Selenium speciation dynamics in the Baltic and North Seas: Competing influences of marine and terrestrial inputs, biological activity, and redox conditions

Despite the importance of coastal areas for the transfer of selenium (Se) from rivers to the open ocean and its bioavailability to food webs, dissolved Se concentrations and speciation in these ecosystems remain poorly understood. We conducted a 3-yr study with monthly water column sampling at five stations spanning from the North Sea to the central Baltic Sea, capturing large gradients of salinity, redox conditions, and biological productivity to quantify dissolved Se species and characterize Se speciation across environmental gradients over time. Despite remarkably similar total dissolved Se concentrations (66 ± 17 ng·L⁻¹) across all sub-basins, Se speciation showed contrasting spatial and vertical patterns. Marine-influenced waters from the North Sea were dominated by bioavailable inorganic oxidized Se species (selenite: Se^IV and selenate: Se^VI), while the brackish Baltic Proper was enriched in reduced organic and elemental Se (Se^−II + Se⁰). Vertical profiles showed consistent Se^IV depletion in surface waters, indicating biological uptake across all stations, while Se^−II + Se⁰ exhibited both production during periods of high phytoplankton biomass and subsequent vertical transport, leading to accumulation in hypoxic and euxinic waters. These contrasting patterns suggest fundamentally different speciation and transformation regimes: well-mixed marine systems maintain efficient recycling of bioavailable inorganic Se through frequent water exchange, whereas the permanently stratified Baltic system creates a significant sink for reduced Se species in oxygen-depleted deep waters. Our results demonstrate how water column structure and redox conditions control Se bioavailability and fate in coastal systems, with implications for understanding micronutrient dynamics in expanding oxygen minimum zones and climate-driven changes in coastal stratification.