Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998.
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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998.
EXPLANATION: For the 2008 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 2002 through 2006 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 66, and 70 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 23rd 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 80% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (16.8 mg/L) was the 70th highest 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 2002-2006 show no violations of the Class B(LW) criterion for ammonia in 14 samples, pH in 20 samples, or dissolved oxygen in 19 samples.
Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Desoto Bend Lake in 1998. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 1998 RAFT sampling conducted at this lake show that levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
