Assessment Comments
Assessment is based on (1) surveys conducted by the DNR Fisheries Bureau and (2) results of IDNR beach monitoring from 2000-02, (3) ISU survey of Iowa lakes from 2000-02, (4) ISU study of lake plankton communities in 2000, and (5) U.S. EPA/IDNR fish tissue monitoring in 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remained assessed (evaluated) as "fully supported / threatened." The Class C (drinking water) uses are "not assessed" due to the lack of water quality information upon which to base an assessment. Fish consumption uses are assessed as "fully supported" based on fish contaminant monitoring conducted in 2001. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2000 through 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 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: 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, 2001, and 2002 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 52 thirty-day periods during summers of 2000-02 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 and 102 orgs/100 ml in 2002. 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. Similarly, in 2002, two samples exceeded this maximum value: July 30, 2000 (3,000 orgs/100 ml) and August 20, 2002 (430 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.
Results from the ISU statewide survey of Iowa lakes also suggest that the Class A uses of Nine Eagles Lake are “fully supported.” Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 53, 45, and 55, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between 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 very low levels of phosphorus, extremely low production of suspended algae, and water transparency that is very good for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable conditions due either to blooms of algae or high levels of inorganic turbidity. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicates that non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Nine Eagles Lake. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Nine Eagles Lake had the 68th highest median level of inorganic suspended solids (3.7 mg/l). While this level of inorganic solids is only slightly elevated, sufficient inorganic material may be present to reduce light penetration and the production of suspended algae. Results of ISU monitoring do not show non-phosphorus limitation due to nitrogen availability or zooplankton grazing. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Nine Eagles Lake is 23, thus suggesting that nitrogen availability does not limit algal production. This ratio, however, is considerably higher than that based on only the first two years of the ISU survey (2000 and 2001): 10. This much lower ratio, which does suggest nitrogen limitation, suggests that algal production at Nine Eagles Lake may fluctuate from being limited by phosphorus and by nitrogen. 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. The 2000 average summer mass of Cladocerans at this lake (3.7 mg/l) was the 29th lowest of the 131 lakes sampled. Thus, grazing of algae by zooplankton does not appear to explain the extremely low levels of chlorophyll-a at Nine Eagles Lake.
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. Despite the apparent dominance of bluegreens, the 2000 average summer mass of bluegreen algae (2.5 mg/l) was very low (25th lowest of the 131 lakes sampled), thus suggesting that the presence of bluegreen algae likely does not cause a water quality impact.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened." The Class C (drinking water) uses are "not assessed" due to lack of monitoring data upon which to base an assessment. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 1.0 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption are assessed (monitored) as "fully supported" based on results of U.S. EPA / IDNR fish tissue monitoring in 2001 (see assessment for the 2002 report for more information).
