Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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. Violations of the Class A1 criterion for pH also contribute to the impairment at this lake. The Class B(LW) aquatic life uses are assessed (monitored) as “partially supported” due to violations of the state’s pH criterion. 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 2002 through 2006 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 67, and 68 respectively for Windmill Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Windmill Lake in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and 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 44th 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 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Windmill Lake, which may contribute to the impairment at this lake due to high levels of algal turbidity. These data show that cyanobacteria comprised 55% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (22.1 mg/L) was the 56th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported.” Data from the ISU and UHL lake surveys show relatively good chemical water quality at Windmill Lake. Results from these surveys show that during 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 13 samples, two violations of the dissolved oxygen criterion in 20 samples (10%), and 2 violations of the pH criterion in 20 samples (10%). 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. However, Windmill Lake was assessed as “partially supporting” in the 2006 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. The pH violations at Windmill Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this 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.
