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.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." The Class C (drinking water) uses are "not assessed." Fish consumption uses remain assessed as "fully supported." The 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, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1999. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal and non-algal turbidity may adversely affect the Class A and Class B(LW) uses of Silver Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 78, 60, and 76, respectively. According to Carlson (1977), the index values total phosphorus and secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is at upper limit of eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, relatively low (and less than expected) levels of chorophyll-a, and very poor water transparency. 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. Of 130 lakes sampled, Silver Lake had the 25th highest median level of inorganic suspended solids (14.6 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to impairments of both the primary contact recreation and aquatic life uses. The excessive levels of total phosphorus in the water column indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. In addition, the presence of nuisance algal species (i.e., bluegreen algae) present an additional cause of impairment. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of Silver Lake and is the only algal group found in mid to late summer). Sampling in 2000 showed the percent wet mass of bluegreens increased from approximately 60% in the mid-June sampling, to comprise 100% of the wet mass in the mid-July sampling and early August samples. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 28. Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses remain assessed as "fully supported / threatened" (see previous assessments). Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The Class C (drinking water) uses are "not assessed" due to the lack of recent information upon which to base an assessment. The previous assessment of the Class C uses was based on monitoring conducted in 1995 (see assessments for the 1996 report), and these data are now considered too old (greater than five years) for characterizing current water quality conditions. Fish consumption uses remain assessed as "fully supported" based on results of U.S. EPA/IDNR fish contaminant monitoring in 1999 (see assessment for the 2000 report).
