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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2001 and 2005.
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 algal blooms. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to the impacts of nutrients and siltation at this lake. 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 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), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2001 and 2005.
Note: A TMDL for siltation and nutrients at Arbor Lake was prepared by IDNR and approved by EPA in 2002. Because all Section 303(d) impairments identified in the 2010 assessment/listing cycle (algal growth, nutrients, and siltation) are addressed by the TMDL, Arbor Lake remains in IR Category 4a (TMDL approved).
EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for Arbor 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 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 68, and 74 respectively for Arbor Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Arbor Lake between the eutrophic and hypereutrophic categories. The total phosphorus value places Arbor Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately 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 and do not suggest impairment related to non-algal turbidity. The median level of inorganic suspended solids in Arbor Lake (5.2 mg/L) was slightly higher than the median concentration of inorganic suspended solids for all lakes monitored by ISU and UHL (4.0 mg/L).
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at Arbor Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 89% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (51.8 mg/L) was also the 24th 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 Arbor Lake are assessed as “fully supported” based on information from the IDNR Fisheries Bureau and results of the ISU and UHL lakes surveys. Nuisance algae blooms and siltation impacts, however, remain concerns at Arbor Lake. Results from the ISU and UHL lake surveys from 2006-2010 show no violations of the Class B(LW) criterion for ammonia in 22 samples, and no violations of the criterion for dissolved oxygen in 22 samples. One of the 22 pH samples violate the Class A1,B(LW) criterion for pH. Based on IDNR’s methodology these results do not suggest an impairment of the Class A1 and Class B(LW) uses of Arbor Lake due to the frequency with which high levels of pH violate state criteria. Therefore the Class B(LW) aquatic life uses are assessed (monitored) as "fully supported."
Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Arbor Lake in 2001 and 2005. While a composite sample of largemouth bass fillets collected from Arbor Lake for the 2001 RAFT contained 0.312 ppm of mercury, the 2005 RAFT follow-up composite sample of fillets from largemouth bass had low levels of mercury (0.17 ppm). These results suggest “full support” of the fish consumption uses. 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 2005 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.
