Minnewashta Lake IA 06-LSR-1650

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 “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  The fish consumption uses are “not assessed” due to a lack of fish contaminant monitoring at this lake.   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: For the 2012 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Minnewashta are assessed as “fully supported” based on information from the ISU lake survey and UHL lake monitoring program.   Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 60, and 69 respectively for Lake Minnewashta.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Minnewashta in between the eutrophic and hypereutrophic categories.   These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.  

Based on data from the ISU and UHL lake surveys, the level of inorganic suspended solids was moderately high at this lake and does not suggest impairment due to high non-algal turbidity.   The median inorganic suspended solids concentration at Lake Minnewashta was 3.2 mg/L, which was the 61st lowest of the 134 monitored lakes.

Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Minnewashta, which does not cause an impairment at this lake.   These data show that cyanobacteria comprised approximately 100% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (32.9 mg/L) was the 44th highest of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Based on data from the ISU and UHL lake surveys from 2006 through 2010, there were no violations of the Class B(LW) criterion for ammonia in 22 samples or for dissolved oxygen in 21 samples.   There was one violation of the Class A1,B(LW) criterion for pH in 22 samples (5%).   Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment 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.