Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to slightly elevated levels of chlorophyll-a (algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to populations of bluegreen algae (nuisance (=noxious) aquatic life). 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: Results from the ISU statewide survey of Iowa lakes suggest that slightly elevated levels chlorophyll (algae) may threaten full support of the Class A and Class B uses of Lake Minnewashta. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 67, 60, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the upper boundary of eutrophic lakes, and the index value Secchi depth is in the middle range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, relatively low (and 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at Lake Minnewashta may be limited by a combination of non-algal turbidity, nitrogen limitation, and inorganic turbidity. None of these factors alone, however, would appear to explain the apparent non-phosphorus limitation on algal production at this lake. The levels of inorganic suspended solids at this lake are relatively low and, while not suggesting the potential for impairing designated uses, do suggest the potential for limiting algal production. 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 Lake Minnewashta was 3.2 mg/l. Nitrogen limitation may also serve as a non-phosphorus limitation to algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Minnewashta is 15. This TN:TP ratio suggests the possibility that nitrogen limitation may, at times, suppress the production of suspended algae at this lake. Data from Downing et al. (2002) show relatively small populations 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 2000 average summer mass of Cladocerans at this lake (9.2 mg/l) was the 46th lowest of the 131 lakes sampled.
The presence of nuisance algal species (i.e., bluegreen algae) at this lake does not appear to either threaten or impair designated uses. 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, the 2000 average summer mass of bluegreen algae at this lake (6.1 mg/l) was the 46th lowest of the 131 lakes sampled and thus does not suggest a significant water quality impact.
Information from the IDNR Fisheries Bureau suggests that Lake Minnewashta has high levels of bluegreen algae that impair the Class B(LW) aquatic life uses of this lake. Based on the relatively low levels of bluegreen algae reported as part of the ISU lake survey in 2000, however, the Class B(LW) uses will remain assessed as “fully supported / threatened.” Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
