Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to poor water transparency that violates Iowa’s narrative water quality standard to protect against aesthetically objectionable conditions. This poor water transparency is likely caused by a combination of blooms of algae and non-algal (inorganic) turbidity. Frequent violations of the water quality criterion for pH also suggest impairment of the Class A uses. The presence of very 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 are assessed (monitored) as "partially supporting" due to frequent violations of the Class B(LW) criterion for pH. Nutrient loading to the water column and non-algal turbidity remain concerns for support of the Class B(LW) uses at this 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A and Class B(LW) uses of Windmill Lake are impaired due to poor water transparency that violates Iowa’s narrative water quality standards protecting against aesthetically objectionable conditions. This poor water transparency is likely due to a combination of algal and non-algal (inorganic) sources of turbidity.
Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 77, 64, and 67, respectively. According to Carlson (1977), the index value for total phosphorus place this lake in range of hyper-eutrophic lakes; the values for chlorophyll-a and Secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high levels of chlorophyll-a (algal production), and relatively 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 limitations 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 2004 was 4.8 mg/l; the median level at Windmill Lake was 6.6 mg/l.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Windmill Lake is 10. This TN:TP ratio is low and suggests a good possibility that algal production at this lake is limited, at times, by nitrogen availability.
The presence of very large populations of zooplankton at Windmill Lake that graze on algae likely contribute to the discrepancy between the TSI value for total phosphorus (77) and that for chlorophyll-a (64). In terms of all Iowa lakes sampled, data from the ISU survey show very large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (165 mg/l) was the 29th highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Windmill Lake.
Thus, the TSI value for Secchi depth indicates poor water transparency and “partial support” of the Class A (primary contact) uses through presence of aesthetically objectionable conditions related both to blooms of algae and to inorganic turbidity.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Windmill Lake. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens (cyanobacteria) was approximately 90%. Also, the median per summer sample mass of bluegreen algae at this lake (33.8 mg/l) was the 29th highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests the potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
According to the IDNR Fisheries Bureau, both grass carp and common carp likely contribute to poor water clarity at this lake. The population of common carp, however, is relatively small, and thus are believed to have a relatively minor impact on this lake's water quality. Nutrient loading to the water column and non-algal turbidity remain water quality concerns at this lake.
The ISU lake survey data suggest an impairment due to high levels of pH at this lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 15 samples collected during summers of 2000 through 2004. Based on IDNR’s assessment methodology, these results do not suggest an impairment of aquatic life uses at this lake. Four of 15 samples, however, violated the Class A,B(LW) criterion for pH (maximum = 10.0; minimum = 7.3 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10% of the samples exceed Iowa’s pH criteria and thus suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations, however, likely reflect primary productivity at Windmill Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
