Desoto Bend IA 06-WEM-1716

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Desoto Bend Lake are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 12 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 66, and 70.5 respectively for Desoto Bend Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Desoto Bend Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 12 samples.

The level of inorganic suspended solids was high at Desoto Bend Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Desoto Bend Lake (10.85 mg/L) was ranked 122nd among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a large population of cyanobacteria exists at Desoto Bend Lake, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (36.8 mg/L) was ranked 109th of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2010-2014. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 12 samples(0%), one violation of the criterion for dissolved oxygen in 12 samples(8%), and no violations of the criterion for pH in 12 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Desoto Bend 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.

Note: According to Iowa DNR Fisheries Biologists, DeSoto Bend was impacted by record flooding in 2011 on the Missouri River include the introduction of silver and bighead carp. The channel catfish population benefited the most from the 2011 flooding. Quality sized white bass have returned to the fishery in 2014. Walleye stocking continues on an annual basis in an effort to restore their numbers. Common carp are common in DeSoto Bend and have a negative impact on the aquatic plant community and water quality.