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, 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 "not supported" due to extremely high levels of chlorophyll-a and the related algal blooms. In addition, the presence of very large populations of nuisance aquatic life (bluegreen algae) likely represents an additional impairment to the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1999. These assessments are consistent with those developed for the 2000 and 2002 reports. 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) and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1999.
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 the extremely poor water transparency caused by algal blooms and by high levels of non-algal 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 86, 79, and 83, respectively, for Silver Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) extremely high levels of phosphorus in the water column (sixth highest of the 131 lakes sampled), (2) extremely high levels of chlorophyll-a (second highest of the 131 lakes), and (3) extremely poor water transparency (third poorest 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. The generally good agreement between the TSI values for total phosphorus, chlorophyll-a, and Secchi depth does not suggest any particularly strong non-phosphorous limitation on algal production at this lake. Nonetheless, both the levels of inorganic suspended solids and the relatively low ratio of total nitrogen to total phosphorus may, at times, limit algal production at this lake. The levels of inorganic suspended solids at Silver Lake are moderately high and could be expected to exert at least some limitation to algal production. 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 (5.1 mg/l) was the 59th highest of the 131 lakes, thus suggesting that non-algal turbidity may limit the production of algae as well as contribute to the impairment of beneficial uses. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 10; this ratio is relatively low and does the suggest the potential for nitrogen availability to limit algal production at this lake. Data from Downing et al. (2002) show relatively small 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 10.2 mg/l at Silver Lake; this level was the 50th lowest of the 131 lakes sampled and thus likely suggests little 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 extremely poor water transparency that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
An additional impairment to the Class A uses designated for this lake is related to presence of nuisance aquatic life (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. The summer 2000 average mass of bluegreen algae at Silver Lake (750 mg/l) is the second highest of the 131 lakes sampled and is an order of magnitude higher than most other Iowa lakes. This level of bluegreen algae violates Iowa’s narrative water quality standard protecting against nuisance aquatic life. 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 "not supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The IDNR Fisheries Bureau states that, although the lake was renovated in fall 2003 (fish populations were removed), water quality conditions have not noticeably improved. 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). Fish consumption remain "fully supported" based on results of U.S. EPA/IDNR fish tissue monitoring in 1999 (see assessment for the 2000 report).
