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), (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 remain assessed (monitored) as “fully supported.” 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), (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 2010 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 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 68, and 76 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 extremely 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 (5.0 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 2004-2008 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 97% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (47.0 mg/L) was also the 18th 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 2004-2008 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 2004-2008 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. Two 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. In the 2006 assessment/listing cycle, Arbor Lake was assessed as "partially supporting" due to significant violations of the pH criteria. According to IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. Because violations of the pH criteria were not significantly greater than 10% of the samples for both the 2008 and 2010 assessment/listing cycles, the pH impairment at Arbor Lake is suggested for delisting. The pH violations at Arbor Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses are assessed (monitored) as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2001 and RAFT follow-up monitoring in 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. 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 follow-up 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 fish consumption advisory.
