Center Lake IA 06-LSR-1663

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 “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 fish contaminant monitoring at this lake.   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: For the 2010 reporting cycle, the Class A1 (primary contact recreation) uses for Center Lake are assessed as “fully supported."  Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program do not suggest impairment of the Class A1 uses at Center Lake.   Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 59, 61, and 69 respectively for Center Lake.   According to Carlson (1977) the Secchi depth value places Center Lake in the eutrophic category while the chlorophyll a and total phosphorus index values 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 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 (3.0 mg/L) was the 51st lowest median of the 134 lakes sampled by the ISU and UHL surveys.

Data from the 2006-2010 ISU and UHL surveys suggest that a moderately large population of cyanobacteria exists at Center Lake.   These data show that cyanobacteria comprised 72% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (22.4 mg/L) was also the 60th highest of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses for Center Lake are assessed (monitored) as “fully supported."  The ISU and UHL lake surveys data from 2006-2010 show one violation of the Class B(LW) criterion for ammonia in 21 samples and one violation of the Class B(LW) criteria for dissolved oxygen in 22 samples.   These violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Center Lake.   The data for pH show 3 violations of the Class A1,B(LW) criterion for pH in 22 samples (14%).   Center Lake was assessed as “partially supporting” in the 2008 assessment/listing cycle due to significant violations of the pH criteria.   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.   Because violations of the pH criteria were not significantly greater than 10% for the 2010 assessment/listing cycle or the current (2012) assessment/listing cycle the pH impairment for Center Lake is suggested for delisting.  

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.