Assessment Comments
Assessment is based on (1) surveys of DNR Fisheries Bureau and (2) results of fish tissue (RAFT) monitoring in 1996.
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 remain assessed (evaluated) as "partially supporting." Fish consumption uses remain assessed (evaluated) 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). 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 algal turbidity threaten the Class A uses of Don Williams 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 66, 64, and 63, respectively. According to Carlson (1977), all three index values places this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, levels of chlorophyll-a (suspended algae), and moderately poor water transparency. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 134, thus suggesting an overabundance of nitrogen in this lake. Based on approximately six samples from the ISU surveys in 2000 and 2001, this lake had the 8th highest median total nitrogen concentration of the 130 lakes sampled. The levels of inorganic suspended solids at this lake are moderately high and suggest at least the potential for increasing in-lake turbidity and limiting production of suspended algae. Of 130 lakes sampled, Don Williams Lake had the 42nd highest median level of inorganic suspended solids (8.1 mg/l), thus suggesting that non-algal turbidity may contribute to turbidity-related impairments of both the primary contact recreation and aquatic life uses. The moderately high TSI values for total phosphorus and chlorophyll-a suggest threats to full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. 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 relatively small portions of the phytoplankton community in early and mid-summer but increase to comprise over 50% of wet mass by late summer.
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 "partially supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake.
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.
Fish consumption uses remain assessed as “fully supported” based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring results of fish contaminant monitoring conducted in 1996. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively lower confidence).
