Assessment Comments
Assessment based on (1) surveys of DNR Fisheries Bureau, (2) results of IDNR beach monitoring in 2000-01, (3) ISU lake survey in 2000-01, (4) ISU report on lake phytoplankton communities, and (5) fish contaminant monitoring in 1994.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are assessed as (evaluated) as "fully supporting / threatened." Fish consumption uses are assessed (evaluated) as "fully supported." The sources of data for this assessment include (1) results of IDNR/UHL 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, (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (5) results of U.S. EPA / IDNR fish contaminant monitoring in 1994.
EXPLANATION: Levels of indicator bacteria at Big Creek 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 Big Creek Lake 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 75 orgs/100ml in 2000 and 12 orgs/100 ml in 2001. Only one of the 34 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (the sample from July 10, 2000 contained 1200 orgs/100 ml; the next highest reading in summer 2000 was 100 orgs/100 ml). 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 results from IDNR beach monitoring that suggest “full support” of primary contact uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of nutrients (phosphorus) and the related potentnial for high levels of algal turbidity may threaten full support of the Class A (primary contact recreation) uses of Big Creek 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 66, 54, and 53, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest relatively high levels of phosphorus, relatively low (and less than expected) levels of chlorophyll-a, and very good (and better than expected) water transparency. According to Carlson (1991), the occurrence of a high TSI values for total phosphorus and chlorophyll-a with a relatively low value for secchi depth indicates 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 Creek Lake is 64.0. This ratio does not suggest a strong potential for nitrogen limitation at this lake. According to Downing et al. (2002), sampling in summer 2000 showed that over half of the late summer zooplankton community at Big Creek Lake (on a dry mass basis) was composed of Cladocerans known to be effective grazers on phytoplankton. This sampling showed that Cladoceran taxa (e.g., Daphnia) were not present in the late July sample but increased to comprise approximately 60% of the dry mass of the zooplankton community in the mid-August sample. The levels of inorganic suspended solids at this lake are 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 Creek Lake was 1.8 mg/l: this median level was the 11th lowest of the 130 lakes sampled. Despite the indications of good water quality, the relatively high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the phytoplankton community of this lake in mid and late summer. Sampling in 2000 showed that over 80% of wet mass of phytoplankton were blulegreen algae in the late July and mid-August samples. Although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is relatively low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact at this lake.
The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" based on review of the previous assessments by the DNR Fisheries Bureau (see assessments for the previous reports above). Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994 (see assessment developed for the 2000 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).
