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 “not supported” due to aesthetically objectionable conditions caused by poor water clarity related to algal blooms and inorganic suspended solids.   High levels of pH that exceed state criteria also contribute to the impairment at this lake.   The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed state criteria, excessive nutrient loading to the water column, and impacts from organic enrichment.   Fish consumption uses remain “not assessed.”  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.

NOTE: A TMDL for algae and turbidity for East Lake was completed by IDNR and approved by EPA in 2008.   Because all Section 303(d) impairments identified for the 2008 and current (2010) assessment/listing cycle (algae and turbidity) are addressed by the TMDL, this waterbody remains in IR Category 4a (impaired; TMDL approved).

EXPLANATION: For the 2010 reporting cycle, the Class A1 (primary contact recreation) uses for East Lake are assessed (monitored) as “not supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program.   Using the median values from these surveys from 2004 through 2008 (approximately 28 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 70, and 87 respectively for East Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place East Lake in the hypereutrophic category.   These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were relatively high and suggest that non-algal turbidity contributes to the impairment at this lake.   The median level of inorganic suspended solids in East Lake (6.0 mg/L) was the 42nd highest median of the 132 lakes sampled by the ISU and UHL programs.

Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at East Lake, which contributes to an impairment due to nuisance aquatic life at this lake.   These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (36.8 mg/L) was also the 26th highest of the 132 lakes sampled.   This median is in the worst 25% of the 132 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 for the 2004-2008 period.    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.     The assessment category for 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 as “partially supported” due to violations of the Class B(LW) criteria for pH.   Information from the IDNR Fisheries Bureau also suggests that the aquatic life uses are “partially supported” due to algae blooms and poor water quality that put the lake at risk for a fish kill.   The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criteria for ammonia in 28 samples.   The data show 4 violations of the criterion for dissolved oxygen in 28 samples (14%).  Based on IDNR’s assessment methodology, however, these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the aquatic life uses due to dissolved oxygen.   Seven of the 28 pH samples (25%) violated the Class A1,B(LW) criterion for pH.   Based on IDNR’s methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class A1,B(LW) uses of East Lake.   These violations, however, likely reflect the excessive primary productivity at East Lake and do not reflect the input of pollutants into this lake.  

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