Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for the algae impairment at this lake was prepared and approved by EPA in September 2014. The approval of this TMDL moves the impairment due to algae from Iowa's list of Section 303(d) waters (Category 5a of the Integrated Report) to IR Category 4a (impaired; TMDL not required).
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 2008-2012 (approximately 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 70, and 65 respectively for Otter Creek Lake. According to Carlson (1977) the total phosphorus value places Otter Creek Lake in between the eutrophic and hypereutrophic categories, while the chlorophyll a and Secchi depth values place Otter Creek Lake in the hypereutrophic cateogry. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is moderately high at this lake but does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Otter Creek Lake was 4.5 mg/L, and ranked 76th of the 134 monitored lakes.
Data from the 2008-2012 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) and ranked 119th 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 2008-2012 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 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 17 samples. There were two violations of the Class B(LW) criterion for dissolved oxygen in 15 samples (13%) and no violations of the Class A1,B(LW) criterion for pH in 15 samples. 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.
