Tuttle Lake (a.k.a. Okamanpeedan Lake) IA 04-EDM-975

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Tuttle 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 83, 72, and 77 respectively for Tuttle Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Tuttle Lake in the Hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and extremely high levels of phosphorus in the water column. The data show 2 violations of the Class A1 criterion for pH in 15 samples.

Note:  A TMDL for algae and turbidity at Tuttle Lake was prepared by IDNR and approved by EPA in 2005, and these impairments for this lake were thus moved to IR Category 4a for the 2006 cycle.   This lake remains in Category 4a of the 2016 cycle.

The level of inorganic suspended solids was extremely high at Tuttle Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Tuttle Lake (22 mg/L) was ranked 132nd among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a large population of cyanobacteria exists at Tuttle Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 92% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (113.1 mg/L) was ranked 131st 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 2010-2014. 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." The ISU lake survey results show poor chemical water quality at Tuttle Lake due to excessive nutrient loading to the water column, organic enrichment, nuisance blooms of algae, and re-suspension of sediment.   Information from the IDNR Fisheries Bureau also suggests poor water quality at Tuttle Lake.  Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 2 violations of the criterion for pH in 15 samples(13%). Based on IDNR'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 Tuttle Lake.

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