Assessment Comments
Assessment based on (1) surveys of DNR Fisheries Bureau, (2) results of IDNR beach monitoring from 2002-04, (3) results of the ISU lake survey from 2000-04, (4) ISU reports on lake plankton communities, 2000-05, and (5) EPA/DNR fish contaminant (RAFT) monitoring in 2004.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" based on results of IDNR beach monitoring. The Class B(LW) aquatic life uses are assessed (evaluated) as “fully supporting". Siltation and nutrient impacts in the upper end of the lake, however, remain of concern. Fish consumption uses are assessed (monitored) as "fully supported” based on fish contaminant monitoring in 2004. The 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 2004.
EXPLANATION: Results of IDNR beach monitoring from 2002 through 2004 suggest that the Class A uses are "not supported." Levels of indicator bacteria at Big Creek Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (29 samples), 2003 (29 samples), and 2004 (23 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 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. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A uses should be assessed as “not supported”. Also, if more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Big Creek Lake beach, the geometric means for 6 of the 19 thirty-day periods during the summer recreation season of 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. None of the 25 geometric means for the recreational seasons of 2002 and 2003 exceeded this standard. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was greater than 10% in the following recreation season: 2004: 22%. Less than 10% of the samples exceeded this standard during the recreational seasons of 2002 (0%) and 2003 (3%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
Despite results of beach monitoring that suggest impairment, results from the ISU statewide survey of Iowa lakes suggest very good water transparency at Big Creek Lake during the assessment period. 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 60, 51, and 50, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range or eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the lower range of eutrophic lakes. These index values suggest relatively low levels of total phosphorus, very low levels of chlorophyll-a, and very good water transparency.
According to Carlson (1991), the occurrence of a high TSI values for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicates that some factor (e.g., nitrogen limitation. zooplankton grazing, or some other factor) limits production of algae. The results of ISU monitoring from 2000-04, however, do not suggest any significant non-phosphorus limitations on algal production at Big Creek Lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Big Creek Lake is 187. This ratio suggests an excess of nitrogen at this lake and that algal production is limited by phosphorus. This very high TN:TP ratio reflects the often exceptionally high levels of total nitrogen in impoundments of the Des Moines Lobe region of north-central Iowa. For example, Big Creek Lake had the 10th highest median summer total nitrogen over the 2000-024 period (8.7 mg/l) of the 131 lakes sampled for the ISU survey.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 55% of the dry mass of the zooplankton community of this lake, the average per summer sample mass of Cladoceran taxa over the 2000-2005 period (120 mg/l) was only the 85th highest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Big Creek Lake.
The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for either limiting algal production, threatening designated uses, or impairing these uses. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000-04 was 5.2 mg/l; the median level at Big Creek Lake was 1.7 mg/l: this median level was the fifth lowest of the 131 lakes sampled.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. While data results of monitoring from 2000-2004 suggest that bluegreen algae (Cyanophyta) comprise approximately 70% of the wet mass of the summertime phytoplankton community of this lake, the median 2000-04 summer per sample mass of bluegreen algae at Big Creek Lake (5.7 mg/l) is the 29th lowest of the 131 lakes sampled. The presence of bluegreen algae at this lake does not appear to either threaten or impair designated uses.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported ". High levels of nutrients and siltation in the upper portions of the lake remain concerns regarding aquatic life uses. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lakes. Results of this monitoring show that only 1 of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 1 of the 15 samples collected exceeded Iowa’s Class A,B(LW) criterioa for pH (maximum = 9.1; minimum = 6.7 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies of greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Big Creek Lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Big Creek Lake in 2004. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish 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 largemouth bass fillets were as follows: mercury: 0.043 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 2004 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.
