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 high levels of chlorophyll a (algae) that violate Iowa’s narrative criteria protecting against aesthetically objectionable conditions. Also, the presence of large populations of cyanobacteria suggests an impairment of due to nuisance aquatic life. 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 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 (primary contact recreation) uses at White Oak Lake are “partially supported” due to high levels of chlorophyll a (algae), nuisance cyanobacteria, and violations of the Class A1 criterion for pH. Using the median values from these surveys from 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 69, and 71 respectively for White Oak Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place White Oak Lake in between the eutrophic and hypereutrophic categories, while the total phosphorus value places White Oak Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
Although historically higher, recent levels of inorganic suspended solids at this lake have been relatively low, thus suggesting that non-algal turbidity does not contribute to the water quality (transparency) problems at this lake. The median inorganic suspended solids concentration for the 2004-08 period at White Oak Lake was 3.0 mg/L, which was the 60th lowest of the 132 monitored lakes. ISS levels have been low in the past 2 assessment/listing cycles. Therefore according to IDNR's assessment/listing methodology 2 consecutive assessment/listing cycles with ISS levels below impairment levels suggest that the turbidity impairment at White Oak Lake should be de-listed.
Data from the 2004-2008 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 97% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (55.1 mg/L) was the 11th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2004-2008. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
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 2004-2008 there was one violation of the Class B(LW) criterion for ammonia in 19 samples (5%). Based on IDNR’s assessment methodology the occurrence of a single violation of the ammonia criterion does not constitute an impairment of the Class B(LW) uses. There was also one violation of the dissolved oxygen criterion in 19 samples (5%). 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, however, 6 violations of the Class A1,B(LW) criterion for pH in 19 samples (32%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment of the Class A1 and Class B(LW) uses at White Oak Lake. 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.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
