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 high levels of chlorophyll a (algae) and turbidity that violate Iowa’s narrative criteria protecting against aesthetically objectionable conditions. Violations of the Class A1 criterion for pH also suggest an impairment at this lake. 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 remain “not assessed” due to the 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: Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at White Oak Lake are “partially supported” due to high levels of chlorophyll a (algae), turbidity, and violations of the Class A1 criterion for pH. Using the median values from these surveys from 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 68, and 73 respectively for White Oak Lake. According to Carlson (1977) the chlorophyll a value places White Oak Lake in between the eutrophic and hypereutrophic categories, while the Secchi depth and total phosphorus values place White Oak Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very high levels of phosphorus in the water column.
Levels of inorganic suspended solids were moderately high and suggest that turbidity contributes to aesthetically objectionable conditions at this lake. The median inorganic suspended solids concentration for the 2006-10 period at White Oak Lake was 4.0 mg/L, which was the 64th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at White Oak Lake, which contributes to the impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 50% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (29.3 mg/L) was the 51st highest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class A1,B(LW) criterion for pH. Data from the ISU and UHL lake surveys show that during 2006-2010 there were no violations of the Class B(LW) criteria for ammonia in 16 samples. There was one violation of the dissolved oxygen criterion in 16 samples (6%). Based on IDNR’s assessment methodology this violation is not significantly greater than 10% of the samples and therefore does not constitute an impairment of the Class B(LW) uses. There were 2 violations of the Class A1,B(LW) criterion for pH in 16 samples (12%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1 and Class B(LW) uses at White Oak Lake. However, White Oak Lake was assessed as “partially supporting” in the 2010 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. The pH violations at White Oak Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
High levels of algae and turbidity at this lake remain a concern for support of aquatic life uses and represent a potential impairment of the Class B(LW) aquatic life uses. This potential impairment, however, is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
