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

Nutrient enrichment and warming cause widespread costal deoxygenation, leading to hypoxia or even anoxia in bottom waters. The benthic-water-column-feedbacks that develop under persistent seasonal oxygen depletion and amplify hypoxia remain poorly constrained. A 15-month study of a seasonally hypoxic to anoxic costal system in the southwestern Baltic Sea combined water-column monitoring, sediment chemistry, metabolic rates and microbial analyses with historic data to reveal pronounced seasonal shifts in sediment processes. During winter oxygenation, organic matter degradation, fermentation, aerobic respiration, nitrate- and iron-reduction and microbial competition increased. Re-ventilation following anoxia did not reset sediment biogeochemistry, but initiated intensified oxygen consumption, microbial biosynthetic activity and anaerobic carbon re-cycling. Reactive ironoxyhydroxides became exhausted, the sulfidic zone shoaled and sulfate reduction and pyrite formation rates increased through fall. These processes reinforced benthic feedbacks, progressively reducing the sediment’s capacity to retain sulfide over years. This weakening of sedimentary resilience exacerbates sulfide accumulation in eutrophic costal environments with growing chances of sulfidic deep-water upwelling.