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 “not supported” due to aesthetically objectionable conditions caused by nuisance algae blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Fish consumption uses are “not assessed.” 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 uses at Otter Creek Lake are “not supported.” Using the median values from these surveys from 2006 through 2010 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 70, and 65 respectively for Otter Creek Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Otter Creek Lake in between the eutrophic and hypereutrophic categories, while the chlorophyll a value places Otter Creek Lake in the hypereutrophic cateogry. These values suggest very 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 relatively low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Otter Creek Lake was 3.0 mg/L, which was the 57th lowest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at Otter Creek Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (50.0 mg/L) was also the 25th 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 “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. The ISU and UHL lake surveys show that from 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 17 samples. There was one violation of the Class B(LW) criterion for dissolved oxygen in 17 samples (6%) and one violation of the Class A1,B(LW) criterion for pH in 17 samples (6%). Based on IDNR’s assessment methodology these violations were not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Otter Creek Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
