Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
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, but also due to algae blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” although siltation remains a concern. Fish consumption uses are assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: For the 2010 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 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 69, 68, and 71 respectively for Desoto Bend Lake. According to Carlson (1977) the index values for Secchi depth and chlorophyll a place Desoto Bend Lake in between the eutrophic and hypereutrophic categories. The index value for total phosphorus places Desoto Bend Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were very 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 (9.4 mg/L) was the 22nd highest median of the 132 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 49% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (8.2 mg/L) was the 43rd lowest of the 132 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 2004-2008 show no violations of the Class B(LW) criterion for ammonia in 19 samples, pH in 19 samples, or dissolved oxygen in 19 samples. 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 “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1998. While these results suggest that levels of contaminants were low at Desoto Bend Lake, they are now too old (greater than 10 years) to be used for an assessment.
