South Prairie Lake IA 02-CED-962

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 “fully supported” and the previous impairment for pH is suggested for de-listing.   [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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 improving at South Prairie Lake.   Using the median values from these surveys from 2008-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 55, and 65 respectively for South Prairie Lake.   According to Carlson (1977) the chlorophyll a value placse South Prairie Lake in the eutrophic category, while the total phosphorus and Secchi depth values place South Prairie Lake in between the eutrophic and the hypereutrophic categories.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.  

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 2.5 mg/L, which was the 44th lowest of the 134 monitored lakes.

Data from the 2008-2012 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 approximately 71% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (16.3 mg/L) was the 55th lowest of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on results from the ISU and UHL lake surveys.   The ISU and UHL lake surveys show that during 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 14 samples, or dissolved oxygen in 14 samples.   There was 1 violation of the pH criterion in 14 samples (7%).   Based on IDNR’s assessment methodology this violation is not significantly greater than 10% of the samples and therefore does not suggest impairment of the Class B(LW) uses.   However, South Prairie Lake was added to the impaired waters list in 2010 due to significantly greater than 10% of the samples exceeding the pH criterion.   Based on IDNR's assessment methodology, two consecutive assessment/listing cycles without significantly greater than 10% of the samples exceeding the criterion are necessary to suggest de-listing of the pH impairment.   Because there were no significant violations for the 2012 assessment/listing cycle or the current 2014 cycle, the pH impairment is suggested for de-listing.    

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.