Hypoxia and Anoxia in the Gdansk Deep (Baltic Sea) in 2021–2023

The central part of Gdansk Deep experienced summer-autumn dissolved oxygen minima within 2021–2023, while along the periphery of the Deep no seasonality was detected. Intrusions of transformed North Sea waters with a low oxygen content can partially aerate the bottom layer and cause anoxic waters to detach from the bottom. In the near-slope southeastern part of the Deep, in autumn-winter storm conditions, amplitude vertical fluctuations in oxygen concentration can occur – from the rise of hypoxic waters above the halocline, to the complete elimination of hypoxia as a result of near-slope downwelling, which acts as an “oxygen pump”. The hydrological and hydrochemical characteristics of intrusions of transformed North Sea waters penetrating into the Gdansk Basin and influencing the dynamics of the nepheloid layer of biogeochemical origin, arising at the boundary of oxidizing and reducing conditions, have been clarified.