Assessment Comments
Assessment is based on results of (1) IDNR/UHL beach monitoring from 2000-02, (2) ISU lake survey from 2000-02, (3) surveys by IDNR Fisheries Bureau, (4) ISU report on lake plankton communities in 2000, (5) results of EPA/DNR fish tissue (RAFT) monitoring in 1997, and (6) a UHL water supply survey in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting" based on results of IDNR beach monitoring and ISU lake survey information. Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses assessed (evaluated) as "fully supporting / threatened" due to potential nutrient-related problems. Fish consumption uses remain assessed (evaluated) as "fully supported" based on fish contaminant monitoring in 1997. The Class C (drinking water) uses remain assessed (evaluated) as "fully supported" based on results of sampling conducted by UHL in January 1995. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) results of the IDNR / UHL beach monitoring program from 2000-02, (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 U.S. EPA / IDNR fish contaminant monitoring in 1997, and (6) results of a drinking water study conducted by UHL in January 1995.
EXPLANATION: Results of monitoring for indicator bacteria at both Crandalls Beach and Marble Beach suggest “full support” of the Class A uses. Results of IDNR / UHL monitoring at Crandalls Beach in 2001 and 2002 suggest that the Class A (primary contact recreation) uses are "fully supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (April through October) of 2001 and 2002. 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 thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a thirty-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 thirty-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 thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any thirty-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 or the 23 thirty-day periods during summer 2002 at Crandall's beach had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric mean was 108 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; two samples exceeded 400 orgs/100 ml. in summer 2002. The maximum weekly sample value for the two-year period was 1,300 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 very low. Results of IDNR / UHL monitoring at Marble Beach in 2002 also suggest that the Class A (primary contact recreation) uses are "fully supported." None of the 22 thirty-day periods during summer 2002 had a geometric mean (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric mean was 47 orgs/100 ml. Two samples exceeded 400 orgs/100 ml. at Marble Beach in summer 2002. The maximum weekly sample value during summer 2002 was 760 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 very 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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 62, 47, and 48, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth are in the middle to upper range of mesotrophic lakes. These index values suggest that, despite the somewhat elevated levels of total phosphorus, the production of suspended algae in this lake 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 conditions due either to blooms of algae or high levels of inorganic turbidity. 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) limits production of algae. Results of ISU monitoring, however, do not immediately suggest a non-phosphorus limitation that can account for the discrepancy between the TSI for total phosphorus (62) and those for chlorophyll-a and Secchi depth (47 and 48, respectively). Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Big Spirit Lake is 16. This ratio suggests that algal production at this lake is likely limited by availability of phosphorus (although the TN:TP ratio for the 2000-01 period was 9.7, thus suggesting that algal production at this lake may be nitrogen-limited in some years). 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 median summer mass of Cladocerans in 2000 (5.9 mg/l) was the 39th lowest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for either threatening or impairing designated uses. 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 Big Spirit Lake was 2.5 mg/l; this median level was the 21st lowest of the 131 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. The 2000 median summer mass of bluegreen algae (8.3 mg/l) was the 55th lowest of the 131 lakes sampled. Thus, because the production of suspended algae at this lake is extremely low (in the mesotrophic range), these populations of bluegreen algae are not believed to either threaten or impair the primary contact recreation use of this lake.
The mesotrophic to 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. 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 and 1998 reports). 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 (evaluated) as "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1997 (see assessment developed for the 1998 report).
