Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, 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 (monitored) as "partially supporting" due to moderately high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column and to moderately high levels of inorganic turbidity. Fish consumption uses remain assessed (monitored) as "fully supported" based on fish contaminant monitoring in 1998. 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, (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.
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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 74, 60, and 66, 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 limit 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) very high levels of phosphorus in the water column, (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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at DeSoto Bend Lake is potentially limited by nitrogen availability, by zooplankton grazing, and by inorganic turbidity. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for DeSoto Bend Lake is 10. This TN:TP ratio suggest the possibility that algal production at this may be limited by nitrogen availability. In addition, Downing et al. (2002) show very large populations of zooplankton at DeSoto Bend Lake (4th highest of the 131 lakes sampled), including a significant populations of species (Cladocerans) that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 15% of the dry mass of the zooplankton community at this lake; the 2000 average mass of Cladocerans (42.8 mg/l),however, was the 31st highest of the 131 lakes sampled. This large population of zooplankton grazers suggest an additional potential non-phosphorus limitation on algal production at this lake. The levels of inorganic suspended solids at this lake are moderately high for Iowa lakes and also suggest the potential for limiting production of suspended algae through light attenuation. 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 DeSoto Bend Lake was 8.7 mg/l; this was the 34th highest level of ISS of the 131 lakes sampled. Thus, the moderately high TSI value for secchi depth (66) suggests potential impairments to the Class A (primary contact) uses through presence of non-algal turbidity. The relatively low TSI value for chlorophyll-a (60) suggests that aesthetically objectionable blooms of algae are not a serious problem at this lake. The IDNR Fisheries Bureau concurs that high levels of inorganic turbidity present an impairment of designated uses 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 2000 average mass of bluegreen algae at this lake (5.1 mg/l) was the 42nd lowest of the 131 lakes sampled and thus does not suggest a significant problem with bluegreen algae.
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. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1998 (see assessment for the 2000 report).
