Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities.
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 / remain 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 phosphorus may threaten full support of the Class A uses of Otter Creek Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 55, and 60, 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 middle range of eutrophic lakes, while the value for secchi depth is at the upper boundary of eutrophic lakes. These index values suggest moderately high levels of phosphorus but relatively low levels of chlorophyll-a and better than expected water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Otter Creek Lake is 18.0. This ratio suggests the possibility that algal production at this lake is limited by nitrogen. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing contributing to increased turbidities. 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 Otter Creek Lake was 1.9 mg/l; this is the 12th lowest median value of the 130 lakes sampled. Despite the moderately high levels of phosphorus, the TSI values for chlorophyll-a and secchi depth suggest relatively good water quality. The moderately high TSI values for total phosphorus, however, suggests a threat to the full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. 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) comprise a relatively small portion of the summertime phytoplankton community of this lake. Sampling in summer 2000 showed that bluegreen algae accounted for one-quarter or less of the wet mass of phytoplankton at 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 should be assessed as "fully supported / threatened" due to potential impacts from siltation and excessive nutrient loading to the water column. 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. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
