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 B(LW) aquatic life uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. [Note: This lake is not designated for Class A1 (primary contact recreation) uses.] Fish consumption uses are “not assessed” due to the lack of fish contaminant monitoring at this lake. 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: Results from the ISU and UHL lake surveys show water quality is degrading at South Prairie Lake. 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 62, 64, and 69 respectively for South Prairie Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place South Prairie Lake in between the eutrophic and the hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column. (NOTE: TSI values for Secchi depth, chlorophyll a, and total phosphorus for the 2008 assessment/listing cycle were 54, 59, and 61 respectively.)
The level of inorganic suspended solids is relatively low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at South Prairie Lake was 3.0 mg/L, which was the 56th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at South Prairie Lake, which does not suggest an impairment at this lake. These data show that cyanobacteria comprised only 62% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (7.0 mg/L) was also the 37th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” based on results from the ISU and UHL lake surveys. The ISU and UHL lake surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 19 samples, or dissolved oxygen in 19 samples. There were, however, 5 violations of the pH criterion in 19 samples (26%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment of the Class B(LW) uses. The pH violations at South Prairie Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
