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 phytoplankton communities, 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 "partially supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported." This assessment is consistent with the assessments of support of aquatic life uses developed for previous Section 305(b) reports (see above). Fish consumption uses are assessed as "fully supported." 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 phytoplankton 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 in 2000 and 2001 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are only "partially supported." Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 83, 58, and 80, respectively, for Storm Lake. According to Carlson (1977), the index values for total phosphorus and secchi depth places this lake in the mid to upper 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 less than expected) levels of chlorophyll-a, and very poor water transparency. Given the high levels of phosphorus, the relatively low index for chlorophyll-a indicates less than expected production of suspended algae, probably due to high levels of turbidity related to suspended inorganic material in the water column or due to nitrogen limitation. Data on inorganic suspended solids from the ISU survey suggest that this lake is subject to high levels of non-algal turbidity. 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 of inorganic suspended solids at Storm Lake (40.6 mg/l) was the second highest of the 130 lakes, thus suggesting that non-algal turbidity limits the production of algae as well as impairs beneficial uses. The relatively low ratio of total nitrogen to total phosphorus (5) also suggests a limitation on the production of chlorophyll. The presence of the extremely high levels of total phosphorus in the water column indicates potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). 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. Based on this information, turbidity-related impacts to the primary contact and aquatic life uses at this lake will be attributed primarily to non-algal turbidity and secondarily to suspended algae.
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.
