Assessment Comments
Assessment is based on (1) surveys by the DNR Fisheries Bureau and (2) results of the IDNR beach monitoring program in 2000 & 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "partially supported." The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported /threatened." Fish consumption remain "not assessed. Sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers 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: Levels of indicator bacteria at Black Hawk 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 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 Black Hawk Lake beach, none of the 22 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 28 orgs/100ml in 2000 and 9 orgs/100 ml in 2001. Only one of the 34 weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (the sample from July 31, 2000 contained 600 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. Despite the low levels of bacteria in the lake, results of monitoring conducted by ISU in 2000 and 2001 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are only "partially supported." 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, 72, and 77, respectively, for Black Hawk Lake. According to Carlson (1977), these index values place this lake in the upper range of hyper-eutrophic lakes and suggest (1) excessive phosphorus loading to the water column, (2) excessive, but less than expected, production of suspended algae, and (3) poor water transparency. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to high levels of non-algal turbidity. 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 of inorganic suspended solids at Black Hawk Lake (21.3 mg/l) was the eighth highest of the 130 lakes, thus suggesting that non-algal turbidity limits the production of algae as well as impairs beneficial uses. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and 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 Black Hawk Lake. Sampling in summer 2000 showed bluegreen algae comprised approximatley 80% of the phytoplankton community in the mid-June sample and 95% in the mid-July and early August samples. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 7. 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 "fully supported / threatened" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment; increasing populations of common carp also represent a threat to support of the aquatic life uses of this lake. Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake (see assessment for the 2000 report).
