Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of the ISU lake survey from 2000-04, (3) ISU reports on lake plankton communities 2002-05, and (4) EPA/DNR fish tissue (RAFT) monitoring in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting”. The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting" based on results of summer monitoring for water quality and plankton communities. Fish consumption uses remain assessed as "fully supported" based on results of fish contaminant monitoring in 2000. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in September 2000.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest “full support” of the Class A uses of Beaver Lake. 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 68, 61, and 62, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest relatively high levels of phosphorus, moderately high levels of chlorophyll-a and good water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low 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 do not suggest any particular non-phosphorus factor that limits algal production at this lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Beaver Lake is 21. This ratio suggests that nitrogen availability does not significantly limit algal production at this lake. Similarly, the levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. 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 at Beaver Lake was 4.8 mg/l.
The suppression of algal production at this lake through zooplankton grazing, however, appears possible. In terms of all Iowa lakes sampled, data from the ISU survey show moderately large populations of zooplankton species at Beaver Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 60% 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 (107 mg/l) was the 55th highest of the 131 lakes sampled.
Based on the moderately good TSI values for chlorophyll-a and Secchi depth, this lake appears to have sufficiently good water quality to support its Class A uses.
In addition, the presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Although data from the ISU survey suggest that bluegreen algae (Cyanophyta) comprise a relatively large portion (approximately 64% wet mass) of the summertime phytoplankton community of this lake, the median summer sample mass of bluegreen algae during the 2000-2004 period at this lake (5.3 mg/l) was the 28th lowest of the 131 lakes sampled. Thus, the presence of bluegreen algae at this lake does not appear to either threaten or impair designated uses.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported." The ISU lake survey data suggest good water quality at Beaver Lake. These results show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected during summers of 2000 through 2004. Two of 14 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.4; minimum = 7.3 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Beaver Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Beaver Lake in 2000. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. 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 2000 RAFT sampling conducted in this lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
