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses due to high turbidities. In addition, data from Downing et al. (2002) show very large populations of zooplankton species at Hannen 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 both the early August and early September samples. These results suggest the that the relatively low algal production at this lake is partially due to zooplankton grazing. 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 Hannen Lake was 2.3 mg/l. Despite the indications of good water quality, the high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through the potential presence of aesthetically objectionable blooms of algae and the potential presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) entirely dominate the summertime phytoplankton community of this lake (greater than 95% by wet mass). Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact at this time. 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 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 (evaluated) as "fully supported / threatened" due to siltation in the lake. Fish consumption uses remain "not assessed " due to the lack of fish contaminant monitoring at this lake.
