Assessing Selenium Cycling and Accumulation in Aquatic Ecosystems

We conducted a joint experimental research and modeling study to develop a methodology for assessing selenium (Se) toxicity in aquatic ecosystems. The first phase of the research focused on Se cycling and accumulation. In the laboratory, we measured the rates and mechanisms of accumulation, transformation, and food web transfer of the various chemical forms of Se that occur in freshwater ecosystems. Analytical developments helped define important Se forms. We investigated lower trophic levels (phytoplankton and bacteria) first before proceeding to experiments for each successive trophic component (invertebrates and fish). The lower trophic levels play critical roles in both the biogeochemical cycling and transfer of Se to upper trophic levels. The experimental research provided the scientific basis and rate parameters for a computer simulation model developed in conjunction with the experiments. The 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 modeled processes include biological uptake, transformation, excretion, and volatilization; oxidation and reduction reactions; adsorption; detrital cycling and decomposition processes; and various physical transport processes within the water body and between the water column and sediments. When applied to a Se-contaminated system (Hyco Reservoir), the model predicted Se dynamics and speciation consistent with existing measurements, and examined both the long-term fate of Se loadings and the major processes and fluxes driving its biogeochemical cycle.