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 "fully supporting / threatened" due to minor reductions in water transparency related to slightly elevated levels of algae and inorganic suspended solids. The Class B(LW) aquatic life uses are / remain assessed (evaluated) as "fully supporting / threatened" due to nutrients and siltation. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. The 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: Using the median values from the ISU survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 58, and 61, 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 upper range of eutrophic lakes, while the value for Secchi depth is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus, relatively low levels of chlorophyll-a, and marginally good 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that while neither nitrogen-limitation nor zooplankton grazing limit algal production, the level of inorganic suspended solids may be sufficiently high to influence algal production. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Otter Creek Lake is 18.0. This ratio suggests does not suggest a strong possibility that algal production at this lake is limited by nitrogen availability. Data from Downing et al. (2002) show relatively small populations of zooplankton species at Otter Creek Lake that graze on algae (Cladocerans). The 2000 average summer mass of Cladocerans at this lake (1.6 mg/l) was the 17th lowest of the 131 lakes sampled, thus suggesting that zooplankton grazing does not limit algal production at this lake. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. These levels may, however, be sufficiently high to contribute to increased turbidities and reduced water transparencies. 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 at Otter Creek Lake was 2.2 mg/l; this is the 12th lowest median value of the 131 lakes sampled. Despite the moderately high levels of phosphorus, the TSI values for chlorophyll-a and secchi depth suggest relatively good water quality. The slight reduction in water transparency, however, suggests a threat to the full support of the Class A (primary contact) uses.
The presence of nuisance algal species (i.e., bluegreen algae) 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 2000 summer average mass of bluegreen algae at this lake (2.9 mg/l) was the 27th lowest of the 131 lakes sampled, thus suggesting little impact from these nuisance algae at this lake.
The 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
