Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain assessed as "fully supported." 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue monitoring in 1998. Further monitoring and investigation are needed to better characterize water quality conditions at this lake.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may adversely affect the Class A and Class B(LW) uses of DeSoto Bend 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 74, 59, and 67, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is at the upper range of eutrophic lakes, and the index value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) excessive levels of phosphorus, (2) relatively low (and less than expected) levels of chlorophyll-a (suspended algae), and (3) moderately poor 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 DeSoto Bend Lake is 11). This TN:TP ratio suggest that algal production at this may be limited by nitrogen. In addition, the levels of inorganic suspended solids at this lake are moderately high for Iowa lakes and suggest the potential for limiting production of suspended algae. 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 DeSoto Bend Lake was 13.5 mg/l; this was the 26th highest level of ISS of the 130 lakes sampled. The relatively high TSI value for secchi depth suggest potential impairments to the Class A (primary contact) uses through presence of non-algal turbidity. The relatively low TSI value for chlorophyll-a (59) suggests that aesthetically objectionable blooms of algae are not a serious problem at this lake.
Also, data from Downing et al. (2002) suggest that, given the relatively low production of chlorophyll-a, the presence of nuisance algal species (i.e., bluegreen algae) does not impair beneficial uses at this lake. Sampling in 2000 showed the percent wet mass of bluegreens ranged from zero in the late June sampling, to approximately 70% in the late July sampling, then down to approximately 50% in the early September sampling.
The eutrophic to 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 excessive nutrient loading to the water column and due to high levels of non-algal turbidity. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. 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 assessed as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1998 (see assessment for the 2000 report).
