Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of fish tissue monitoring in 2000, (3) ISU lake survey in 2000-01, (4) ISU report on lake phytoplankton communities & (5) EPA/DNR fish tissue monitoring in 2000.
Basis for Assessment
[Note: This lake was added to the "Water Use Designations" of the Iowa Water Quality Standards in 2000.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting." Fish consumption uses are assessed as "fully supported." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in September 2000. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class A uses of Beaver Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 68, 55, and 56, 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 middle range of eutrophic lakes. These index values suggest relatively high levels of phosphorus but relatively low levels of chlorophyll-a and relatively 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 or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Beaver Lake is 16.7. This ratio suggests the potential for algal production at this lake to be nitrogen-limited. 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 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Beaver Lake was 4.6 mg/l. Despite the indications of good water quality, the high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) are a very minor component of the summertime phytoplankton community of this lake. Sampling conducted during 2000 did not produce evidence of bluegreen algae at this lake. Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported." Although results of ISU lake monitoring in 2000 and 2001 suggest a threat to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses are assessed as "fully supported." Results of U.S. EPA/IDNR fish tissue (RAFT) monitoring at Beaver Lake in September 2000 show that levels of all contaminants in the composite samples of fillets from channel catfish and largemouth bass were well-below ½ of the respective FDA action levels and IDNR levels of concern. Contaminant levels in these samples were extremely low, with levels of most contaminants below the analytical limits of detection.
