Assessment Comments
Assessment is based on (1) surveys conducted by the DNR Fisheries Bureau and (2) results of IDNR beach monitoring in 2000 & 2001, and (3) U.S. EPA/IDNR fish tissue monitoring in 2001.
Basis for Assessment
Note: The original 2002 assessment for Nine Eagles Lake was incomplete. This assessment was updated in August 2004 and the corrected/updated information has been entered into ADB+.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting/threatened." The Class B(LW) aquatic life uses also are assessed (evaluated) as "fully supported/threatened." The Class C (drinking water) uses remain "not assessed." Fish consumption uses are assessed (monitored) as "fully supported." The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2000 and 2001, (2) results of the statewide survey of Iowa lakes conducted from 2000 and 2001 by Iowa State University (ISU), (3) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2001, (4) information from the IDNR Fisheries Bureau, and (5) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: The results of IDNR beach monitoring suggest “full support” of the Class A uses. Levels of indicator bacteria at Nine Eagles beach were monitored once per week during the primary contact recreation seasons (May through October) of 2000 and 2001 as part of the IDNR beach monitoring program. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria level from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At Nine Eagles beach, none of the 27 thirty-day periods during summers of 2000-01 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum 30-day geometric means were 57 orgs/100ml in both 2000 and 2001. None of the 18 weekly samples collected during summer 2000 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample value was 270 orgs/100 ml.). In summer 2001, two samples exceeded the single sample maximum value: the sample from June 18, 2001 contained 3,200 orgs/100 ml, and the sample from August 6, 2001 contained 1,400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if the 30-day geometric means based on at least five samples are less than 200 orgs/100ml, the primary contact recreation uses are "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are generally very low.
Despite the results of IDNR beach monitoring that show “full support” of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Nine Eagles Lake are “fully supported / threatened.” Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 45, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a is in the middle range of mesotrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes. These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is extremely low and water transparency is very good for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae. The moderately high levels of total phosphorus, however, suggest a slight threat from such blooms. 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 in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Nine Eagles Lake is 9.6. This ratio suggests that the availability of nitrogen limits production of algae at this lake. The levels of inorganic suspended solids at this lake are typical for Iowa lakes suggest a slight potential for contributing to in-lake turbidity. Both the TSI values for chlorophyll-a and secchi depth, however, suggest that water transparency at this lake is good for Iowa lakes. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Nine Eagles Lake was nearly equal to this median (5.1 mg/l).
This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid and late summer. Sampling in 2000 showed that bluegreen algae comprised approximately 25% of the wet mass of the phytoplankton community in late June but increase to over 80% of the community wet mass in the late July and late August samplings. Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is extremely low at this lake, and the presence of bluegreen algae likely does not present a water quality impact.
Although large populations of zooplankton grazers can limit algal production, data from Downing et al. (2002) show relatively small populations of these zooplankton species at Nine Eagles Lake. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) were not present in the late July sample and comprised only 10% of the zooplankton population in the late August sample. Thus, grazing of algae by zooplankton does not appear to explain the extremely low levels of chlorophyll-a at Nine Eagles Lake.
The slightly eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be remain assessed as "fully supported / threatened."
Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses remain "fully supported" based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2001.
