Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to poor water transparency caused by algae blooms and non-algal turbidity that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(LW) aquatic life uses are assessed (monitored) as “partially supported" due to violations of the Class B(LW) criterion for dissolved oxygen. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring at this lake. Sources of data for this assessment include: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at Windmill Lake are “partially supported” due to poor water transparency caused by algae blooms and non-algal turbidity. Using the median values from these surveys from 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 74, 66, and 72 respectively for Windmill Lake. According to Carlson (1977) the Secchi depth value places Windmill Lake in between the eutrophic and hypereutrophic categories while the chlorophyll a and total phosphorus values place Windmill Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
The level of inorganic suspended solids is moderately high at Windmill Lake and suggests that non-algal turbdity may contribute to the impairment at this lake. The median inorganic suspended solids concentration at Windmill Lake was 6.4 mg/L, which was the 37th highest of the 134 monitored lakes. The inorganic suspended solids levels are sufficiently high to continue the turbidity impairment for this lake and do not suggest de-listing of the turbidity impairment.
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at Windmill Lake, which causes a potential impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 50% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (45.0 mg/L) was the 31st highest of the 134 lakes sampled. This median is in the worst 25% of the 134 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values for the 2006-2010 period. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for dissolved oxygen. Results from these surveys show that during 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 16 samples. There were four violations of the dissolved oxygen criterion in 16 samples (25%). Based on IDNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment of the Class B(LW) uses. There was 1 violation of the Class A1,B(LW) pH criterion in 16 samples (6%). Based on IDNR’s assessment methodology this violation is not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses at Windmill Lake.
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 is 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.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
