Bottom trawling represents the most widespread anthropogenic physical disturbance to seafloor sediments on continental shelves. While trawling-induced changes to benthic ecology have been widely recognized, the impacts on long-term organic carbon storage in marine sediments remains uncertain. Here we combined datasets of sediment and bottom trawling for a heavily trawled region, the North Sea, to explore their potential mutual dependency. A pattern emerges when comparing the surface sediment organic carbon-to-mud ratio with the trawling intensity represented by the multi-year averaged swept area ratio. The organic carbon-to-mud ratio exhibits a systematic response to trawling where the swept area ratio is larger than 1 yr⁻¹. Three-dimensional physical–biogeochemical simulation results suggest that the observed pattern is attributed to the correlated dynamics of mud and organic carbon during transport and redeposition in response to trawling. Both gain and loss of sedimentary organic carbon may occur in weakly trawled areas, whereas a net reduction of sedimentary organic carbon is found in intensely trawled grounds. Cessation of trawling allows restoration of sedimentary carbon stock and benthic biomass, but their recovery occurs at different timescales. Our results point out a need for management of intensely trawled grounds to enhance the CO₂ sequestration capacity in shelf seas.

A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km², up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 μmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of postglacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

The contribution of sediments to nutrient cycling of the coastal North Sea is strongly controlled by the intensity of fluxes across the sediment water interface. Pore‐water advection is one major exchange mechanism that is well described by models, as it is determined by physical parameters. In contrast, biotransport (i.e., bioirrigation, bioturbation) as the other major transport mechanism is much more complex. Observational data reflecting biotransport, from the German Bight for example, is scarce. We sampled the major sediment provinces of the German Bight repeatedly over the years from 2013 to 2019. By employing ex situ whole core incubations, we established the seasonal and spatial variability of macrofauna‐sustained benthic fluxes of oxygen and nutrients. A multivariate, partial least squares analysis identified faunal activity, in specifically bioturbation and bioirrigation, alongside temperature, as the most important drivers of oxygen and nutrient fluxes. Their combined effect explained 63% of the observed variability in oxygen fluxes, and 36–48% of variability in nutrient fluxes. Additional 10% of the observed variability of fluxes were explained by sediment type and the availability of plankton biomass. Based on our extrapolation by sediment provinces, we conclude that pore‐water advection and macrofaunal activity contributed equally to the total benthic oxygen uptake in the German Bight.

Benthic oxygen fluxes consist mostly of advective and diffusive terms. Both terms in the German Bight exhibit a prominent annual cycle but with opposite variation patterns. To understand the driving mechanisms quantitatively, a novel 3‐D benthic‐pelagic coupled model resolving interactions among macrobenthos, bioturbation, oxygen consumption, and carbon early diagenesis was applied to reconstruct the benthic states. Simulation results show a satisfactory agreement with field data and reveal that the benthic oxygen flux is determined by not only pelagic drivers but also by internal dynamics associated with the interaction between organic carbon and macrobenthos, and bedform morphodynamics. Variation of advective flux, characterized by summer‐low and winter‐high, is mainly driven by hydrodynamics and bedform morphodynamics, while variation of diffusive flux, featured by summer‐high and winter‐low, is a compound effect of pelagic and benthic drivers with a dominant control by macrobenthos through bioturbation. The role of bioturbation in benthic oxygen consumption is twofold: (a) on the one hand, it alters the particulate organic carbon (POC) distribution in surface sediments, thereby changing the availability of POC to oxygen consumption; (b) on the other hand, it mixes oxygen down into sediments, thereby facilitating oxygen consumption. Our results indicate that the first role prevails in sandy seafloor characterized by energetic hydrodynamics, while the second role becomes increasingly important along with a weakening of bottom currents. We found that bioturbation contributes up to 87% 4% and 55% 8% of the total benthic oxygen fluxes in muddy seabed and at a regional scale (the German Bight), respectively.