Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are also assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain "not assessed." 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may threaten full support of the Class A uses of Slip Bluff Lake. Using the median values from the Iowa State University statewide surveys of Iowa lakes in 2000 and 2001 (approximately six samples), Carlson's trophic state indices for total phosphorus and chlorophyll-a at Slip Bluff Lake are 66, 41, and 63 , respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range of mesotrophic lakes, and the index value for secchi depth is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus, extremely low (and much less than expected) production of suspended algae, and moderately poor water transparency. According to Carlson (1991), the occurrence of a high TSI values for total phosphorus and secchi depth, with a relatively low value for chlorophyll-a, suggest that non-algal turbidity limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Slip Bluff Lake is 11.5. This ratio suggests that the availability of nitrogen limits production of algae. The levels of inorganic suspended solids at this lake are moderately high and suggest the potential for contributing to problems with in-lake turbidity. 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; the median level at Slip Bluff Lake was 9.2 mg/l: this is the 39th highest median value of the 130 lakes sampled. Despite the indications of good water quality in terms of low chlorophyll-a, the high TSI value for total phosphorus, and the moderately high value for secchi depth, suggest threats to the full support of the Class A (primary contact) uses through presence of non-algal turbidity. The presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta)in general, comprise a relatively small portion of the summertime phytoplankton community of this lake. Sampling in 2000 showed no bluegreens present in the late June, an increase to over 70% of wet mass in bluegreens in late July, and a decrease to approximately 40% of wet mass in late August. These values, given the relatively extremely low levels of chlorophyll-a (i.e., suspended algae) at this lake, do not suggest significant problems with bluegreen algae at this lake. Data from Downing et al. (2002) show relatively small populations of zooplankton species at Slip Bluff Lake that graze on algae, thus suggesting a limited ability of zooplankton to reduce algal production through grazing. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 5% of the dry mass of the zooplankton community in the late July sample but increased to approximately 30% in the late August sample. The slightly eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported" threatened" due to siltation impacts at the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. 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.
