Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2000.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to the presence of aesthetically objectionable conditions caused by algae blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2000.
EXPLANATION: For the 2008 reporting cycle, the Class A1 (primary contact recreation) uses for Beaver Lake are assessed as “partially supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2002 through 2006 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 69, and 68 respectively for Beaver Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values place Beaver 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 levels of inorganic suspended solids at this lake were moderately high but do not suggest an impairment due to non-algal turbidity. The median level of inorganic suspended solids in Beaver Lake (4.3 mg/L) was the 62nd highest median of the 132 lakes monitored by ISU and UHL.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Beaver Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 91% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (55.9 mg/L) was also the 20th 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 for the 2002-2006 period. 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 for Beaver Lake are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau, and results of the ISU and UHL lake surveys. Data from the ISU and UHL lake surveys from 2002-2006 show no violations of the Class B(LW) criterion for ammonia in 14 samples or the Class B(LW) criterion for dissolved oxygen in 18 samples. The data show 4 violations of the Class A1,B(LW) criterion for pH in 19 samples (21%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not constitute an impairment of the Class B(LW) uses of Beaver Lake. These pH violations likely reflect the excessive primary productivity at Beaver Lake and do not reflect the input of pollutants to this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Beaver Lake in 2000. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2000 RAFT sampling conducted in this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
