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. An increasing number of dead zoning (hypoxia) has been reported as aconsequence of declining levels of dissolved oxygen in coastal oceans all overthe globe. Despite substantial efforts a quantitative description of hypoxiaup to a level enabling reliable predictions has not been achieved yet for mostregions of societal interest. This does also apply to Eckernförde Bight (EB)situated in the Baltic Sea, Germany. The aim of this study is to dissectunderlying mechanisms of hypoxia in EB, to identify key sources ofuncertainties, and to explore the potential of existing monitoring programs topredict hypoxia by developing and documenting a workflow that may beapplicable to other regions facing similar challenges. Our main tool is anultra-high spatially resolved general ocean circulation model based on a codeframework of proven versatility in that it has been applied to variousregional and even global simulations in the past. Our model configurationfeatures a spacial horizontal resolution of 100 m (unprecedented inthe underlying framework which is used in both global and regionalapplications) and includes an elementary representation of the biogeochemicaldynamics of dissolved oxygen. In addition, we integrate artificial “clocks”that measure the residence time of the water in EB along with timescales of(surface) ventilation. Our approach relies on an ensemble of hindcast modelsimulations, covering the period from 2000 to 2018, designed to cover a rangeof poorly known model parameters for vertical background mixing (diffusivity)and local oxygen consumption within EB. Feed-forward artificial neuralnetworks are used to identify predictors of hypoxia deep in EB based on dataat a monitoring site at the entrance of EB. Our results consistently show that the dynamics of low (hypoxic) oxygenconcentrations in bottom waters deep inside EB is, to first order, determinedby the following antagonistic processes: (1) the inflow of low-oxygenatedwater from the Kiel Bight (KB) – especially from July to October – and (2) thelocal ventilation of bottom waters by local (within EB) subduction andvertical mixing. Biogeochemical processes that consume oxygen locally areapparently of minor importance for the development of hypoxic events. Reversereasoning suggests that subduction and mixing processes in EB contribute,under certain environmental conditions, to the ventilation of the KB byexporting recently ventilated waters enriched in oxygen. A detailed analysisof the 2017 fish-kill incident highlights the interplay between westerly windsimporting hypoxia from KB and ventilating easterly winds which subductoxygenated water.

Retracing hypoxia in Eckernförde Bight (Baltic Sea)