This study combined three years of monthly water column sampling at five stations from the North Sea to the central Baltic Sea, capturing gradients in salinity, redox conditions, and biological productivity to characterize Se speciation over time.
Despite remarkably similar total dissolved Se concentrations across all sub basins, speciation showed contrasting spatial and vertical patterns. Marine influenced waters from the North Sea were dominated by bioavailable oxidized Se species (selenite and selenate), while the brackish Baltic Proper was enriched in reduced organic and elemental Se. Surface waters showed consistent selenite depletion, indicating biological uptake, while reduced and elemental Se formed during phytoplankton blooms and were transported downward, accumulating in hypoxic and euxinic waters.
These patterns point to fundamentally different regimes: well mixed marine systems efficiently recycle bioavailable Se through frequent water exchange, while the permanently stratified Baltic acts as a significant sink for reduced Se in oxygen depleted deep waters. This highlights how water column structure and redox conditions govern Se bioavailability, with implications for micronutrient dynamics as oxygen minimum zones expand and coastal stratification shifts with climate change.
The authors of this publication included Esther Breuninger (EDGE), Elyssa Beyrouti, Julie Tolu, Zoé Le Bras, Sylvain Bouchet and Lenny H. E. Winkel. The facilities involved in the study were the Institute of Biogeochemistry and Pollutant Dynamics at ETH Zurich, the Swiss Federal Institute of Aquatic Science and Technologynstitute of Biogeochemistry and Pollutant Dynamics at ETH Zurich, and the Swiss Federal Institute of Aquatic Science and Technology at Dübendorf.
