Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-04, (2) surveys by IDNR Fisheries Bureau (3) ISU informatin on lake plankton communities, 2000-05, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2001 and 2005.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "partially supported" due to presence of aesthetically objectionable blooms of algae and due to high levels of pH that exceed state criteria. The Class B(LW) aquatic life are assessed (monitored) as "partially supported" primarily due to high levels of pH that exceed state criteria and secondarily due to blooms of algae and impacts of nutrients and siltation in the lake. Fish consumption remain assessed (monitored) as "fully supported". Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes from 2000 through 2004 from the ISU statewide lake survey, and (4) 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; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, nutrients, pH, and siltation) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: For the 2006 reporting cycle, the level of support of the Class A (primary contact recreation) uses are assessed as "partially supported" based on results from the ISU statewide survey of Iowa lakes. Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 79, 66, and 58, respectively, for Arbor Lake. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes. The index value for chlorophyll-a is between eutrophic and hyper-eutrophic, and the index value for secchi depth is in the upper range of eutrophic lakes. Compared to other Iowa lakes, these index values suggest (1) extremely high levels of phosphorus in the water column, (2) high levels of chlorophyll-a and suspended algae, and (3) and relatively good water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Arbor Lake is 8, thus suggesting that nitrogen availability may limit algal production at times at this lake.
In addition, data from the 2000-2005 ISU statewide lakes survey show a somewhat less than average population of zooplankton grazers at Arbor Lake, with the average (2000-05) per summer sample mass of these (Cladoceran) grazers (81.6 mg/l) ranking only 71st highest of the 131 lakes sampled. The presence of these algal grazers may, in part, explain the discrepancy between the TSI values for total phosphorus and chlorophyll-a.
Carlson (1991) also suggests that a high TSI for chlorophyll-a relative to the value for secchi depth suggests that the algal community is composed of large particulate algae such as the bluegreen alga Aphanizomenon. Such a phenomenon may explain the high TSI for chlorophyll-a (66) relative to the TSI for secchi depth (58) at this lake.
The levels of inorganic suspended solids at this lake are relatively low and do not suggest potential problems with high levels of inorganic turbidity. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l. The median level of inorganic suspended solids during this period at Arbor Lake was 3.6 mg/l, thus suggesting that non-algal turbidity contributes relatively little to either limitation on algal production or in-lake turbidity. These conditions indicate impairments to the Class A (primary contact) uses through presence of blooms of algae that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
Data from five years of plankton analysis by Iowa State University conducted as part of the statewide lake survey suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the phytoplankton community of Arbor Lake). Sampling from 2000 through 2004 showed that the median summer mass of bluegreen algae at this lake (4.3 mg/l) was the 20th lowest of the 131 lakes sampled. This low level of bluegreen algae does not suggest either a threat or impairment to support of designated uses at this lake.
Information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are fully supported. Nuisance blooms of algae and siltation impacts, however, remain concerns at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004 or for ammonia-nitrogen in the 8 samples collected during summers of 2002 through 2004. Four of 13 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.2; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria. Thus, these results suggest an impairment (partial support/monitored) of the Class A and Class B(LW) uses of this lake due to the frequency with which high levels of pH violate state criteria. These violations, however, likely reflect the excessive primary productivity at Arbor Lake and the large populations of suspended algae and do not reflect the input of pollutants into this lake.
Fish consumption uses are assessed 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 a low level 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. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2005 RAFT follow-up sampling conducted in this lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory.
