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 “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 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 Roberts Creek Lake are “partially supported” due to poor water transparency caused by inorganic turbidity and algae. Using the median values from these surveys from 2006 through 2010 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 65, and 67 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 21st highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Roberts Creek Lake, which does not cause an impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 53% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (10.9 mg/L) was the 30th lowest of the 134 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 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 17 samples, 3 violations of the Class B(LW) criterion for dissolved oxygen in 17 samples (18%), and no violations of the Class B(LW) criterion for pH in 17 samples. Based on IDNR’s assessment methodology the dissolved oxygen violations are not significantly greater than 10% of the samples and therefore do 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.
