Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to extremely large populations of suspended algae and extremely high levels of inorganic turbidity that contribute to aesthetically objectionable conditions at this lake. An additional impairment to the Class A uses is due to the extremely high populations of nuisance aquatic life (bluegreen algae) at this lake. The Class B(LW) aquatic life uses remain assessed (monitored) as "partially supported" due to the increasingly poor recreational fishery related to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The assessment of the Class B(LW) uses is consistent with the assessments developed for previous Section 305(b) reports. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
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" due to extremely poor water transparency at this lake related to both algal and non-algal turbidity. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices at this lake for total phosphorus, chlorophyll-a, and secchi depth are 92, 81, and 87, respectively. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest extremely high levels phosphorus in the water column, extremely high production of suspended algae, and extremely poor water transparency (the TSI values for total phosphorus and chlorophyll-a at Swan Lake are the highest of the 131 Iowa lakes sampled from 2000 through 2002 as part of the ISU survey; the TSI value for Secchi depth is the second worst of the 131 lakes).
Despite the extremely high levels of chlorophyll-a at this lake, some non-phosphorus limitation on algal production due to nitrogen limitation and inorganic turbidity likely occurs at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 5, thus suggesting a strong likelihood that algal production at Swan Lake is limited by nitrogen availability. In addition, data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to very high levels of non-algal turbidity that may limit algal production. 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 Swan Lake (49.0 mg/l) was the second 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 of this lake. 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. Zooplankton grazing does not appear to limit algal production at Swan 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 (3.8 mg/l) was the 30th 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 chlorophyll (suspended algae) and inorganic turbidity—and the related reductions in water transparency—that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
An additional impairment to the Class A uses designated for this lake is related to presence of nuisance aquatic life (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. The summer 2000 average mass of bluegreen algae (2200 mg/l) was the highest of the 131 lakes sampled, thus suggesting a violation of Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life.
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; all of these problems are believed worsened by the large population of common carp in this lake. According 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. As part of the planned fall 2004 renovation and re-stocking of the Swan Lake fish populations, IDNR Fisheries has requested that a fish barrier be constructed at the lake outlet to prevent common carp from re-entering the lake (as has happened during high-flow periods in the past).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. This lake was, however, sampled in 2003 as part of the U.S. EPA/IDNR RAFT fish contaminant monitoring program. The results from this sampling are not yet available.
