Biogeochemistry of Selenium in different aquatic sytems (Lakes and estuaries)

Selenium (Se) exists in aquatic systems in multiple oxidation states (VI, IV, 0, -II) in a wide variety of species. Selenate and selenite are generally found in natural waters but may not account for total Se content indicating the co-existence of unidentified reduced Se compounds. Due to the role of Se as a micro-nutrient for living organisms, the characterization of Se compounds must be done to better constrain Se biotic and abiotic transformations and fate in aquatic systems. Selenium occurring in trace amounts in most aquatic environments, sensitive analytical methods are required to achieve this goal. In addition, the number of studies reporting on selenium speciation and cycling in non-polluted aquatic environments is still limited claiming for more field investigations. In this work, the optimization of chromatographic separation based on mixed-mode stationary phase combining reverse phase and anionic exchange allowed the simultaneous separation of six inorganic and organic compounds. The developed method together with a sensitive method for the determination of volatile Se compounds, were applied in parallel for the first time to provide new insights on the Se cycle in different aquatic systems as function of biogeochemical and seasonal variations. In a stratified eutrophic lake (Lake Kinneret, Israel), the existence of reduced and most probably organic Se containing compound actively produced by phytoplankton was revealed and suggested as a precursor for Se volatilization. Meanwhile under anaerobic conditions, the reduction of oxidized Se species to reduced forms could result in Se removal to sediments. In oligotrophic alpine lakes (Pyrenees lakes, France - Spain), selenate was the major compound in water and represented 63% of total Se. Downstream in the Adour estuary, the influence of agricultural land use on Se inputs was demonstrated with a clear relationship between Se and nitrates water concentrations, while selenium uptake and transformation to reduced and volatile compounds was promoted seaward during warmer productive periods.