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 poor water transparency caused by high levels of inorganic turbidity and algae. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to high levels of inorganic turbidity. Fish consumption uses remain “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: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Roberts Creek Lake are “partially supported” due to poor water transparency caused by inorganic turbidity. Using the median values from these surveys from 2004 through 2008 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 68, 68, and 69 respectively for Roberts Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Roberts Creek Lake in between the eutrophic and hypereutrophic categories. These values suggest high 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 very high at this lake and suggests impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Roberts Creek Lake was 9.0 mg/L, which was the 23rd highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Roberts Creek Lake, which may contribute to aesthetically objectionable conditions caused by algae blooms. These data show that cyanobacteria comprised 60% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (13.0 mg/L) was the 59th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. Based on information from the IDNR Fisheries Bureau, inorganic turbidity contributes to impairment of the Class B(LW) uses of Roberts Creek Lake. The ISU and UHL lake surveys, however, show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 19 samples, one violation of the Class B(LW) criterion for dissolved oxygen in 20 samples (5%), and no violations of the Class B(LW) criterion for pH in 20 samples. Based on IDNR’s assessment methodology the dissolved oxygen violation is less than 10% of the samples and therefore does not suggest impairment of the Class B(LW) uses of Roberts Creek Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
