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 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to aesthetically objectionable blooms of algae and non-algal (inorganic) turbidity that contributes to very poor water transparency. The presence of extremely large populations of nuisance aquatic life (i.e., bluegreen algae) suggests an additional impairment to the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column and non-algal turbidity in the lake. Assessments based on results from the ISU lake survey do not agree well with information provided by the IDNR Fisheries Bureau. 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 high levels of algal and non-algal (inorganic) turbidity impair the Class A and Class B(LW) uses of Windmill Lake. 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 77, 70, and 70, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the lower range of hyper-eutrophic lakes and suggest extremely high levels of phosphorus in the water column, very high levels of chlorophyll-a (algal production), and very poor 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 of ISU monitoring suggest that the moderately high level of inorganic suspended solids, as well as nitrogen availability and zooplankton grazing, are the most likely non-phosphorus limitation on algal production at this lake. The levels of inorganic suspended solids at Windmill Lake are moderately high and thus suggest the potential for limiting algal production as well as contributing to in-lake turbidity and reductions in water transparency. 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 Windmill Lake was 6.8 mg/l. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Windmill Lake is 11. This TN:TP ratio is low and suggests a strong possibility that algal production at this lake is limited by nitrogen availability. Data from Downing et al. (2002) show moderately large populations of zooplankton species at this lake that graze on algae. The summer 2000 average mass of Cladocerans (26.3 mg/l) was the 49th highest of the 131 lakes sampled, thus suggesting the potential for zooplankton grazers to limit algal production at this lake. Thus, the TSI values for chlorophyll-a and Secchi depth indicate potential impairments (partial support) of the Class A (primary contact) uses through presence of aesthetically objectionable conditions related to blooms of algae and to inorganic turbidity that leads to very poor water transparency.
In addition to the impairments suggested by TSI values for chlorophyll-a and Secchi depth, the presence of extremely large populations of nuisance algal species (i.e., bluegreen algae) suggests an additional impairment to the Class A uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) entirely dominate the summertime phytoplankton community of Windmill Lake. The summer 2000 average mass of bluegreen algae at this lake (190 mg/l) was the 6th highest of the 131 lakes sampled and thus strongly suggests a violation of Iowa’s narrative water quality standard protecting against nuisance aquatic life.
In contrast to the impairments suggested by trophic state index values, information from the IDNR Fisheries Bureau, suggest that Windmill Lake has good water quality relative to other lakes in southwest Iowa and that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported / threatened" due to nutrient loading to the water column and non-algal turbidity in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
