Assessment Comments
Assessment based on (1) surveys of DNR Fisheries Bureau, (2) results of IDNR beach monitoring from 2000-02, (3) results of the ISU lake survey from 2000-02, (4) ISU report on lake phytoplankton communities in 2000, and (5) EPA/DNR fish contaminant (RAFT) monitoring in 1994.
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. The Class B(LW) aquatic life uses are assessed (evaluated) as fully supporting / threatened" due to siltation and nutrient impacts in the upper end of the lake. Fish consumption uses remain assessed (evaluated) as "fully supported” based on fish contaminant monitoring in 1994. The sources of data for this assessment include (1) results of IDNR/UHL beach monitoring from 2000-2002, (2) results of the statewide survey of Iowa lakes conducted from 2000-02 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, 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 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. At Big Creek Lake beach, none of the 47 thirty-day periods during summers of 2000, 2001, and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric means were 75 orgs/100ml in 2000, 12 orgs/100 ml in 2001, and 32 orgs/100 ml in 2002. Only one of the 63 weekly samples collected from 2000-2002 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). 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.
Results from the ISU statewide survey of Iowa lakes suggest very good water transparency at Big Creek Lake. 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 58, 54, and 54, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range or eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest relatively low levels of total phosphorus, very low levels of chlorophyll-a, and very good water transparency.
According to Carlson (1991), the occurrence of a high TSI values for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicates that some factor (e.g., nitrogen limitation. zooplankton grazing, or some other factor) limits production of algae. The results of ISU monitoring from 2000-02, however, do not suggest any significant non-phosphorus limitations on algal production at Big Creek Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Big Creek Lake is 128. This ratio suggests an excess of nitrogen at this lake and that algal production is limited by phosphorus. Big Creek Lake had the 13th highest median summer total nitrogen over the 2000-02 period (5.4 mg/l) of the 131 lakes sampled for the ISU survey. 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 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 median Cladoceran mass for summer 2000 was less than 1 mg/l and suggests little if any non-phosphorus limitation due to zooplankton grazing at this lake. 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 2000-02 was 4.8 mg/l; the median level at Big Creek Lake was 2.5 mg/l: this median level was the 23rd lowest of the 131 lakes sampled. 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 on a percentage basis but that the overall mass of bluegreens in Big Creek Lake is relatively low. Sampling in 2000 showed that over 80% of wet mass of phytoplankton were bluegreen algae in the late July and mid-August samples. The 2000 median summer mass of bluegreen algae (5 mg/l) was the 43rd lowest of the 131 lakes sampled. Thus, 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 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 and due to siltation in the upper portions of the lake.
Fish consumption uses remain assessed (evaluated) 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 above). 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).
