Assessment Comments
Assessment based on results of (1) IDNR beach monitoring from 2000 through 2002, (2) the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton communities at Iowa lakes in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to results of IDNR beach monitoring that show high levels of indicator bacteria at this lake’s beach area. In addition, results of ISU lake monitoring from 2000-02 suggest “partial support” of the Class A uses due to high levels of turbidity caused by a combination of algae blooms and moderately high levels of inorganic suspended solids. The presence of moderately high populations of bluegreen algae (nuisance aquatic life) presents an additional threat to support of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Drinking water uses and fish consumption uses remain "not assessed" due to the lack of information upon which to base an assessment. Fish contaminant monitoring was conducted at this lake in 2003. The results from the monitoring, however, are not yet available. The sources of data for this assessment include (1) results of IDNR beach monitoring from 2000 through 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results of IDNR beach monitoring suggest "nonsupport" 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, 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 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. During summer 2002, however, three geomeans exceeded 200 orgs/100 ml, thus suggesting that the Class A uses are not supported. These high geomeans reflect the very high levels of fecal coliforms at the Prairie Rose beach during August 2002. The maximum 30-day geometric means were 13 orgs/100ml in 2000, 11 orgs/100 ml in 2001, and 417 orgs/100 ml in 2002. 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; four samples exceeded this maximum in summer 2002. These results suggest nonsupport of primary contact recreation uses at this beach.
In addition to the results of IDNR beach monitoring that suggest "nonsupport" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that moderately high levels of algal and non-algal turbidity combine to potentially impair the Class A uses of Prairie Rose Lake. Using the median values from the ISU lakes survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 70, 66, and 68, respectively. According to Carlson (1977), the index value for total phosphorus places this lake at the lower range of hyper-eutrophic lakes, the index values for chlorophyll-a and secchi depth are in the middle to upper range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, moderately high levels of chlorophyll-a (suspended algae), and moderately 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring suggest that nitrogen limitation, zooplankton grazing, and light attenuation through inorganic turbidity may limit algal production at Prairie Rose Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Prairie Rose Lake is 15. This TN:TP ratio is near the generally-accepted boundary that separates nitrogen-limited from phosphorus-limited lakes and suggests that nitrogen may, at times, 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 (Cladocerans) known as algal grazers. Sampling in 2000 showed that the Cladoceran 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 summer 2000 average mass of Cladocerans at this lake (68.5 mg/l) was the 22nd highest of the 131 lakes sampled and is sufficiently high to suggest the potential for zooplankton grazing to limit algal production. The relatively high TSI value for secchi depth suggests potential impairments to the Class A (primary contact) uses through presence of turbidity that constitutes an aesthetically objectionable condition. 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 thus suggest the potential for contributing to in-lake turbidity and poor water transparency. 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 Prairie Rose Lake was 4.4 mg/l. This is the 47th highest value of the 131 lakes sampled. Although not high relative to other Iowa lakes, and although probably not solely responsible for creating aesthetically objectionable levels of turbidity and moderately poor water transparency, the level of inorganic suspended solids at Prairie Rose Lake appears sufficiently high to (1) cause light attenuation and some reduction in algal production and (2) contribute to reductions in water transparency. Thus, the relatively high TSI value for secchi depth (68) suggests an impairment to the Class A (primary contact) uses through reductions in water transparency caused by both algal turbidity and non-algal turbidity. These conditions appear to violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The presence of nuisance algal species (i.e., bluegreen algae) suggest a potential threat to the full support of 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 summer 2000 average mass of bluegreen algae at Prairie Rose Lake (23.3 mg/l) was the 40th highest of the 131 lakes sampled. This mass of bluegreens is high relative to other Iowa lakes and suggests a potential threat to the full support of the designated uses of this lake. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to update this assessment and improve the accuracy of future water quality assessments for this lake.
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. Drinking water uses remain "not assessed" due to lack of information 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 very low at Prairie Rose Lake (maximum value = 1.5 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 "not assessed" due to the lack of recent fish contaminant monitoring at this lake (see results for the 2000 report). This lake was sampled in 1992 as part of the U.S. EPA/IDNR fish contaminant (RAFT) monitoring program. These data are now considered too old (greater than 10 years) for characterizing current conditions. This lakes was sampled again for the 2003 RAFT; the results from this monitoring are not yet available.
