Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 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 “fully supported." 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 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 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 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 67, and 72 respectively for Windmill Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Windmill Lake in between the eutrophic and hypereutrophic categories while the total phosphorus value places Windmill Lake in the hypereutrophic category. These values suggest 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 6.0 mg/L, which was the 40th highest of the 132 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 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Windmill Lake, which does not cause to an impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 67% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (13.4 mg/L) was the 64th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Results from these surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 18 samples. There were three violations of the dissolved oxygen criterion in 19 samples (16%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses. There were also 3 violations of the Class A1,B(LW) pH criterion in 19 samples (16%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses at Windmill Lake. The lake was listed as Section 303(d) impaired for the 2006 IR cycle due to violations of the pH criterion, however significantly greater than 10% of the samples did not violate the pH criterion in the 2008 assessment and now the 2010 assessment. Therefore, based on IDNR's assessment methodology the pH impairment is suggested for de-listing for the 2010 assessment/listing cycle and the Class B(LW) uses are assessed as "fully supported."
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.
