Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> An increasing number of dead zoning (hypoxia) has been reported as a consequence of declining levels of dissolved oxygen in coastal oceans all over the globe. Despite substantial efforts a quantitative description of hypoxia up to a level enabling reliable predictions has not been achieved yet for most regions of societal interest. This does also apply to Eckernförde Bight (EB) situated in the Baltic Sea, Germany. The aim of this study is to dissect underlying mechanisms of hypoxia in EB, to identify key sources of uncertainties, and to explore the potential of existing monitoring programs to predict hypoxia by developing and documenting a workflow that may be applicable to other regions facing similar challenges. Our main tool is an ultra-high spatially resolved general ocean circulation model based on a code framework of proven versatility in that it has been applied to various regional and even global simulations in the past. Our model configuration features a spacial horizontal resolution of <span class="inline-formula">100 m</span> (unprecedented in the underlying framework which is used in both global and regional applications) and includes an elementary representation of the biogeochemical dynamics 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 model simulations, covering the period from 2000 to 2018, designed to cover a range of poorly known model parameters for vertical background mixing (diffusivity) and local oxygen consumption within EB. Feed-forward artificial neural networks are used to identify predictors of hypoxia deep in EB based on data at a monitoring site at the entrance of EB. Our results consistently show that the dynamics of low (hypoxic) oxygen concentrations in bottom waters deep inside EB is, to first order, determined by the following antagonistic processes: (1) the inflow of low-oxygenated water from the Kiel Bight (KB) – especially from July to October – and (2) the local ventilation of bottom waters by local (within EB) subduction and vertical mixing. Biogeochemical processes that consume oxygen locally are apparently of minor importance for the development of hypoxic events. Reverse reasoning suggests that subduction and mixing processes in EB contribute, under certain environmental conditions, to the ventilation of the KB by exporting recently ventilated waters enriched in oxygen. A detailed analysis of the 2017 fish-kill incident highlights the interplay between westerly winds importing hypoxia from KB and ventilating easterly winds which subduct oxygenated water.

Highlights

  • The impact of humans on the Earth system has reached a level of magnitude comparable to natural influences

  • This makes sense because the longest residence times can be found at the surface (Fig. 8), suggesting that, on average, water enters the bight at depth and leaves the bight at the surface

  • Oxygen concentrations are controlled by the antagonistic interplay of respiration and ventilation processes, both of which may respond antagonistically to climate change and improved management of water resources (e.g., Lennartz et al, 2014; Hoppe et al, 2013)

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Summary

Introduction

The impact of humans on the Earth system has reached a level of magnitude comparable to natural influences. Among the changes apparently accompanying our way into the Anthropocene are decreasing oxygen concentrations in the global oceans. This decrease in oxygen is manifesting itself most prominently in coastal regions: in the 1960s only 42 of the so-called “dead zones”, which no longer permit the survival of higher animals, were reported. In 2008 this number already increased to 400 (Diaz and Rosenberg, 2008). The Baltic Sea in central northern Europe is a prominent example of a coastal region that has been exposed to intermittent dead zoning (i.e., hypoxic events) in the past (Zillén et al, 2008). Hypoxia has increased over time in response to anthropogenic nutrient inputs and ocean warming (Jonsson et al, 1990; Carstensen et al, 2014).

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