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 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to extremely poor water transparency caused by high levels of inorganic turbidity and sediment re-suspension. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. This assessment is consistent with the assessments of support of aquatic life uses developed for previous Section 305(b) reports. Fish consumption uses are assessed as "fully supported" based on results of fish contaminant monitoring in 2001. 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 (RAFT) monitoring in 2001.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported." 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 78, 59, and 79, respectively, for Storm Lake. According to Carlson (1977), the index values for total phosphorus and secchi depth places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a, and extremely poor water transparency (the three-year median Secchi depth was 0.27 m). According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and Secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal (inorganic) particles do likely serve as the primary non-phosphorus factor limiting algal production at Storm Lake. 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 of inorganic suspended solids at Storm Lake (40.5 mg/l) was the third highest of the 131 lakes, thus suggesting that non-algal turbidity limits the production of algae as well as contributes to the extremely poor water transparency that impairs the Class A uses. The ratio of total nitrogen to total phosphorus (7) is very low and also suggests a limitation on the production of chlorophyll at this lake. Zooplankton grazing, however, does not appear to limit algal production at Storm Lake. 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 the average 2000 summer mass of Cladoceran grazers (0.9 mg/l) was the 12th lowest of the 131 lakes sampled. At these population levels, grazing of algae by zooplankters likely does not affect algal production. These conditions suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity, and the related reductions in water transparency, that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. According to the IDNR Fisheries Bureau, the ongoing dredging project at Storm Lake may reduce the levels of suspended sediment and improve water transparency.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), tend to dominate the summertime phytoplankton community of Storm Lake, especially in late summer). Sampling in 2000 showed the percent wet mass of bluegreens increased from just above 60% in the mid-June sampling to approximately 90% in early August sample. The 2000 average summer mass of bluegreen algae at this lake (1.2 mg/l), however, is the 15th lowest of the 131 lakes sampled and thus does not suggest a water quality impact.
The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption are assessed as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in August 2001. Results of this monitoring show that levels of all contaminants in the composite samples of fillets of channel catfish and walleye were far below ½ of the respective FDA action levels and IDNR levels of concern.
