Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 2008 assessment/listing cycles (algal growth and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) for the 2008 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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 80, 70, and 76 respectively for Silver Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all 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 28.5 mg/L, which was the 2nd highest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Silver Lake, which could contribute to the impairment at this lake. These data show that cyanobacteria comprised only 54% of the phytoplankton wet mass at this lake. However, the median cyanobacteria wet mass (23.4 mg/L) was the 53rd highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed 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. During 2002-2006 there was 1 violation of the Class B(LW) criterion for ammonia in 16 samples. Based on IDNR’s assessment methodology a single violation of the Class B(LW) criterion for ammonia does not suggest impairment of the Class B(LW) uses. Data from the ISU and UHL lake surveys show no violations of the Class B(LW) criterion for dissolved oxygen in 23 samples and 2 violations of the Class A1,B(LW) criterion for pH in 23 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. According to IDNR’s assessment/listing methodology, 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 (2004-2007) 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.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
