Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities from 2000-05, and (4) results of EPA/IDNR fish contaminant monitoring in 2003.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to extremely poor water transparency related to high levels of suspended algae and extremely high levels of inorganic turbidity. Additional impairments to the Class A uses are due to (1) the extremely high populations of nuisance aquatic life (bluegreen algae) at this lake and (2) frequent violations of the Iowa water quality criterion for pH. 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 Class B(LW) uses are also impaired by frequent violations of the Iowa water quality criterion for pH. With the exception of the pH impairments, the 2006 assessments of the Class A and Class B(LW) uses are consistent with the assessments developed for previous Section 305(b) reports. Fish consumption uses are assessed as “fully supported” based on results of fish contaminant monitoring in 2003. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S. EPA / IDNR fish contaminant monitoring in 2003. A renovation and re-stocking of the fish populations at Swan Lake was conducted in 2004. In addition, a new wetland was constructed at the upper end of the lake. These acivities may improve water quality at Swan Lake.
Note: A TMDL for algae and tubidity at Swan Lake was prepared by IDNR and approved by EPA in 2004; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, turbidity, and pH) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2004 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 that violates Iowa’s narrative criterion protecting against aesthetically objectionable conditions. Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices at this lake for total phosphorus, chlorophyll-a, and secchi depth are 89, 73, 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, very high production of suspended algae, and extremely poor water transparency (the TSI values for total phosphorus and Secchi depth at Swan Lake are the worst of the 131 Iowa lakes sampled from 2000 through 2004 as part of the ISU survey; the TSI value for chlorophyll-a is the fourth worst of the 131 lakes).
Despite the very 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 2004, the ratio of total nitrogen to total phosphorus for this lake is 6, 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 2004 was 5.2 mg/l. The median level of inorganic suspended solids at Swan Lake (27.6 mg/l) was the fifth 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 the ISU lake survey show the presence of moderately large populations of zooplankton at Swan Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 60% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (106 mg/l) was the 56th highest of the 131 lakes sampled.
These conditions suggest impairments to the Class A (primary contact) uses primarily due to presence extremely poor water transparency that violates Iowa’s narrative criterion protecting against aesthetically objectionable conditions. The poor water transparency at Swan Lake is caused by a combination of high levels of chlorophyll (suspended algae) and inorganic turbidity.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 98%. In addition, Swan Lake had the highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 364 mg/l. The presence of this extremely large population of bluegreen algae at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels greater than the 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
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).
Similar to other data, results of chemical water quality monitoring also suggest impairments of Swan Lake: the 2000-04 ISU chemical water quality data for Swan Lake suggest “nonsupport” of the Class B(LW) uses. Although none of 14 samples collected during the 2000-2004 period violated the Class B(LW) criterion for dissolved oxygen, eleven of 15 samples (73%) violated the Class B(LW) criteria for pH (maximum level of 10.3 pH units; minimum of 8.5 units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria. Thus, these results suggest an impairment (partial support/monitored) of the Class A and Class B(LW) uses of this lake. These violations likely reflect the extremely high levels of primary productivity at Swan Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Swan Lake in 2003. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.028 ppm; total PCBs: 0.0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.102 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2003 RAFT sampling conducted at Swan Lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
