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Intrusions of large amounts of dense and oxygen‐rich waters during so‐called Major Baltic Inflows (MBIs) form an essential component of the Baltic Sea overturning circulation and deepwater ventilation. Despite their importance, however, detailed observations of the processes occurring in the central basins during an MBI are virtually lacking. Here data from a long‐term deployment of an autonomous profiling platform located in the center of one of the main basins are presented, providing the first direct and detailed view of the deepwater modifications and dynamics induced by one of the largest MBIs ever recorded (MBI 2014/2015). Approximately, 21 Gmol of oxygen were imported during three distinct inflow phases with an unexpectedly large contribution of oxic intrusions at intermediate depth. Oxygen consumption rates during the stagnation period immediately following the inflow phase was found to be 87 g m−2 yr−1 with a dominant contribution of sedimentary oxygen demand. The most energetic deepwater processes (topographic and near‐inertial waves) were only marginally affected by the inflow; however, subinertial energy levels associated with intrusions and eddies were strongly enhanced. Turbulence microstructure data revealed that the deep interior regions remain essentially nonturbulent even during the energetic conditions of an MBI, emphasizing the importance of boundary mixing. Warm intrusions frequently showed a temperature fine structure with vertical scales of the order of 0.1 m, without any signs of active turbulence. At the upper flanks of these intrusions, double‐diffusive staircases were often found to develop, suggesting an important alternative mixing process during inflow conditions.

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Many coastal seas suffer from expanding bottom‐water deoxygenation and hypoxia primarily because of excessive nutrient loads from land. The Baltic Sea in northern Europe has one of the largest anthropogenically induced oxygen‐deficient bottom zones in the world. Despite the decrease of nutrient supply after the 1980s, recently observed oxygen consumption rates are higher than ever observed, limiting the impact of natural ventilation by oxygen‐enriched saltwater intrusions. We have estimated oxygen consumption rates after saltwater inflows during subsequent stagnation periods from monitoring observations and model results for 1850–2015. In recent years, ventilating water that originates mainly from the surface layer has contained higher concentrations of organic matter, zooplankton, and higher trophic levels. As a result, oxygen consumption in the water column has increased relatively more than oxygen consumption in the sediment, primarily due to respiration of zooplankton and higher trophic levels. Subsequently, natural ventilation has become less effective in alleviating hypoxia, instead amplifying deoxygenation of the deep water. We propose that such a detrimental, positive feedback may also affect other coastal seas with nutrient excess and with an intense, internal recirculation. Other drivers of oxygen consumption, like warming, were found to be less important under contemporary conditions.