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 "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 phytoplankton 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 and Class B uses of Lake Minnewashta. 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 69, 56, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a, and relatively good (and 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 Lake Minnewashta is 12). This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show moderately large of zooplankton species at this lake that are potential grazers of suspended algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 25% of the dry mass of the zooplankton community in both the mid-July and early August samples. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. 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 Lake Minnewashta was 2.6 mg/l. Despite the indications of good water quality, the high TSI value for total phosphorus suggests potential threats to the full support of the Class A (primary contact) and/or Class B uses through presence of aesthetically objectionable blooms of algae and due to the presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake with greater than 95% of wet mass in bluegreen algae during all three samplings in summer 2000. However, these values, given the relatively low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae at this lake. 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
