Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to extremely high levels of inorganic turbidity and due to algal blooms. In addition, the presence of nuisance (noxious) aquatic life (bluegreen algae) presents a threat to full support of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "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 fish contaminant monitoring at this lake. 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported" due to algal blooms and high levels of inorganic turbidity. 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 89, 73, and 83, respectively, for Silver Lake. According to Carlson (1977), all three index values place this lake in the range of hyper-eutrophic lakes and suggest extremely high levels of phosphorus (second highest of the 131 lakes sampled), (2) very high, but somewhat less than expected, production of suspended algae (eighth highest of the 131 lakes), and (3) extremely poor water transparency (second worst of the 131 lakes sampled). These TSI values suggest that Silver Lake has some of the poorest water quality of any lake in the state. 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. The median level of inorganic suspended solids at Silver Lake (26.0 mg/l) was the seventh highest of the 131 lakes, thus suggesting that inorganic (non-algal) turbidity may limit the production of algae as well as cause reductions in water transparency that impair beneficial uses. In addition to the non-phosphorus limitation on algal production created by the very high levels of inorganic turbidity at this lake, algal production may also be limited, although likely to a lesser extent, by nitrogen availability and by zooplankton grazing. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 9. This relatively low ratio suggests that algal production at Silver Lake is also limited by nitrogen availability. 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 20.3 mg/l at Silver Lake; this level was the 51st highest of the 131 lakes sampled and thus suggests at least the potential for zooplankton grazers to limit algal production. Thus, the water quality conditions at Silver Lake indicate impairments to the Class A (primary contact) uses through presence of algal blooms and levels of inorganic turbidity that contribute to extremely poor water transparency at this lake, thus violating Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs with this assessment.
The presence of nuisance algal species (i.e., bluegreen algae) at Silver suggest a potential threat to the full support of Class A uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 35%) of the summertime phytoplankton community of this 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. The summer 2000 average mass of bluegreen algae at Silver Lake (30.2 mg/l) was the 32nd highest of the 131 lakes sampled. This mass of bluegreens is high relative to most other Iowa lakes and strongly suggests a threat to the full support of the designated uses of this lake. The amount of data available for characterizing algal populations at this lake (one season), however, 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 update this assessment and improve the accuracy of future water quality assessments for this lake.
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 fish contaminant monitoring at this lake.
