Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "partially supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported." Fish consumption remain "not assessed." 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). 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 90, 72, and 79, respectively, for Silver Lake. According to Carlson (1977), these index values place this lake in the upper range of hyper-eutrophic lakes and suggest excessive levels of phosphorus, (2) excessive, but somewhat less than expected, production of suspended algae, and (3) 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), comprise a significant portion (approximately 35%) of the summertime phytoplankton community of Silver Lake. Sampling in summer 2000 showed that bluegreen algae comprised approximately 15% of wet mass of the phytoplankton community in the mid-June sample, 55% of the mid-July sample, and 30% of the early August sample. 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 (21.4 mg/l) was the seventh 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 "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, re-suspension of sediment, and organic enrichment. Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
