Minnewashta Lake IA 06-LSR-1650

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: For the 2014 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 2008-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 56, 55, and 61 respectively for Lake Minnewashta.   According to Carlson (1977) the Secchi depth and chlorophyll a values place Lake Minnewashta in the eutrophic category, while the total phosphorus value places Minnewashta Lake in betweent the eutrophic and hypereutrophic categories.   These values suggest relativley low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately 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 3.6 mg/L, and ranked 67th of the 134 monitored lakes.

Data from the 2008-2012 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Lake Minnewashta, which does not cause an impairment at this lake.   These data show that cyanobacteria comprised approximately 59% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (14.6 mg/L) and rankedk 47th 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 20008-2012, there were no violations of the Class B(LW) criterion for ammonia in 14 samples or for dissolved oxygen in 14 samples.   There were no violations of the Class A1,B(LW) criterion for pH in 14 samples.   These results suggest "full support" 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.