Assessment Comments
Assessment based on (1) results of the IDNR beach monitoring program form 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) results from the ISU lake survey from 2000-02, & (4) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to (1) aesthetically objectionable conditions caused by high levels inorganic turbidity and (2) results of IDNR beach monitoring that show high levels of indicator bacteria at this lake’s beach area. 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" due to the lack of water quality information upon which to base an assessment. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The sources of data for this assessment include (1) results of the IDNR beach monitoring program in summer seasons of 2000, 2001, and 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 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 from 2000 through 2002 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 2000-02 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 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 Bob White beach, none of the 43 thirty-day geometric means for the summers of 2000 and 2002 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 weekly beach monitoring in early June through mid-July of 2001. According to U.S. EPA guidelines for Section 305(b) reporting, if a thirty-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 this 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 from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 84, 55, and 80, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is low (and much less than expected) for Iowa lakes, and is in the middle of the range of eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, much less than expected production of suspended algae, and extremely poor water transparency.
According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data strongly suggest that non-algal particles do likely limit algal production at Bob White Lake. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. Of 131 lakes sampled, Bob White Lake had the sixth highest median level of inorganic suspended solids (25.9 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is the primary cause of the extremely poor water transparency. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The level of chlorophyll-a at Bob White Lake is relatively low (42nd lowest of the 131 lakes sampled) and does not appear to contribute to impairments at this lakes. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake. Other factors may limit algal production at Bob White Lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 10.5; this ratio suggests the possibility that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show very large zooplankton populations at this lake, including a large population (approximately 52% dry mass) of species known as algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (25 mg/l) was the 51st highest of the 131 lakes sampled. The extremely high level of inorganic suspended solids at this lake, however, remains the primary non-phosphorus limitation to algal production at this lake.
Nuisance aquatic (algal) species (i.e., bluegreen algae) do not appear to be a problem at Bob White Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion (approximately 4%) of the summertime phytoplankton community of this lake. 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 average mass of bluegreen algae in summer 2000 at this lake (1.7 mg/l) was the 17th lowest of the 131 lakes sampled. These conditions do not suggest any impairments due to presence of nuisance aquatic (e.g., algal) species.
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 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. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 4.9 mg/l; median = 1.3 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
