Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities, (4) results of fish tissue monitoring in 1997, and (5) UHL water supply survey in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting." The Class B(LW) aquatic life uses assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain assessed (monitored) as "fully supported." The Class C (drinking water) uses are assessed (evaluated) as "fully supported." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) results of the IDNR / UHL beach monitoring program, (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), (5) results of IDNR / UHL beach monitoring in 2000 and 2001, (6) results of U.S. EPA / IDNR fish contaminant monitoring in 1997, and (7) results of a 1995 survey of 19 water supply reservoirs (Miller and Kennedy 1995). EXPLANATION: Results of IDNR / UHL monitoring at Crandalls Beach in 2001 suggest that the Class A (primary contact recreation) uses are "fully supported." Levels of indicator bacteria at Crandall's Beach were monitored once per week during the primary contact recreation season (April through October) of 2001. 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. None of the 13 thirty-day periods during summer 2001 at Crandall's beach had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum 30-day geometric mean was 27 orgs/100 ml. None of the 17 weekly samples collected at this beach in 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. The maximum weekly sample value was 150 orgs/100 ml. These results suggest "full support" of primary contact recreation uses at this beach and suggest that levels of indicator bacteria at Big Spirit Lake are extremely low.
Similarly, results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Big Spirit Lake are "fully supported." 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 68, 48, and 46, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of mesotrophic lakes, and the index value for secchi depth is in the middle range of mesotrophic lakes. These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is extremely low, and water transparency is exceptional for Iowa lakes. These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae. 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 Big Spirit Lake is 9.7. This ratio suggests that algal production at this lake is limited, at least in part, by nitrogen availability. Data from Downing et al. (2002) show relatively small populations of zooplankton species at Big Spirit Lake that graze on algae, thus suggesting little potential for algal grazers to limit algal production. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 10% of the dry mass of the zooplankton community in both the mid July and early August samples. The levels of inorganic suspended solids at this lake are relatively 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 Big Spirit Lake was 2.6 mg/l; this median level was the 28th lowest of the 130 lakes sampled. Despite the dominance of the phytoplankton community of this lake by bluegreen algae, this lake does appear to have problems due to presence of nuisance algal species. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a large portion of the summertime phytoplankton community of this lake. Sampling in 2000 showed that the percent wet mass of bluegreen algae ranged from approximately 70% in the mid-June sampling to 100% in the early August sampling. Because, however, the production of suspended algae at this lake is extremely low (in the mesotrophic range), these populations of blugreen algae are not believed to either threaten or impair the primary contact recreation use of this lake. The slightly eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to high levels of nutrients. 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 an 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.
The Class C (drinking water) uses remain assessed (evaluated) as "fully supported" based on results of sampling conducted by UHL in January 1995 (see assessment for the 1996 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence). Fish consumption uses remain assessed as "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1997 (see assessment developed for the 1998 report).
