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 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. Violations of the state’s pH criterion also contribute to impairment of the Class A1 uses at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed state criteria and secondarily 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 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 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 2008 assessment/listing cycle (algal growth, nutrients, pH and siltation) are addressed by the TMDL, Arbor Lake remains in IR Category 4a (TMDL approved).
EXPLANATION: For the 2008 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 2002 through 2006 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 67, 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 (4.2 mg/L) was equal to the median for the 132 lakes sampled by ISU and UHL.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Arbor Lake, which contributes to impairment at this lake by contributing to the aesthetically objectionable conditions. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (30.8 mg/L) was also the 38th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses for Arbor Lake are assessed 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 suggest that the Class B(LW) uses are “fully supported.” Nuisance algae blooms and siltation impacts, however, remain concerns at Arbor Lake. Results from the ISU and UHL lake surveys from 2002-2006 show no violations of the Class B(LW) criterion for ammonia in 14 samples, and no violations of the criterion for dissolved oxygen in 20 samples. Two of the 18 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. However, Arbor Lake was assessed as “partially supporting” in the 2006 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on 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. 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.
