Baltic Sea Sediments Lose Their Power to Trap Sulfide as Hypoxia Persists

A 15-month study of a seasonally hypoxic coastal basin in the southwestern Baltic Sea reveals that repeated summer oxygen depletion does not simply pause life on the seafloor—it rewires it. Published in Communications Earth & Environment, the research combined water-column monitoring, sediment chemistry, measured metabolic rates and microbial analyses with historic data to track how benthic processes shift as bottom waters swing between oxygenated and anoxic states. The authors found that these swings progressively erode the sediment’s ability to keep toxic sulfide locked away, creating a feedback that can accelerate and intensify coastal hypoxia over time.

The team documented pronounced seasonal changes in sediment metabolism. During winter, when oxygenated waters return, organic matter degradation ramps up, along with fermentation, aerobic respiration, nitrate reduction, iron reduction and intense microbial competition. Perhaps the most striking finding, however, is what winter re-ventilation does not do: it fails to reset sediment biogeochemistry. Instead, each return of oxygen triggers intensified oxygen consumption, heightened microbial biosynthetic activity and renewed anaerobic carbon recycling—meaning the sediment emerges from anoxia primed to consume oxygen faster than before.

Below the surface, a longer-term transformation was underway. Reactive iron oxyhydroxides—minerals that can bind sulfide and keep it out of the water column—became exhausted as the sulfidic zone shoaled closer to the sediment surface. Sulfate reduction and pyrite formation rates increased through the fall, and the study concluded that these processes reinforce benthic feedbacks that progressively reduce the sediment’s sulfide-retention capacity over years. In practical terms, eutrophic coastal sediments that once acted as a buffer against sulfide are steadily losing that protective function.

The implications extend beyond the seafloor itself. As sediments weaken, sulfide accumulates more readily in bottom waters, raising the probability of sulfidic deep-water upwelling—events that can deliver toxic, oxygen-poor water into shallower habitats. Because nutrient enrichment and warming are driving widespread coastal deoxygenation worldwide, the mechanisms documented here suggest a self-amplifying loop: oxygen loss alters sediment chemistry, which in turn makes future oxygen loss harder to reverse. The study’s scope is a single seasonally hypoxic coastal system observed over 15 months, so the authors’ conclusions speak most directly to environments with similar seasonal oxygen swings, but they offer a mechanistic explanation for why hypoxia in such settings can deepen year after year even when individual seasons of recovery might have been expected to restore conditions.

Subject of Research: Benthic biogeochemical feedbacks under persistent seasonal deoxygenation in the southwestern Baltic Sea

Article Title: Persistent seasonal deoxygenation alters benthic metabolic processes and accelerates hypoxia feedbacks in the Baltic Sea

Article References: Alampalli, S. V., Böhnke-Brandt, S., Adam-Beyer, N., Laufer-Meiser, K., Wallmann, K., Bange, H. W., Hepach, H., Thiele, M., Scholz, F., Wyler, E., Landthaler, M., Diercks, I., Fuß, J., & Perner, M. (2026). Persistent seasonal deoxygenation alters benthic metabolic processes and accelerates hypoxia feedbacks in the Baltic Sea. Communications Earth & Environment, 7(1), Article 844. https://doi.org/10.1038/s43247-026-04120-8

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04120-8

Keywords: Baltic Sea, hypoxia, anoxia, sediment biogeochemistry, sulfide, iron oxyhydroxides, sulfate reduction, pyrite, de Oxygenation, eutrophication, microbial metabolism, coastal upwelling