Minnewashta Lake IA 06-LSR-1650

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 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 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: For the 2010 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 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 63, and 71 respectively for Lake Minnewashta.   According to Carlson (1977) the Secchi depth value places Lake Minnewashta in the eutrophic category while the chlorophyll a value places Lake Minnewashta in between the eutrophic and hypereutrophic categories and the total phosphorus value places Lake Minnewashta in the hypereutrophic categories.   These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and very 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 4.0 mg/L, which was the 65th highest of the 132 monitored lakes.

Data from the 2004-2008 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 46% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (12.7 mg/L) was the 58th lowest of the 132 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 2004 through 2008, there was one violation of the Class B(LW) criterion for ammonia in 24 samples (4%).   Based on IDNR’s assessment methodology this violation is not significantly greater than 10% of the samples and therefore does not suggest impairment of the Class B(LW) uses of Lake Minnewashta.   There were no violations of the Class B(LW) criterion for dissolved oxygen in 25 samples and one violation of the Class A1,B(LW) criterion for pH in 25 samples (4%).   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.