Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake phytoplankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Using the median values from the ISU lakes survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 76, 55, and 50, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth, however, place this lake in the middle and lower ranges, respectively, of eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus in the water column, (2) low, and much less than expected, production of suspended algae, and (3) very good, 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results from ISU monitoring suggest non-phosphorus limitation at Hannen Lake is due to nitrogen limitation and to large populations of zooplankton that graze on algae. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Hannen Lake is 10. 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. 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 Hannen Lake (2.3 mg/l) was the 32nd lowest of the 131 lakes sampled and does not suggest the likelihood that inorganic turbidity limits algal production at this lake. However, 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. The 2000 average summer mass of Cladocerans (111 mg/l) was the 10th highest of the 131 lakes samples and thus suggest that the that the relatively low algal production at this lake is partially due to zooplankton grazing. Despite the high TSI value for total phosphorus, the index values for chlorophyll-a and Secchi depth suggest relatively good water quality and full support of the Class A (primary contact) uses.
Results of ISU monitoring suggest that the level of bluegreen algae at this lake are somewhat elevated but do not either threaten or impair designated uses. 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). The 2000 average summer mass of bluegreen algae (8.6 mg/l) was the 57th lowest of the 131 lakes sampled. This level does not appear to threaten the Class A or Class B uses. 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.
