Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau (3) ISU report on lake plankton communities in 2000, (4) results of U.S. EPA/IDNR fish tissue monitoring in 1999, and (5) results of a drinking water study in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to high levels of inorganic turbidity that contribute to the extremely poor water transparency at this lake. In addition, the presence of nuisance (noxious) aquatic species (bluegreen algae) present a threat to full support of these uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." The Class C (drinking water) uses are assessed (evaluated) as “fully supported.” Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1999. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), (4) results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1999, and (5) results of a drinking water study conducted by UHL in January 1995.
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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 77, 58, and 80, 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 in the upper range of eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a, and extremely poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Silver Lake. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Silver Lake had the 25th highest median level of inorganic suspended solids (19.0 mg/l), thus suggesting that non-algal turbidity is the primary factor that limits the production of algae and that contributes to the turbidity-related impairments of both the primary contact recreation and aquatic life uses. In addition to inorganic turbidity, other factors appear to limit algal production at Silver Lake. For example, algal production at this lake may also be limited by nitrogen limitation. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 12. This ratio suggest that algal production at Silver Lake is, at times, limited by nitrogen availability. In addition, the presence of large populations of zooplankton grazers may limit algal production. Data from Downing et al. (2002) show moderately large populations of zooplankton species (Cladocerans) at this lake that graze on algae. Sampling in 2000 showed that the average summer mass of Cladoceran taxa (e.g., Daphnia) was 31.8 mg/l at Silver Lake; this level was the 41st highest of the 131 lakes sampled and thus suggests at least the potential for zooplankton grazers to limit algal production.
The presence of nuisance algal species (i.e., bluegreen algae) at Silver Lake presents a threat to the full support of designated uses at this lake. 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. The 2000 summer average mass of bluegreen algae at this lake (21.7 mg/l) was the 42nd highest of the 131 lakes sampled and suggests a threat to full support of the designated uses for this lake. Although results of plankton sampling in 2000 suggest that designated uses are threatened by these nuisance aquatic algae, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses remain assessed as "fully supported / threatened" (see previous assessments). The Class C (drinking water) uses remain assessed (evaluated) as "fully supported" based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence). The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 2.3 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a more recent assessment of support of the Class C uses. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of U.S. EPA/IDNR fish contaminant monitoring in 1999 (see assessment for the 2000 report).
