South Prairie Lake IA 02-CED-962

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), 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.”   [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 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), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: Results from the ISU and UHL lake surveys show relatively good water quality exists at South Prairie Lake.   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 54, 59, and 61 respectively for South Prairie Lake.   According to Carlson (1977) the Secchi depth and chlorophyll a values place South Prairie Lake in the eutrophic category, while the total phosphorus value places 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, very good water transparency, and moderately 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 3.0 mg/L, which was the 50th lowest of the 132 monitored lakes.

Data from the 2002-2006 ISU and UHL surveys suggest a 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 66% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (7.5 mg/L) was also the 26th lowest of the 132 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys.   The ISU and UHL lake surveys show that during 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 14 samples, or dissolved oxygen in 20 samples.   There were 4 violations of the pH criterion in 20 samples (20%).   However, based on IDNR’s assessment methodology these violations were not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses.   These results suggest full support of the Class B(LW) uses at South Prairie Lake.

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