Mud depocenters in shelf seas serve as a key element in the source-to-sink system of sediment transport on the Earth surface. Despite their undoubtful importance, physical mechanisms for their formation and functioning, sediment budgeting and cycling of localized depocenters remain largely unknown. This study aims to investigate the development of a spatially confined mud depocenter (“Helgoland Mud Area”) in the southern North Sea characterized by energetic hydrodynamics. By combining field observation with 3-dimensional numerical simulations, we analysed hydrodynamics and sediment dynamics over 2012–2014. Our results indicate a persistent transport of fine-grained sediments toward the depocenter and subsequent trapping resulting in accumulation, with distinct seasonal and spatial variations in the net depositional rate. The interaction of wind-driven coastal circulation with two distinct frontal systems—a salinity front and a tidal mixing front—emerges as a key mechanism of sediment dynamics. While the salinity front remains persistently over the HMA, promoting sediment deposition year-round, the tidal mixing front appears primarily in summer, limiting sediment deposition. Sediment inflows particularly from the paleo-Elbe valley and southwest coasts, dominate sediment supply to the HMA, while contemporary Elbe River sediment outflows contribute marginally due to a net landward transport. Southwesterly winds enhancing erosion and northerly winds promoting deposition. Additionally, short-term extreme events significantly contribute to annual net sedimentation. Our work highlights the influence of persistent tidal and wind forcing, sediment sources, and extreme events on mud depocenter development and underscores the need for further research into anthropogenic impacts on future fate of depocenters.

Bottom trawling represents the most widespread anthropogenic physical disturbance to shelf sea sediments. While trawling-induced mortality in benthic fauna has been extensively investigated, its impacts on ecosystem functioning and carbon cycling at regional scales remain unclear. Using the North Sea as an example, we address these issues by synthesizing a high-resolution dataset of bottom trawling impact on sediments, feeding this dataset into a 3-dimensional physical–biogeochemical model to estimate trawling-induced changes in biomass, bioturbation and sedimentary organic carbon, and assessing model results with field samples. Results suggest a trawling-induced net reduction in macrobenthic biomass by 10-27%. Trawling-induced resuspension and reduction of bioturbation jointly and accumulatively reduce the regional sedimentary organic carbon sequestration capacity by 21-67%, equivalent to 0.58-1.84 Mt CO2 yr-1. Our study emphasizes the need for proper management of trawling on muddy seabeds, if the natural capacity of shelf seas for carbon sequestration should be conserved and restored.

Seagrass meadows fulfil many essential ecological functions of which an important one is to stabilize sediment. Therefore, they are perceived as a nature-based alternative or addition to conventional rigid coastal protection. The magnitude of the impact by seagrass meadows depends on their morphology such as canopy height, stem density and spatial extent. However, deciduous, intertidal seagrass species are often simplified in modelling studies by adopting their annual mean height and density. This can lead to an erroneous estimate of their impact on hydro-morphodynamics and misconceptions about their contribution to coastal protection. Here, we assess the importance of seasonal change of seagrass properties for morphological development of a tidal basin in the Wadden Sea as an exemplary study. We applied numerical modeling to simulate the annual growth cycle of seagrass meadows and their interaction with hydro-morphodynamics. Based on validated seasonal change of seagrass properties by field surveys and comparison between scenarios of seagrass growth, our results show that adopting static seagrass parameters in modeling can lead to over- or underestimation of morphological changes induced by the seagrass meadows and even predict contrary results to simulations considering seasonal change of seagrass properties for the net sediment volume change in the intertidal zone. This points out the essential necessity of considering natural growth and decline cycles of seagrass meadows when assessing their role in coastal protection, especially in temperate zones where seasonal change of seagrass properties is distinct.

We present the first global ocean-biogeochemistry model that uses a telescoping high resolution for an improved representation of coastal carbon dynamics: ICON-Coast. Based on the unstructured triangular grid topology of the model, we globally apply a grid refinement in the land-ocean transition zone to better resolve the complex circulation of shallow shelves and marginal seas as well as ocean-shelf exchange. Moreover, we incorporate tidal currents including bottom drag effects, and extend the parameterizations of the model’s biogeochemistry component to account explicitly for key shelf-specific carbon transformation processes. These comprise sediment resuspension, temperature-dependent remineralization in the water column and sediment, riverine matter fluxes from land including terrestrial organic carbon, and variable sinking speed of aggregated particulate matter. The combination of regional grid refinement and enhanced process representation enables for the first time a seamless incorporation of the global coastal ocean in model-based Earth system research. In particular, ICON-Coast encompasses all coastal areas around the globe within a single, consistent ocean-biogeochemistry model, thus naturally accounting for two-way coupling of ocean-shelf feedback mechanisms at the global scale. The high quality of the model results as well as the efficiency in computational cost and storage requirements proves this strategy a pioneering approach for global high-resolution modeling. We conclude that ICON-Coast represents a new tool to deepen our mechanistic understanding of the role of the land-ocean transition zone in the global carbon cycle, and to narrow related uncertainties in global future projections.