Assessment Comments
Assessment is based on results of (1) IDNR beach monitoring from 2002 through 2004, (2) the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) ISU information on plankton communities at Iowa lakes from 2000-05, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2003.
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 2000-04 suggest “partial support” of the Class A uses due to poor water transparency caused by a combination of algae blooms and moderately high levels of inorganic suspended solids. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Drinking water uses remain "not assessed" due to the lack of information upon which to base an assessment. Fish consumption uses are assessed (monitored) as “fully supported” based on fish contaminant monitoring conducted in 2003. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2002, 2003, and 2004, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey, and (5) results of U.S. EPA / IDNR fish tissue monitoring in 2003.
EXPLANATION: Results of IDNR beach monitoring at Prairie Rose Lake from 2002 through 2004 suggest that the Class A uses are "not supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (30 samples), 2003 (29 samples), and 2004 (23 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A uses should be assessed as “not supported”. Also, if more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Prairie Rose Lake beach, the geometric means of 5 of the 26 thirty-day periods during the summer recreation seasons of 2002 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. None of the geometric means exceeded this standard during the recreational seasons of 2003 (25 geometric means) or 2004 (19 geometric means). Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was greater than 10% in the 2002 recreation season (13%). Less than 10% of the samples exceeded this standard during the recreational seasons of 2003 (0%) and 2004 (4%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 67, 65, and 67, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the middle to upper range between eutrophic and hyper-eutrophic lakes. These index values suggest relatively high levels of phosphorus in the water column, moderately high levels of chlorophyll-a (suspended algae), and moderately poor water transparency.
The general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Prairie Rose Lake. 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 22. This TN:TP ratio does not suggest significant nitrogen limitation of algal production at this lake.
Sampling from 2000 through 2005 showed that while Cladoceran taxa (e.g., Daphnia) comprised only about 30% of the dry mass of the zooplankton community of this lake, the average per summer sample mass of Cladoceran taxa over the 2000-2005 period (112 mg/l) was the 50th highest of the 131 lakes sampled. The potential for zooplankton grazing to limit algal production would appear to exist at this lake. The general agreement between the TSI values for total phosphorus (67) and that for chlorophyll-a (65), however, suggests that this type of non-phosphorus limitation does not occur at Prairie Rose Lake.
The relatively high TSI value for secchi depth (67) suggests potential impairments to the Class A (primary contact) uses through presence of turbidity that constitutes an aesthetically objectionable condition. 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 2004 was 5.2 mg/l; the median level at Prairie Rose Lake was 7.2 mg/l. This is the 42nd 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 contribute to reductions in water transparency.
These conditions suggest impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions (poor water transparency) caused primarily by high levels of inorganic suspended solids and secondarily by high levels of suspended algae.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Prairie Rose Lake (13.2 mg/l) was only the 59th highest of the 131 lakes sampled. This levels is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of the moderately high population of bluegreen algae at this lake does not immediately suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
The Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported " based on information from the DNR Fisheries Bureau. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Three of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.6; minimum = 8.0 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Prairie Rose Lake and do not reflect the input of pollutants into this lake.
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 2000-04 show that nitrate levels are very low at Prairie Rose Lake (maximum value = 3.8 mg/l; median = 0.4 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Prairie Rose Lake in 2003. The composite samples of fillets from channel catfish and largemouth bass had very low levels of contaminants. 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 largemouth bass fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2003 RAFT sampling conducted at Prairie Rose Lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
