Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01 and (2) ISU report on lake plankton communities.
Basis for Assessment
Not assessed as part Section 305(b) reporting prior to the 2002 cycle. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting." Fish consumption uses are "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) and (2) 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 the Class B(LW) uses of South Prairie Lake are "fully supported." 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, 49, and 54, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a in the upper range of mesotrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes. These index values suggest relatively high levels of phosphorus, extremely low (and much lower than expected) levels of chlorophyll-a, and very good (and better than expected) water transparency. These results suggest that this lake does not have threats or impairments due to aesthetically objectionable blooms of algae or to nuisance aquatic species (e.g., bluegreen algae). 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 South Prairie Lake is 9). This ratio suggests that algal production at this lake is limited by nitrogen. Data from Downing et al. (2002) show relatively small populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised less than 10% of the dry mass of the zooplankton community in the early August and early July samples. At these population levels, grazing of phytoplankton by zooplankters likely does not affect water transparency. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. 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 South Prairie Lake was 3.8 mg/l. This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid to late summer. Sampling in 2000 showed that bluegreen algae comprised approximately 10% of the wet mass of the phytoplankton community in the early June sample but increased to 70% and 90% in the early August and early September samples, respectively. Although bluegreen algae comprise a significant portion of the summertime phytoplankton community, the production of chlorophyll is very low at this lake, and the presence of bluegreen algae likely does not present a water quality impact. 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 are "not assessed" due to the lack of fish contaminant monitoring at this lake.
