Silver Lake IA 04-UDM-1229

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Silver Lake (Palo Alto) are assessed (monitored) as "not supported" due to poor water transparency, aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 72, and 80 respectively for Silver Lake (Palo Alto). According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Silver Lake (Palo Alto) 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 data show 5 violations of the Class A1 criterion for pH in 15 samples (33%).

Note: A TMDL for algae and turbidity at Silver Lake was prepared by DNR and approved by EPA in 2005. Thus, these impairments are moved from IR Category 5a (impaired; TMDL required) to Category 4a (impaired; TMDL approved). Because not all the impairments identified for this lake are covered by this TMDL (pH), this lake remains in IR Category 5a.

The level of inorganic suspended solids was extremely high at Silver Lake (Palo Alto), and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Silver Lake (Palo Alto) (19 mg/L) was ranked 132nd among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at Silver Lake (Palo Alto), which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 83% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (41.3 mg/L) was ranked 114th of the 138 lakes sampled. This median is in the worst 25% of the 138 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 2012-2016. 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 (monitored) as "not supported" due to violations of the Class B(LW) criterion for pH and (evaluated) as “partially supported” due to organic enrichment and due to a fish kill in 2004. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 5 violations of the criterion for pH in 15 samples(33%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Silver Lake (Palo Alto).

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 DNR 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, 2012, 2014, and 2016 assessment/listing cycles. The continuance of the IR Category 3b listing for this impairment is based on DNR's 2018 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 (2012-2015) 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 DNR 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 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 2018 Integrated Report.

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