Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities from 2000-05, and (4) an IDNR investigation of a fish kill in May 2004.
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. 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of an IDNR investigation of a fish kill in May 2004.
Note: A TMDL for algae and turbidity at Silver Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, this waterbody is moved from IR Category 5a from the 2004 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2006 cycle.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2004 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 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 80, 72, and 80, 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 (17th 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 (sixth 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 2004 was 5.2 mg/l. The median level of inorganic suspended solids at Silver Lake (36.7 mg/l) was the third highest of the 131 lakes, thus suggesting that inorganic (non-algal) turbidity limits 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 2004, the ratio of total nitrogen to total phosphorus for this lake is 15. This moderately low ratio suggests that algal production at Silver Lake may, at times, be limited by nitrogen availability.
In addition, the presence of very large populations of zooplankton at Silver Lake that graze on algae may explain the discrepancy between the TSI value for phosphorus (80) and that for chlorophyll-a (72). In terms of all Iowa lakes sampled, data from the ISU survey show large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately just about one-third of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (180 mg/l) was the 22nd highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Silver Lake.
Thus, the water quality conditions at Silver Lake indicate impairments to the Class A (primary contact) uses through presence of severe algal blooms and extremely high 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 levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (about 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Silver Lake (22.8 mg/l) was the 39th highest of the 131 lakes sampled. This levels is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
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. The ISU lake survey data also show generally good chemical water quality at this lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 15 samples collected (7%) during summers of 2000 through 2004. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as dissolved oxygen nonetheless suggests "full support" of aquatic life uses. Thus, the percentage of violations of the dissolved oxygen criterion at this station does not suggest an impairment of aquatic life uses at this lake. Three of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.4; minimum = 7.7 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Silver Lake and do not reflect the input of pollutants into this lake.
A fish kill occurred at Silver Lake on or before May 18, 2004. The cause of the kill was identified as “natural/unknown”. An estimated 100 fish were killed; no estimated value of the fish killed was provided. According to the IDNR investigation, the kill almost entirely affected bullheads; only a few dead buffalo were observed. No other game or rough fish species was observed. According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).
Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
