Assessment Comments
Assessment based on results of (1) IDNR beach monitoring program from 2000-02, (2) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (evaluated) as "partially supported" due to large populations of bluegreen algae at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supported / threatened." The Class C (drinking water) uses remain "not assessed" due to a lack of monitoring information. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of U.S. EPA/IDNR fish contaminant monitoring in 1996. The sources of data for this assessment include (1) results of the IDNR beach monitoring program in summer seasons of 2000, 2001, and 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 1996.
EXPLANATION: Levels of indicator bacteria at Lake Geode beach were monitored once per week during the primary contact recreation seasons (April through October) of 2000, 2001, and 2002 as part of the IDNR beach monitoring program. Results of this monitoring indicate “full support” of the Class A uses. 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 thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a thirty-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 thirty-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 thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any thirty-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 Lake Geode beach, none of the 52 thirty-day periods during summers of 2000, 2001, and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric means were 67 orgs/100ml in 2000, 64 orgs/100 ml in 2001, and 102 orgs/100ml in 2002. One of the weekly samples in 2000 and in 2001 (6% of samples) exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml; two samples exceeded this level in 2002. 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.
Similar to results of IDNR beach monitoring, results from the ISU statewide survey of Iowa lakes from 2000-02 suggest full support of the Class A (primary contact recreation) uses of Lake Geode. Using the median values from this survey from 2000-02 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 50, 50, and 52, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the lower range of eutrophic lakes. These index values suggest exceptional water quality for an Iowa impoundment: extremely low levels of phosphorus, very low production of suspended algae, and very good water transparency. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Lake Geode is 133. This ratio does not suggest that nitrogen limits production of algae at this lake but does indicate an excess of total nitrogen: for the 2000-02 period, the median level of total nitrogen at Lake Geode (3.1 mg/l) was the ninth highest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for either impairing designated uses or contributing to in-lake turbidity. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000-02 was 4.8 mg/l; the median level at Lake Geode was 3.1 mg/l. In addition, data from Downing et al. (2002) show moderately large populations of zooplankton species at Lake Geode that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 45% of the dry mass of the zooplankton community in the late July sample but only about 15% of the mid-August sample. The 2000 average summer mass of Cladoceran grazers at Lake Geode (18.8 mg/l) was the 65th lowest of the 131 lakes sampled.
Despite the indications of good water quality from IDNR beach monitoring and from results of the ISU lake survey, data from Downing et al. (2002) suggest that levels of bluegreen algae at this lake are sufficiently high to potentially impair designated uses. Downing et al. (2000) showed (1) that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake and (2) that the mass of bluegreen algae at this lake is relatively high. Sampling in 2000 showed that bluegreen algae comprised 100% of the wet mass of the phytoplankton samples in late July in mid-August. The 2000 average summer mass of bluegreen algae at this lake (99.5 mg/l) was the ninth highest of the 131 lakes sampled. This level suggests a potential impairment of designated uses due to violation of the state’s narrative water quality standard protecting against nuisance aquatic life. Although results of plankton sampling in 2000 suggest impairment of designated uses, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
The previous assessment of support of the Class B(LW) uses ("fully supported / threatened") was reviewed and approved by the DNR Fisheries Bureau. The Class C (drinking water) uses remained "not assessed" due to the lack of water quality information. 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 at this lake (maximum value = 5.7 mg/l; median = 3.1 mg/l) are low relative to the drinking water MCL (10 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1996 (see assessment developed for the 2000 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).
