Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of the ISU lake survey from 2000-02, (3) ISU report on lake phytoplankton communities in 2002, and (4) EPA/DNR fish tissue (RAFT) 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" due to high levels of nutrients (phosphorus) that may contribute to nutrient-related water quality problems (e.g., algal blooms). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting" based on results of summer monitoring for water quality and plankton communities. Fish consumption uses are 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton 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 from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 68, 58, and 58, 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, 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 2002, the ratio of total nitrogen to total phosphorus for Beaver Lake is 18. 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 2002 was 4.8 mg/l; the median level at Beaver Lake was 4.3 mg/l. Data for zooplankton were not reported for this lake in Downing et al. (2002). Based on the TSI values for chlorophyll-a and Secchi depth, this appears to have relatively good water quality. In addition, 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.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should remain assessed as "fully supported." 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.
