Slip Bluff Lake IA 05-GRA-1362

Assessment Comments

Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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 “fully supported.”  The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Fish consumption uses are “not assessed” due to a lack of information upon which to base an assessment.   Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Slip Bluff Lake are “fully supported.”  Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 49, 45, and 53 respectively for Slip Bluff Lake.   According to Carlson (1977) the Secchi depth and chlorophyll a values place Slip Bluff Lake in the mesotrophic category, while the total phosphorus value places Slip Bluff Lake in the eutrophic category.   These values suggest extremely low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and low 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 Slip Bluff Lake was 2.0 mg/L, which was the 40th lowest of the 134 monitored lakes.

Data from the 2006-2010 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Slip Bluff Lake, which does not suggest impairment at this lake.   These data show that cyanobacteria comprised approximately 99% of the phytoplankton wet mass at this lake.   However, the median cyanobacteria wet mass (18.7 mg/L) was the 60th lowest of the 134 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 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 22 samples, dissolved oxygen in 22 samples, or pH in 22 samples.   These results suggest full support of the Class B(LW) uses at Slip Bluff Lake.

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