Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, (3) results of the IDNR beach monitoring program from 2000-02, (4) the ISU 2001 diagnostic/feasbility study, (5) surveys of the DNR Fisheries Bureau, and (6) EPA/DNR fish contaminant monitoring in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses were assessed (monitored) as “not supporting” due to high levels of indicator bacteria at swimming beaches. In addition, results of the ISU lakes survey suggest "partial support" of Class A uses due to aesthetically objectionable conditions (poor water transparency) related primarily to high levels of inorganic turbidity and secondarily to blooms of algae. The presence of nuisance (=noxious) aquatic life (bluegreen algae) present an additional threat to full support of the Class A uses. The Class B(LW) aquatic life uses remained assessed as “fully supporting / threatened.” Drinking water uses remain “not assessed” due to lack of water quality information. Fish consumption uses are assessed as “fully supported” based on results of fish contaminant monitoring in 2002. Sources of data for this assessment include results of (1) IDNR beach monitoring from 2000-02, (2) ISU lake surveys from 2000-02, (3) surveys by IDNR Fisheries Bureau, (4) the ISU report on lake plankton communities (Downing et al. 2002), (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2002, and (6) the ISU diagnostic/feasibility study in 2001.
EXPLANATION: Results of IDNR beach monitoring from 2000 through 2002 suggest that the Class A (primary contact recreation) uses are “not supported” at Clear Lake Beach but suggest "full support" of the Class A uses at McIntosh Woods Beach. Levels of indicator bacteria at both beaches 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. 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 Clear Lake beach, one of the 49 thirty-day periods during summers of 2000, 2001, and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml: this geometric mean (224 orgs/100 ml) reflected elevated levels of fecal coliforms during early and mid-June, 2002. This single violation of Iowa’s water quality criterion indicates not-support of the Class A uses at Clear Lake. Four of the 64 weekly samples collected during the 2000-02 period exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml; three of the four samples were collected during the 2002 recreational season. At McIntosh Woods beach, however, none of the 48 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 111 orgs/100ml in 2000, 15 orgs/100 ml in 2001, and 39 orgs/100 ml in 2002. Only one of the 63 weekly samples collected during the 2000-02 period 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 McIntosh Woods Beach.
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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 63, and 70, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a are in the range between eutrophic and hyper-eutrophic lakes; the value for Secchi depth is at the boundary between eutrophic and hyper-eutrophic lakes. These TSI values suggest excessive levels of phosphorus in the water column, somewhat elevated levels of chlorophyll-a, and poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Clear 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, Clear Lake had the 30th highest median level of inorganic suspended solids (9.6 mg/l), thus suggesting that these very high levels of non-algal turbidity limit the production of algae as well as contributes to poor water transparency. In addition to inorganic sources of turbidity, the somewhat elevated levels of chlorophyll-a at Clear Lake suggest that suspended algae also contribute to poor water transparency. The overall 2000-02 median chlorophyll-a level at Clear Lake (27.8 ug/l) is the 40th highest of the 131 lakes sampled. Neither nitrogen limitation nor zooplankton grazing appear to limit algal production at Clear Lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 20; this ratio suggests that algal production at this lake is limited by availability of phosphorus as opposed to nitrogen. Data from Downing et al. (2002) show relatively small zooplankton populations at this lake, including a relatively small population (approximately 40% dry mass) of species known as algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (11.9 mg/l) was the 51st lowest of the 131 lakes sampled. At this density, zooplankton grazers are not believed to exert a non-phosphorus limitation on algal production. These conditions indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions due primarily to high levels of inorganic turbidity and secondarily to blooms of 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. The 2000 average summer mass of bluegreen algae at Clear Lake (26.5 mg/l) was the 37th highest of the 131 lakes sampled. This level suggests a threat to full support of the designated uses of this lake due to the potential for violating Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest a potential impact at this lake, 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. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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 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 (Class C) uses were not assessed due to the lack of water quality information needed for this 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 = 0.4 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 uses are assessed as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002. The composite samples of fillets from channel catfish and walleye had very low levels of contaminants. Neither sample contained levels of contaminants that approached even one-half the respective FDA action levels or IDNR levels of concern. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: <0.0181 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of walleye fillets were as follows: mercury: 0.045 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.
