Otter Creek Lake IA 02-IOW-720

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Otter Creek Lake are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 18 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 71, 70, and 69 respectively for Otter Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Otter Creek Lake in between the Eutrophic and the Hypereutrophic categories. 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 data show 2 violations of the Class A1 criterion for pH in 18 samples (11%).

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).

The level of inorganic suspended solids was moderately high at Otter Creek Lake, and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Otter Creek Lake (5.35 mg/L) was ranked 97th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at Otter Creek Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 97% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (62.8 mg/L) was ranked 126th of the 138 lakes sampled. This median is in the worst 25% of the 138 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 from 2012-2016. 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. 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." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 18 samples(0%), one violation of the criterion for dissolved oxygen in 18 samples(6%), and 2 violations of the criterion for pH in 18 samples(11%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Otter Creek Lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.