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 the presence of aesthetically objectionable conditions caused by algae blooms 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 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 2012 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 2006 through 2010 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 69, 68, and 71 respectively for Beaver Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Beaver Lake in between the eutrophic and hypereutrophic categories, while the chlorophyll a value places Beaver 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.
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 (5.0 mg/L) was the 57th highest median of the 134 lakes monitored by ISU and UHL.
Data from the 2006-2010 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 84% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (61.5 mg/L) was also the 15th highest of the 134 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 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 2006-2010 show no violations of the Class B(LW) criterion for ammonia in 17 samples and one violation of the Class B(LW) criterion for dissolved oxygen in 17 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 17 samples (24%). 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 “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 2000. While these results suggest that levels of contaminants were low at Beaver Lake, they are now too old (greater than 10 years) to be used for an assessment.
