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 violations of the Class A1 criterion for pH. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. Fish consumption uses are “not assessed” due to a lack of fish contaminant monitoring at this lake. 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 Center Lake are assessed as “partially supported” due to violations of the Class A1 criterion for pH. Additional results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program, however, do not suggest impairment of the Class A1 uses at Center Lake. 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 58, 63, and 70 respectively for Center Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus index values all place Center Lake in the between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and very high levels of phosphorus in the water column.
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 Center Lake (3.1 mg/L) was the 61st highest 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 Center Lake. These data show that cyanobacteria comprised 98% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (20.6 mg/L) was also the 47th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses for Center Lake are assessed (monitored) as “partially supported” due to violations of the Class A1,B(LW) criterion for pH. The ISU and UHL lake surveys data from 2004-2006 show no violations of the Class B(LW) criterion for ammonia in 24 samples, or dissolved oxygen in 25 samples. The data for pH, however, show 4 violations of the Class A1,B(LW) criterion for pH in 25 samples (16%). Based on IDNR’s methodology these results do not suggest an impairment of the Class A1 and Class B(LW) uses of Arbor Lake due to the frequency with which high levels of pH violate state criteria. However, Center Lake was assessed as “partially supporting” in the 2008 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. These violations likely reflect the high levels of primary productivity at Center Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
