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 “partially supported” due to aesthetically objectionable conditions caused by algae blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are “not assessed.” 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: For the 2010 reporting cycle, the Class A1 (primary contact recreation) uses for Crawford Creek Lake are assessed as “partially supported” due to aesthetically objectionable conditions caused by algal turbidity. Using the median values from the ISU statewide survey of lakes and the UHL ambient lake monitoring program from 2004 through 2008 (approximately 24 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 62, and 63 respectively for Crawford Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus index values all place Crawford Creek Lake in the between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column.
While the TSI values for Crawford Creek Lake are below the impairment trigger of 65 for the 2010 reporting cycle, Crawford Creek Lake was listed as "partially supporting" the Class A1 uses for the 2008 reporting cycle due to algal turbidity. Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll-a and Secchi depth must be 63 or less for two consecutive summer seasons before a lake can be removed from the state’s Section 303(d) list (IR Category 5). Therefore, Crawford Creek Lake remains assessed as "partially supported" for the 2010 assessment cycle.
The levels of inorganic suspended solids at this lake were moderately high and do not suggest an impairment due to non-algal turbidity. The median level of inorganic suspended solids in Crawford Creek Lake (3.2 mg/L) was the 62nd lowest median of the 132 lakes sampled by the ISU and UHL surveys.
Data from the 2004-2008 ISU and UHL surveys suggest that a moderately large population of cyanobacteria exists at Crawford Creek Lake. These data show that cyanobacteria comprised 89% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (23.3 mg/L) was also the 42nd highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses for Crawford Creek Lake are assessed (monitored) as “fully supported.” Data from the ISU and UHL lake surveys from 2004-2008 show no violations of the Class B(LW) criterion for ammonia in 24 samples, dissolved oxygen in 24 samples, or pH in 24 samples. These results suggest “full support” of the Class B(LW) uses at Crawford Creek Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
