Assessment Comments
Assessment based on (1) results of the IDNR beach monitoring program in 2000 & 2001 and (2) surveys by IDNR Fisheries Bureau, (3) ISU lake survey in 2000-01, & (4) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" primarily due to high levels of (1) indicator bacteria and (2) non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "not supporting" primarily due to non-algal turbidity and siltation. The Class C (drinking water) uses and fish consumption uses remain "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 in 2000 and 2001 suggest "nonsupport" of the Class A uses. Levels of indicator bacteria at Bob White 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 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 Bob White beach, none of the 14 thirty-day geometric means for summer 2000 exceeded the Iowa Water Quality Standard of 200 orgs/100 ml (maximum geometric mean = 103 orgs/100 ml; maximum sample value = 3,200 orgs/100 ml). In summer 2001, however, the geometric means for seven of the 12 thirty-day periods exceeded 200 orgs/100 ml; these geometric means were as follows: 256, 727, 873, 1,318, 878, 857, and 302 orgs/100 ml. All seven geometric means were based on beach monitoring in early June through mid-July of 2001. According to U.S. EPA guidelines for Section 305(b) reporting, if a 30-day geometric mean based on at least five samples is greater than 200 orgs/100ml, the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, based on multiple geometric mean values greater than 200 orgs/100 ml in summer 2001, the Class A uses of this lake are assessed as "not supported." Results from the ISU statewide survey of Iowa lakes also suggest impairments to Class A uses. Data from ths study suggest that high levels of non-algal turbidity may affect the Class A uses of Bob White Lake. 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 90, 55, and 79, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the upper and middle range of hyper-eutrophic lakes, respectively; the index value for chlorophyll-a is low (and much less than expected) for Iowa lakes. These index values suggest extremely high levels of phosphorus in the water column, mush less than expected production of suspended algae, and very poor water transparency. The low levels of chlorophyll-a relative to the extremely high levels of phosphorus suggest a limitation to the production of suspended algae. Such limiatation may be related to high levels of inorganic suspended solids or to a low ratio of total nitrogen to total phosphorus (TN:TP). Of 130 lakes sampled in 2000 and 2001, Bob White Lake had the fifth highest median level of inorganic suspended solids (25.3 mg/l), thus suggesting a limitation to the production of algae as well as contributing to impairments of both the primary contact recreation and aquatic life uses from non-algal turbidity. The relatively low TN:TP ratio (6) for this lake also suggest a limitation to production of suspended algae. Based on the relatively low TSI for chlorphyll-a, neither the Class A or Class B uses appear impaired due to aesthetically objection blooms of algae. In addition, data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of Bob White Lake. Thus, nuisance algal species do not appear to be a problem at this lake. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 5% in the late June sampling, up to approximately 40% in the late July sampling, and back down to approximately zero% in the late August sampling. The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "not supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. The Class C (drinking water) uses and fish consumption uses remain "not assessed" due to lack of information upon which to base an assessment of these uses.
