Assessment Comments
Assessment is based on results of (1) ISU survey of Iowa lakes from 2000-02, (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000, (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1996, and (5) results of TMDL monitoring conducted by Iowa DNR in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "fully supporting / threatened" due to minor reductions in water transparency due to turbidity from algae and/or inorganic sources. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to nutrients in the water column and siltation impacts, especially in the upper portions of the lake. Fish consumption uses remain assessed (evaluated) as “fully supported.” based on fish contaminant monitoring in 1996. 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), (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1996, and (5) results of TMDL monitoring conducted by Iowa DNR in 2002.
EXPLANATION: 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 65, 58, and 60, respectively. According to Carlson (1977), the index value for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the upper range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, relatively low levels of chlorophyll-a (suspended algae), and relatively good water transparency. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 186, thus suggesting an overabundance of nitrogen in this lake. Based on approximately nine summer samples from the ISU surveys from 2000 through 2002, this lake had the third highest median total nitrogen concentration of the 131 lakes sampled. The levels of inorganic suspended solids at this lake during the 2000-02 period were somewhat elevated and suggest at least some potential for increasing in-lake turbidity and limiting production of suspended algae. Of 131 lakes sampled, Don Williams Lake had the 57th lowest median level of inorganic suspended solids (4.0 mg/l), thus suggesting that non-algal turbidity likely contributes to slight reductions in water transparency that threaten full support of the designated uses for this lake. Downing et al. (2002) show relatively small populations of zooplankton at Don Williams Lake as well as small populations of zooplankton (Cladocerans) known to graze on algae. Sampling in 2000 showed that the average mass of Cladocerans (0.3 mg/l) was the 7th lowest of the 131 lakes sampled. This small population of zooplankton grazers suggest little potential for non-phosphorus limitation on algal production at this lake. The slightly elevated TSI value for Secchi depth suggests a threat to full support of the Class A (primary contact) uses through presence of reductions in water transparency. 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 2000 average summer mass of bluegreen algae at this lake (4.3 mg/l) was the 39th lowest of the 131 lakes sampled, thus suggesting little potential for creating nuisance conditions that violate Iowa’s water quality standards.
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. Despite these impairments of Class B(LW) uses, results of monitoring conducted by IDNR/UHL in support of TMDL development at this lake from March to October 2002 showed no violations of Class B(LW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 10 samples collected. Fish consumption uses remain assessed (evaluated) 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).
