Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to aesthetically objectionable conditions primarily caused by the presence of large amounts of suspended sediment. High levels of algae in the past also cause an impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” although siltation remains a concern. Fish consumption uses assessed (monitored) as "fully supported" based on results of fish tissue monitoring in 2010. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 2010.
EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for Desoto Bend Lake are assessed as “partially supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2006 through 2010 (approximately 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 60, and 67 respectively for Desoto Bend Lake. According to Carlson (1977) the index values for Secchi depth, chlorophyll a, and total phosphorus all place Desoto Bend Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.
While the TSI value for chlorophyll a at DeSoto Bend Lake is below the impairment trigger of 65 for the 2012 reporting cycle, DeSoto Bend Lake was listed as "partially supporting" the Class A1 uses for the 2010 reporting cycle due to algal turbidity (as well as non-algal turbidity). Based on IDNR's assessment methodology, the median-based TSI value for chlorophyll-a must be 63 or less for two consecutive summer seasons before an algae impairment can be removed from the state’s Section 303(d) list (IR Category 5). Therefore, the algae impairment at DeSoto Bend Lake remains for the 2012 assessment/listing cycle, along with the turbidity impairment.
The levels of inorganic suspended solids at this lake were high and suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Desoto Bend Lake (6.0 mg/L) was the 43rd highest median of the 134 lakes sampled by the ISU and UHL surveys.
Data from the ISU and UHL surveys suggest that cyanobacteria populations were relatively low at Desoto Bend Lake and do not contribute to impairment at this lake. These data show that cyanobacteria comprised 50% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (11.0 mg/L) was the 31st lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses for Desoto Bend Lake are assessed (monitored) as “fully supported.” Data from the ISU and UHL surveys from 2006-2010 show no violations of the Class B(LW) criterion for ammonia in 15 samples or pH in 15 samples. There were two violations of the Class B(LW) criterion for dissolved oxygen in 15 samples (13%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses. Information from the IDNR Fisheries Bureau, however, suggests that turbidity and algae remain concerns and common carp affect the fishery and water quality at this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at DeSoto Bend in 2010. The composite samples of fillets from common carp and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.106 ppm; total PCBs: 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: 0.164 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. The fish contaminant data generated from the 2010 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
