Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau (3) ISU report on lake phytoplankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1999. See attached document for details.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "partially supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supported." Fish consumption remain "fully supported." These assessments are consistent with those developed for the 2000 report (see above). Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002) and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1999.
EXPLANATION: Results of monitoring conducted by ISU in 2000 and 2001 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are only "partially supported." Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 87, 80, and 83, respectively, for Silver Lake. According to Carlson (1977), these index values place this lake in the upper range of hyper-eutrophic lakes and suggest (1) excessive phosphorus loading to the water column, (2) excessive, although somewhat less than expected, production of suspended algae, and (3) very poor water transparency. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), entirely dominate the summertime phytoplankton community of Silver Lake. Sampling in 2000 showed that bluegreens comprised approximately 100% of the wet mass of the phytoplankton during the three summer samplings. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 8. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to occasional episodes of high levels of non-algal turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l. The median level of inorganic suspended solids at Silver Lake (18.9 mg/l) was the thirteenth highest of the 130 lakes, thus suggesting that non-algal turbidity may limit the production of algae as well as impair beneficial uses. Based on this information, turbidity-related impacts to the primary contact and aquatic life uses at this lake will be attributed to both suspended algae and re-suspended inorganic material.
The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The impacts to the aquatic life of this lake are demonstrated well by the inability of fish other than bullheads to survive in the lake (see assessment for the 2000 report above). Fish consumption remain "fully supported" based on results of U.S. EPA/IDNR fish tissue monitoring in 1999 (see assessment for the 2000 report above).
