| Summary: | The Baltic Sea and North Sea are two highly productive, connected marginal seas in Northern Europe. Both are strongly influenced by inputs of terrestrial carbon but differ fundamentally in character. The Baltic Sea is a wind-driven, brackish water system that is almost completely enclosed by land and has residence times on the order of decades. In contrast, the North Sea is a tidally-driven, marine system, on the edge of the North Atlantic with residence times on the order of months. Episodic deep inflows of salty, oxygenated North Sea water penetrate the deep basins of the Baltic Sea, providing temporary oxygen supply to otherwise persistent hypoxic zones, while brackish, surface Baltic Sea water drains into the North Sea carrying with it a net export of carbon. Both systems have densely populated and intensively used coastlines, exposed to climate change and ever-increasing anthropogenic pressures. To understand the net carbon uptake behaviour of the coupled system, and how this might change in response to perturbations in atmospheric and river forcing, we reconstruct the seasonal and inter-annual variability in carbon pools and fluxes in the Baltic – North Sea continuum from 1993 to 2019 using a three-dimensional biophysical model. To our knowledge, this is the first hindcast of bulk biogeochemical and physical properties which explicitly models the flux of carbon between the seas and accounts for the observed increase in atmospheric CO 2 from 360 to 420 ppm in recent decades.
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