Slip Bluff Lake IA 05-GRA-1362

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 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 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 suggest that the Class A1 uses at Slip Bluff Lake are “fully supported.”  Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 49, 45, and 50 respectively for Slip Bluff Lake.   According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Slip Bluff Lake in the mesotrophic 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 33rd lowest of the 132 monitored lakes.

Data from the 2004-2008 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 75% of the phytoplankton wet mass at this lake.   However, the median cyanobacteria wet mass (8.0 mg/L) was the 42nd 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 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 25 samples, dissolved oxygen in 25 samples, or pH in 25 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.