Assessment Comments
Assessment is based on (1) surveys by the DNR Fisheries Bureau, (2) results of the IDNR beach monitoring program from 2002-04, (3) results from the ISU lake survey from 2000-04, and (4) results from the ISU lake plankon studies from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to aesthetically objectionable conditions caused by poor water clarity; this impairment appears due primarily to inorganic turbidity and secondarily to large populations of suspended algae. Results of IDNR beach monitoring, however, show “full support” of primary contact uses during the 2002-04 period. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported " although nuisance blooms of algae, re-suspension of sediment, and the increasing population of common carp remain water quality concerns at this lake. Fish consumption are assessed as “fully supported” based on results of fish contaminant monitoring in 2003. Sources of data for this assessment include (1) results of IDNR/UHL beach monitoring from 2002-2004, (2) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (3) surveys by IDNR Fisheries Bureau, (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 2003.
EXPLANATION: Results of IDNR beach monitoring at Black Hawk Lake from 2002 through 2004 suggest that the Class A uses are "fully supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (27 samples), 2003 (28 samples), and 2004 (16 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Black Hawk Lake beach (Campground Beach), the geometric means of all 59 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 were below the Iowa water quality standard of 126 orgs/100 ml. Also, none of the 71 samples collected during the three recreational seasons exceeded Iowa’s single-sample maximum criterion of 235 E. coli orgs/100 ml. According to U.S. EPA guidelines and IDNR’s assessment methodology, these results suggest “full support” of the Class A (primary contact recreation) uses.
Despite the very low levels of bacteria in the lake, results of monitoring conducted by ISU from 2000 through 2004 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported" due to very poor water transparency. Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 77, 64, 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) moderately high, and 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 primarily by inorganic turbidity. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for Black Hawk Lake is 15. This ratio, although somewhat low, does not suggest a strong nitrogen limitation at this lake. During the 2000-2002 period, however, the data suggested a TN:TP ratio of 7 for this lake, this lower ratio more strongly suggests that algal production at this lake is at times limited by nitrogen availability.
In addition, data from the ISU survey show moderately large populations of zooplankton species at this that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 30% of the dry mass of the zooplankton community and that Black Hawk Lake had the 47th highest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled. This level of zooplankton grazers may be able to suppress algal production.
Data on inorganic suspended solids from the ISU survey suggest that algal production at this lake most strongly affected by the extremely high levels of non-algal turbidity. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l. The median level of inorganic suspended solids at Black Hawk Lake (18.4 mg/l) was the 14th highest of the 131 lakes, thus suggesting that non-algal turbidity limits the production of algae and accounts for the discrepancy between the TSI for phosphorus (77) and the TSI for chlorophyll-a (64).
Based on overall summer median values the 2000-04 period, Black Hawk Lake had the 17th 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 poor water transparency caused primarily by high levels of inorganic turbidity and secondarily by algae suspended in the water column. The IDNR Fisheries Bureau concurs that high levels of algae present an impairment of designated uses at this lake.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may threaten support of the Class A uses. Sampling during this time showed the percent wet mass of bluegreens comprised nearly 80% of the phytoplankton community. Black Hawk Lake, however, had only the 38th highest per summer sample mass of bluegreen algae (24.4 mg/l) of the 131 lakes sampled. While elevated relative to most Iowa lakes, this median is not in the worst 25% of the 131 Iowa lakes sampled.
Information from the IDNR Fisheries Bureau suggest that the Class B(LW) aquatic life uses are "fully supported". The following factors, however, remain of concern at this lake: nuisance blooms of algae, 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. The ISU lake survey data also show good chemical water quality at this lake. Results of this monitoring show no violations of the Class A,B(LW) criteria for pH in the 15 samples collected during summers of 2000 through 2004 (maximum = 8.7; minimum = 6.8 pH units). One of the 14 samples collected exceeded the Class B(LW) criterion for dissolved oxygen. Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(LW) uses of Black Hawk Lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Black Hawk Lake in 2003. The composite samples of fillets from common carp and black crappie had low levels of contaminants. Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of black crappie fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2003 RAFT sampling conducted at this lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
