Assessment Comments
Assessment based on (1) surveys by IDNR Fisheries Bureau, (2) results from the IDNR beach monitoring program in 2000 & 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Drinking water uses and fish consumption uses remain "not assessed." The sources of data for this assessment include (1) results of IDNR beach monitoring in 2000 and 2001, (2) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results of IDNR beach monitoring suggest "full support" of the Class A uses. Levels of indicator bacteria at Prairie Rose Lake 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 Prairie Rose 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 13 orgs/100ml in 2000 and 11 orgs/100 ml in 2001. None 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. The single sample maximum values were very low: 130 orgs/100 ml in 2000 and 120 orgs/100 ml in 2001. These are some of the lower seasonal levels of indicator bacteria reported for the IDNR beach monitoring program. 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. Despite the results of IDNR beach monitoring that suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A uses of Prairie Rose 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 72, 64, and 68, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes, the index values for chlorophyll-a and secchi depth are in the range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, moderately low levels of chlorophyll-a (suspended algae), and relatively poor 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) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Prairie Rose Lake is 17. This TN:TP ratio suggests that nitrogen may limit production of algae at this lake. In addition, data from Downing et al. (2002) show that the zooplankton community of Prairie Rose Lake has relatively large populations of species known as algal grazers. Sampling in 2000 showed that Daphnia comprised just below 20% of the dry mass of the zooplankton community in the mid-July sample but increased to approximately 80% in the early September sample. The relatively high TSI value for secchi depth suggests potential impairments to the Class A (primary contact) uses through presence of non-algal turbidity. According to Carlson (1991), the occurrence of a high TSI value for secchi depth relative to that for chlorophyll-a suggests that non-algal turbidity limits algal production. The levels of inorganic suspended solids at this lake are moderately high and suggest at least the potential for contributing to in-lake turbidity 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; the median level at Prairie Rose Lake was 7.0 mg/l. This is the 47th highest value of the 130 lakes sampled. Also, the presence of nuisance algal species (i.e., bluegreen algae) suggest potential impairments to Class A uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake, with greater than 80% of the wet mass in bluegreen algae in mid to late summer. Sampling in 2000 showed the percent wet mass of bluegreens increased from approximately 65% in the late June sampling to approximately 85% in the mid-July sampling and approximately 90% in the early September sampling. The Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported / threatened " based on information from the DNR Fisheries Bureau. Although results of ISU lake monitoring in 2000 and 2001 suggest that nutrient-related impairments may exist, neither the amount of data available for characterizing water quality nor the magnitude of the TSI values are sufficient for identifying impairments with the level of confidence necessary to justify adding this lake to Iowa's Section 303(d) list. Thus, further investigation and monitoring are needed to (1) better characterize current water quality conditions and (2) verify that such impairments exist. Drinking water uses remain "not assessed" due to lack of information upon which to base an assessment. Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake (see results for the 2000 report above).
