Assessment Comments
Assessment based on (1) results of IDNR beach monitoring program, (2) ISU lake survey in 2000-01, (3) surveys by IDNR Fisheries Bureau, and (4) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to high levels of (1) indicator bacteria and (2) non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to turbidity, nutrients, and siltation. Fish consumption uses are "not assessed." The sources of data for this assessment include (1) results of the IDNR beach monitoring program in summer seasons of 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 phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results of IDNR beach monitoring at Lake Darling during 2000 and 2001 suggest that the Class A uses are "not supported." Levels of indicator bacteria at Lake Darling 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 levels 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 Darling beach, the geometric mean of three of the 13 thirty-day periods during the summer of 2001 exceeded 200 orgs/100 ml; these geometric means were as follows: 786, 469, and 252 orgs/100 ml. All three geometric means are based on monitoring in May and early June of 2001. According to IDNR Section 305(b) assessment methodology, these results suggest non-support of the Class A (primary contact recreation) uses. Levels of these indicator bacteria tended to be lower in summer 2000, with a maximum 30-day geometric mean of 82 orgs/100 ml. In both summers, three samples from this beach contained levels of fecal coliforms that exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 1,100 orgs/100 ml in 2000 and 5,800 orgs/100 ml in 2001). Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses are also impaired by high levels of non-algal turbidity. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 85, 51, and 74, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range of eutrophic lakes, and the index value for secchi depth is in the lower range of hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, low (and much less than expected) levels of chlorophyll-a, and poor water transparency. The low TSI value for chlorophyll-a relative to that for total phosphorus and secchi depth suggests that non-algal turbidity limits production of suspended algae. Of 130 lakes sampled as part of the ISU survey in 2000 and 2001, Lake Darling had the sixteenth highest median level of inorganic suspended solids (17.6 mg/l), thus suggesting both limitation to production of suspended algae and contribution to turbidity-related impairments of the primary contact recreation uses. Due to the low levels of chlorophylll-a in this lake, impairments due to either aesthetically objectionable blooms of algae or nuisance algal species (e.g., bluegreen algae) are not likely. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of Lake Darling, especially in mid to late summer). However, the low levels of chlorophyll-a at this lake suggst that the presence of bluegreens does not currently present a threat to suppport of the Class A uses. The hyper-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 "partially supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Fish consumption uses remain "not assessed" due to lack of recent fish contaminant monitoring at this lake.
