Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of the IDNR beach monitoring program in 2000 & 2001, (3) ISU lake survey in 2000-01, & (4) ISU report on lake phytoplankton communities.
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 supporting / threatened." The Class C (drinking water) uses remain "not assessed." Fish consumption uses remain "not assessed." 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 and 2001, (3) ISU lake survey in summers of 2000 and 2001, and (4) ISU report on lake plankton communities.
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 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 Lake Wapello beach, none of the 27 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 18 orgs/100ml in 2000 and 17 orgs/100 ml in 2001. None of the weekly samples collected during 2000 and 2001 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 and 73 orgs/100 ml in 2001). These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are extremely low.
The results of IDNR beach monitoring that suggest "full support" of the Class A uses are consistent with results from the ISU statewide survey of Iowa lakes that show exceptional water transparency and low levels of nuisance algal species at this lake. Using the median values from this survey in 2000 and 20 01 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 57, and 47, 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 middle to upper range of eutrophic lakes, and the index value for secchi depth is in the upper range of mesotrophic lakes. These index values suggest relatively high levels of phosphorus in the water column, relatively low (and much less than expected) levels of chlorophyll-a, and much better than expected 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 or zooplankton grazing) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake Wapello is 12. This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show a relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed 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 levels of inorganic suspended solids at this lake are low and do not suggest the potential for impairing designated uses. 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; the median level at Lake Wapello was 2.2 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 eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remian 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. Fish consumption remain assessed "not assessed" due to the lack of fish contaminant monitoring at this lake.
Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing a more accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments.
