Assessment Comments
Assessment is based on (1) surveys by the DNR Fisheries Bureau, (2) results of the IDNR beach monitoring program from 2000-02, (3) results from the ISU lake survey from 2000-02, and (4) results from the ISU lake plankon studies in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to aesthetically objectionable conditions caused by large algal populations and by high levels of inorganic turbidity; levels of bluegreen algae were relatively high at this lake in 2000, thus suggesting an additional potential impairment of Class A uses. Results of IDNR beach monitoring show full support of primary contact uses during the 2000-02 period. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported /threatened" due to nuisance blooms of algae, resuspension of sediment, and the increasing population of common carp which contributes to these problems. Fish consumption remain "not assessed” due to the lack of recent fish contaminant monitoring at this lake. Sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers 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: 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, 2001, and 2002 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 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 Black Hawk Lake beach, none of the 47 thirty-day periods during summers of 2000, 2001, and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric means were 28 orgs/100ml in 2000, 9 orgs/100 ml in 2001, and 34 orgs/100 ml in 2002. Only one of the 34 weekly samples collected during this three-year period 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 very low levels of bacteria in the lake, results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported." 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 88, 72, and 75, respectively, for Black Hawk Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) extremely high levels of phosphorus in the water column, (2) very high, but somewhat less than expected, production of suspended algae, and (3) very poor water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) limits production of algae. The ISU lake data suggest that algal production at Black Hawk Lake is limited by nitrogen availability, zooplankton grazing, and by inorganic turbidity. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Black Hawk Lake is 7; this ratio strongly suggests that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show that the zooplankton community of this lake has a large population (approximately 50% dry mass) of species known as algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (96 mg/l) was the 16th highest of the 131 lakes sampled. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to high levels of non-algal turbidity that may limit algal production. 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. The median level of inorganic suspended solids at Black Hawk Lake (20.0 mg/l) was the eleventh highest of the 131 lakes, thus suggesting that non-algal turbidity limits the production of algae. Based on overall summer median values the 2000-02 period, Black Hawk Lake had the eleventh highest median chlorophyll-a concentration and the fourteenth poorest Secchi depth of the 131 lakes sampled. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions due to blooms of algae and due to high levels of inorganic turbidity that reduce water transparency. The IDNR Fisheries Bureau concurs that high levels of algae present an impairment of designated uses at this lake.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to impairment at this lake. 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 approximately 80% of the phytoplankton community in the mid-June sample and 95% in the mid-July and early August samples. The average mass of bluegreen algae in summer 2000 at this lake (46 mg/l) ranked 21st highest of the 131 lakes sampled. The presence of relatively large populations of bluegreen algae suggests an impairment of support of the designated uses of this lake due to the potential for violating Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest that designated uses are impaired, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
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 nuisance blooms of algae and to re-suspension of sediment; the increasing population of common carp, and their tendency to increase levels of turbidity through re-suspension of sediment and algal nutrients, 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). This lake was, however, sampled in 2003 as part of the U.S. EPA/IDNR RAFT fish contaminant monitoring program. The results from this sampling, however, are not yet available.
