Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of the IDNR beach monitoring program from 2000-02, (3) ISU lake survey from 2000-02, and (4) ISU report on lake phytoplankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to slightly elevated levels of chlorophyll (algae). The Class B(LW) aquatic life uses remained assessed (evaluated) as fully supporting / threatened" due to moderate impacts from nutrients in the water column and due to minor siltation impacts. The Class C (drinking water) uses remain "not assessed" due to lack of monitoring information. Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring at this lake. The sources of data for this assessment include (1) surveys by the IDNR Fisheries Bureau, (2) results of the IDNR-UHL beach monitoring program in summers of 2000, 2001, and 2002, (3) ISU lake survey in summers of 2000, 2001, and 2002, and (4) 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 Lake Wapello beach were monitored once per week during the primary contact recreation seasons (May through September) 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 Lake Wapello beach, none of the 47 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 18 orgs/100ml in 2000, 17 orgs/100 ml in 2001, and 35 orgs/100ml in 2002. None of the 63 weekly samples collected from 2000-02 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 82 orgs/100 ml in 2000, 73 orgs/100 ml in 2001, and 240 orgs/100 ml 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 very low.
The results of IDNR beach monitoring that suggest "full support" of the Class A uses are generally consistent with results from the ISU statewide survey of Iowa lakes that show exceptional water transparency and low levels of nuisance algal species. Levels of chlorophyll-a at this lake, however, are slightly elevated, thus suggesting a potential threat to the full support of designated uses. 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 65, 59, and 55, respectively. According to Carlson (1977), the index value for total phosphorus is in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of eutrophic lakes, and the index value for secchi depth is in the middle range of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and very good water transparency. 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, zooplankton grazing or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that nitrogen availability may limit algal production while zooplankton grazing and inorganic turbidity do not. The ISU lake data does suggests that availability of nitrogen may limit algal production, at times, at Lake Wapello. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Wapello is 13. This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. The relatively low small populations of zooplankton grazers at this lake likely do not influence algal populations. Downing et al. (2002) show that Cladoceran taxa (e.g., Daphnia) comprised approximately 55% of the dry mass of the zooplankton community in the late July sample and 60% of the mid-August sample. The 2000 average summer mass of Cladocerans (5.4 mg/l) was the 37th lowest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are relatively low and also do not suggest the potential for impairing designated uses. 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; the median level at Lake Wapello was 2.5 mg/l.
Presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise relatively small portions of the phytoplankton community. Sampling in 2000 showed the percent wet mass of bluegreens ranged from zero in the late June sampling, to approximately 5% in the late July sampling, and up to approximately 40% in the mid-August sampling. The 2000 average mass of bluegreen algae (3.1 mg/l) was the 30th lowest of the 131 lakes sampled.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened" due to moderately high nutrient loading to the water column and minor impacts from siltation in the lake. The Class C (drinking water) uses remain "not assessed" due to a lack of relevant water quality information for this lake 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 extremely 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 remain assessed "not assessed" due to the lack of fish contaminant monitoring at this lake.
