Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, (3) results of the IDNR beach monitoring program, (4) ISU 2001 diagnostic/feasbility study, and (5) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses were assessed (monitored) as "partially supporting" due to aesthetically objectionable blooms of algae and to presence of nuisance aquatic species (bluegreen algae). The Class B(LW) aquatic life uses remained assessed as fully supporting / threatened. Drinking water uses were not assessed. Fish consumption uses remain "not assessed." Assessment is based on results of (1) IDNR beach monitoring, (2) ISU lake survey in 2000-01, (3) surveys by IDNR Fisheries Bureau, and (4) ISU report on lake phytoplankton communities. EXPLANATION: Results of IDNR beach monitoring in 2000 and 2001 suggest "full support" of the Class A uses. Levels of indicator bacteria at Clear Lake Beach and McIntosh Woods Beach were monitored once per week during the primary contact recreation seasons (April 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 Clear Lake beach, none of the 24 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml; the maximum 30-day geometric means were 14 orgs/100 ml in 2000 and 44 orgs/100ml in 2001. Only one of the 35 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. At McIntosh Woods beach, none of the 23 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml.; the maximum 30-day geometric means were 111 orgs/100ml in 2000 and 15 orgs/100 ml in 2001. Only one of the 35 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. These results suggest full support of primary contact recreation uses at the Clear Lake beaches. Despite the indications of full support of Class A uses based on IDNR beach monitoring, results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A uses of Clear Lake. 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 73, 67, and 71, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the range of hyper-eutrophic lakes and suggests excessive levels of phosphorus in the water column, nuisance blooms of algae, and poor water transparency. 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. Of 130 lakes sampled, Clear Lake had the 28th highest median level of inorganic suspended solids (12.2 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to impairments of both the primary contact recreation and aquatic life uses. These conditions indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tends to dominate the summertime phytoplankton community of Clear Lake, especially in late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 5% in the early July sampling, to approximately 50% in the late July sampling, and up to approximately 70% in the early September sampling. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 16. The Class B(LW) aquatic life uses remain assessed as "fully supporting / threatened" based on review of the previous (2000) assessments by the DNR Fisheries Bureau. Drinking water uses were not assessed due to the lack of water quality information needed for this assessment. Fish consumption uses were previously assessed as fully supported (=FS) based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1993 (see above) Results of this monitoring are now considered too old (greater than five years) to accurately assess the current levels of contaminants in fish. Thus, the assessment of fish consumption uses was changed from "fully supporting" to "not assessed."
