ASSESSING SELENIUM CYCLING AND ACCUMULATION IN AQUATIC ECOSYSTEMS

We conducted a joint experimental research and modeling study to devel op a methodology for assessing selenium (Se) toxicity in aquatic ecosy stems. The first phase of the research focused on Se cycling and accum ulation. In the laboratory, we measured the rates and mechanisms of ac cumulation, transformation, and food web transfer of the various chemi cal forms of Se that occur in freshwater ecosystems. Analytical develo pments helped define important Se forms. We investigated lower trophic levels (phytoplankton and bacteria) first before proceeding to experi ments for each successive trophic component (invertebrates and fish). The lower trophic levels play critical roles in both the biogeochemica l cycling and transfer of Se to upper trophic levels. The experimental research provided the scientific basis and rate parameters for a comp uter simulation model developed in conjunction with the experiments. T he model includes components to predict the biogeochemical cycling of Se in the water column and sediments, as well as the accumulation and transformations that occur as Se moves through the food web. The model ed processes include biological uptake, transformation, excretion, and volatilization; oxidation and reduction reactions; adsorption; detrit al cycling and decomposition processes; and various physical transport processes within the water body and between the water column and sedi ments. When applied to a Se-contaminated system (Hyco Reservoir), the model predicted Se dynamics and speciation consistent with existing me asurements, and examined both the long-term fate of Se loadings and th e major processes and fluxes driving its biogeochemical cycle.