Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 “not supported” due to the presence of aesthetically objectionable conditions caused by algae blooms, low water clarity, and 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 recent information upon which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for algae and pH in this lake was prepared and approved by EPA in October 2014. The approval of this TMDL moves the impairments due to algae and pH from Iowa's list of Section 303(d) waters (Category 5a of the Integrated Report) to IR Category 4a (impaired; TMDL not required).
EXPLANATION: For the 2014 reporting cycle, the Class A1 (primary contact recreation) uses for Beaver Lake are assessed as “not 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 2008 through 2012 (approximately 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 71, and 77 respectively for Beaver Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values place Beaver Lake in the hypereutrophic categority. These values suggest very 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.
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 (5.3 mg/L) was ranked 82nd of the of the 134 lakes monitored by ISU and UHL.
Data from the 2008-2012 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 100% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (196.0 mg/L) was also the highest of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2006-2010 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 “partially supported” based on information from the IDNR Fisheries Bureau, and results of the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau suggests that Beaver Lake has no vegetation and excessive algae and nutrients. Data from the ISU and UHL lake surveys from 2008-2012 show no violations of the Class B(LW) criterion for ammonia in 15 samples and one violation of the Class B(LW) criterion for dissolved oxygen in 15 samples. The single violation of the Class B(LW) dissolved oxygen criterion is not significantly greater than 10% of the samples and therefore, according to IDNR's assessment methodology, does not suggest an impairment of the aquatic life uses. The data show 4 violations of the Class A1,B(LW) criterion for pH in 15 samples (27%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore constitute an impairment of the Class A1 and 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 (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Beaver Lake in 2011 and 2012. The composite samples of fillets from Largemouth Bass and channel catfish had low levels of contaminants. Levels of primary contaminants in the 2011 composite sample of largemouth bass fillets were as follows: mercury: 0.257 ppm. in 2012, the composite samples of fillets from largemouth bass contained 0.220 ppm mercury. Levels of primary contaminants in the 2011 composite sample of channel catfish fillets were as follows: mercury: RESULT 0.0564; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. 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 2011 and 2012 RAFT sampling conducted at this Beaver Lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
