Minnewashta Lake IA 06-LSR-1650

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Minnewashta are assessed (monitored) as "partially supported" due to 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 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 56, 65, and 65 respectively for Lake Minnewashta. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Minnewashta in the Eutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was relatively low at Lake Minnewashta, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Minnewashta (2.8 mg/L) was ranked 59th 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 Lake Minnewashta, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 73% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (43.8 mg/L) was ranked 115th 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 (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the DNR Fisheries Bureau, this lake was infested with Zebra mussels in 2012 and the population exploded in 2015. This invasive species will likely cause water quality to degrade in the future. This potential impairment is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation.Results of the ISU lake survey from 2012-2016 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 no violations of the criterion for pH in 15 samples(0%). 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 Lake Minnewashta.

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