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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in May 2004.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency caused by inorganic suspended solids and algae blooms that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to a fish kill in 2004 and also due to excessive nutrient loading to the water column, nuisance blooms of algae, re-suspension of sediment, and organic enrichment. Fish consumption uses are “not assessed.” 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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in May 2004.
Note: A TMDL for algae and turbidity at Silver Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2014 assessment/listing cycles (algal growth and turbidity) are addressed by the TMDL, this waterbody will remain in IR Category 4a (impaired; TMDL approved) for the 2014 cycle.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Silver Lake are “not supported.” Using the median values from these surveys from 2009-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 70, and 76 respectively for Silver Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values place Silver Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and extremely high levels of phosphorus in the water column.
The level of inorganic suspended solids is extremely high at Silver Lake and suggests an impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Silver Lake was 23.9 mg/L, and ranked 132nd of the 134 monitored lakes.
Data from the 2008-2012 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Silver Lake, which contributes to the aesthetically objectionable conditions at this lake. These data show that cyanobacteria comprised 88% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (38.8 mg/L) and ranked 108th of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2008-2012. 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” due to excessive nutrient loading to the water column, nuisance blooms of algae, re-suspension of sediment, and organic enrichment and due to a fish kill in 2004. Information from the IDNR Fisheries Bureau suggests that Silver Lake has poor water quality and needs renovation and carp barriers to restore water quality. During 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 14 samples. Data from the ISU and UHL lake surveys show no violations of the Class B(LW) criterion for dissolved oxygen in 14 samples. The data show 2 violations of the Class A1,B(LW) criterion for pH in 22 samples (9%). 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 Silver Lake.
A fish kill occurred at Silver Lake on or before May 18, 2004. The cause of the kill was identified as “natural/unknown.” An estimated 100 fish were killed; no estimated value of the fish killed was provided. According to the IDNR investigation, the kill almost entirely affected bullheads; only a few dead buffalo were observed. No other game or rough fish species was observed. This is the same assessment as that developed for the 2008, 2010, and 2012 assessment/listing cycles. The continuance of the IR Category 3b listing for this impairment is based on IDNR's 2014 assessment methodology that states the following: the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2008-2011) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. Thus, this impairment will remain in Category 3b of Iowa’s 2014 Integrated Report.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
