Assessment Comments
Assessment is based on (1) results of IDNR beach monitoring in 2000-01, (2) ISU lake survey in 2000-01, (3) ISU report on lake phytoplankton communities, and (4) surveys by IDNR Fisheries Bureau.
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 remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "not assessed." The sources of data for this assessment include (1) results of IDNR 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, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results of IDNR beach monitoring in suggest full support of the Class A uses. Levels of indicator bacteria at Lake MacBride Beach were monitored once per week during the primary contact recreation seasons (April through October) 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 Lake MacBride Beach, none of the 28 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 20 orgs/100ml in 2000 and 18 orgs/100 ml in 2001. None of the 35 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 80 orgs/100 ml in 2000 and 120 orgs/100 ml in 2001). These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are extremely low.
Despite the results of IDNR beach monitoring that suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that moderately high levels of phosphorus may threaten full support of the Class A uses of Lake MacBride. 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 64, 55, and 60, 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 value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is at the upper boundary of eutrophic lakes. These index values suggest moderately high levels of phosphorus, relatively low levels of chlorophyll-a, and relatively good water transparency. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake MacBride is 40.1. This ratio suggests that algal production at this lake is phosphorus-limited. The levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong 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 Lake MacBride was 4.1 mg/l. Despite the indications of good water quality, the moderately 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 the presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in summer 2000 showed that bluegreen algae accounted for from 70 to 95% of the wet mass of the phytoplankton community of this lake. 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. 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 excessive nutrient loading to the water column, high levels of non-algal turbidity, siltation in the lake, and the presence of exotic species (e.g., common carp). 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. Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring at this lake (see assessment for the 2000 report).
