Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported." The assessment of the Class B(LW) uses is consistent with the assessments developed for previous Section 305(b) reports (see above). Fish consumption remain "not assessed." 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 phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). 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 at this lake for total phosphorus, chlorophyll-a, and secchi depth are 97, 84, and 92, respectively. According to Carlson (1977), these index values place this lake in the upper range of hyper-eutrophic lakes and suggest extremely high levels phosphorus in the water column, excessive production of suspended algae, and extremely poor water transparency (all three TSI values for Swan Lake are the highest of the 130 Iowa lakes sampled in 2000 and 2001 as part of the ISU survey). These conditions indicate 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 completely dominate the summertime phytoplankton community of Swan Lake. Sampling in 2000 showed that bluegreens comprised 100% of the wet mass of the phytoplankon community during all three sample events. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 4. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to occasional episodes of very 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 Swan Lake (46.9 mg/l) was the highest of the 130 lakes, thus suggesting that non-algal turbidity limits the production of algae as well as impairs beneficial uses. Much of the suspended inorganic material in the water column of Swan Lake is believed due to a large population of common carp that re-suspend sediments and nutrients during feeding and spawning activities. Based on this information, turbidity-related impacts to the primary contact and aquatic life uses at this lake will be attributed to both suspended algae and re-suspended inorganic material. 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. According the the IDNR Fisheries Bureau, a new wetland has been constructed in the upper end of the lake; this wetland is expected to improve water quality. IDNR Fisheries has requested that a fish barrier be constructed at the lake outlet to prevent common carp from entering the lake (as has happened during high-flow periods in the past). Once this barrier is constructed, plans are to chemically renovate and restock the lake. Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
