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Morphological change of coastal and shelf seas is controlled jointly by physical, biological and anthropogenic processes and their interactions. While physical and anthropogenic drivers are normally regarded to exert a primary control on morphodynamics, the role of biota, especially benthos, in guiding long-term and large-scale evolution of coastal landscape/seascape is often overlooked and has received less attention. It was not until recent decades that research has revealed the importance of benthos in coastal protection. Further, as an indispensable part of the ecosystem, benthos not only consists of a significant part of organic carbon, but also plays a vital role in mediating carbon and nutrient fluxes across the sediment-water interface. The interaction between benthos and its ambient environment is twofold and dynamic. On one hand, coastal morphology, oceanographic forcing (e.g. tides, waves) and carbon/nutrient availability exert a first-order control on the type of habitats, as well as abundance and trait expression of benthos. On the other hand, benthos actively modifies its habitats and beyond to attain an optimized fitness for living conditions. Understanding such dynamic interactions is essential in management of coasts and shelf seas against present and future climatic threats and for sustainable use of coastal resources.In this talk, we demonstrate the quantitative importance of fluid-sediment-benthos interactions in (1) guiding morphological development of a tidal embayment in the Wadden Sea and (2) carbon cycling in the great North Sea as exemplary case studies. We applied a novel 3-Dimensional physics-ecosystem coupled numerical model to resolve dynamic interactions among fluid forcing, organic carbon, sediment and benthos. Specifically, we aim to address two different but interrelated questions:How important are fluid-sediment-benthos interactions in coastal morphological development? and To what extent can such interactions affect carbon sequestration at a regional scale?

Bottom trawling has a direct impact on biogeochemical cycling and benthic-pelagic coupling. Despite extensive trawling activities in many shelf seas, the effects on benthic-pelagic exchanges is often not considered in marine ecosystem simulations. In this study, in order to gauge the effects of bottom trawling on North Sea ecosystem productivity, two model simulations were carried out for the period 2000-2005 using the 3D, fully coupled ecosystem model SCHISM-ECOSMO, which allows for coupled simulation of the benthic and pelagic ecosystem. The first simulation includes only natural resuspension while the second experiment also considers a parametrization for bottom trawling induced resuspension. Daily forcing for bottom trawling resuspension rates was generated based on available data of fishing activity including the position, size and engine power of individual vessels, in combination with estimated resuspension rates of various fishing gears in different types of sediment. The results of these simulations were then analyzed with regard to the importance of bottom trawling for the pelagic primary production of the North Sea ecosystem. In addition, we analyzed trawling-induced changes in bottom water oxygen and sedimentary carbon, phosphorus and nitrogen pools.

Many continental shelves host sediment depocenters which act as natural, long-term (>100 yr) carbon sinks. Human activities can strongly affect the efficiency with which carbon is sequestered in these depocenters, either through direct disturbances of the seafloor, or indirectly through climatic, light- or nutrient-induced changes, thereby affecting habitat and ecosystem functioning. In this study, we address the short- and long-term impacts of sea-use on carbon burial in the North Sea. Specifically, we focus on the role of bottom trawling as a crucial disturbance of seafloor sediments and benthic biota. In order to quantify the large-scale impact on carbon sequestration, we employ a numerical coastal ocean model to simulate the effects of demersal fishing gear on sediment transport, bioturbation efficiency and their interactions. Based on the results, the effects of potential management scenarios are discussed.

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.

Bottom trawling, a fishing practice that entails the dragging of heavy fishing gear along the seafloor, is one of the most direct human interferences with the marine environment. Bottom trawling is known to resuspend a large amount of sediment, thereby releasing nutrients, organic carbon and contaminants to the water column. However, the large-scale effects of bottom trawling on sediment and carbon fluxes have remained difficult to quantify. We aim to quantify these fluxes through numerical modeling of bottom trawling activity in the North Sea. A particular focus is put on muddy areas rich in organic carbon, as chronic bottom trawling could inhibit their function as carbon sinks. By combining a three-dimensional, coupled hydrodynamics and sediment transport model with data of trawling effort in the study area, we parametrize the resuspension caused by bottom trawling on the basis of individual vessels and the subsequent transport of resuspended material by ocean currents. Results show that bottom trawling has potentially large impacts on the inventories and fluxes of suspended matter and organic carbon of the North Sea. The results may be useful in determining areas worthy of protection in the context of marine spatial planning.

Mud depocenters (MDCs) represent major proximal-marine sinks for fine-grained terrigenous material, carbon, and contaminants on modern continental shelves. Throughout the past decades, several studies have shed light on the physical processes controlling MDC development at various timescales, ranging from controlled flume experiments and in-situ oceanographic monitoring, to stratigraphic analyses of recent and ancient deposits based on seismo-acoustic and sediment-core data. Thereby, key mechanisms related to the formation and maintenance dynamics of MDCs have been discovered: a) cross-shore bottom transport of suspended mud through gravity flows, b) interaction of mud with density gradients associated with oceanic fronts, c) resuspension and dispersal control of mud by internal waves, d) bedload deposition of mud forming laminated bedding under energetic flow conditions, and e) mud resuspension resulting from chronic bottom trawling. Among the physical processes identified or proposed, three conceptual paradigms for MDC development can be distinguished: 1. continuous supply, associated with a steady sediment supply and hemipelagic settling in relatively calm conditions; 2. continual resuspension-deposition cycles, wherein parts of an MDC area are subject to multiple cycles of resuspension, redeposition and reworking before ultimate burial; and 3. episodic sedimentation and erosion, in which extreme events such as riverine floods and atmospheric storms dominate the total, long-term sediment flux. Although the predominance of each of these paradigms within a single MDC depends to a large degree on the timescales considered, case studies tend to emphasize processes associated with only one of these three paradigms. As a result, the relative, long-term contribution of individual processes remains largely uncertain for many MDCs. The ability of numerical models to accurately predict medium to long-term mud accumulation is restricted not only by computational costs, but also by insufficient parametrizations of the muddy sedimentation process. These remain challenging to constrain due to the multiplicity and complexity of factors affecting the cohesive properties of mud, including its state of consolidation, and the amount and type of organic matter present. Bridging the gap between individual events and long-term accumulation is the key to a more complete understanding of sedimentation processes in MDCs.

The development of two Holocene muddy depocenters in the southwestern Baltic Sea is investigated using sediment budget analysis and numerical modeling. Material derived from the erosion of coastal cliffs surrounding the study area is shown to dominate the supply of fine-grained sediment to the depocenters, while the riverine contribution is an order of magnitude smaller. Comparison with the sink terms, compiled from published geological data, reveals that a substantial additional source of at least 900 kt/yr is required to close the budget, and high-salinity dense inflows from the North Sea carrying suspended sediment are proposed as an additional source mechanism. Seismo-acoustic data show the long-term impact of strong bottom currents, likely linked to dense-water inflows, which produce contouritic deposits in flow-confining channels. We reproduce two distinct inflow events using a coupled hydrodynamic-sediment transport coastal ocean model. The simulations confirm that major inflows are capable of advecting a significant amount of fine-grained sediment into the study area. A scaling relationship based on the simulated fluxes estimates the average amount of sediment imported in this way to the order of 100–900 kt/yr, which is in agreement with the lower limit of the gap in the budget. The amount of sediment advected seems to scale non-linearly with the intensity of the inflow. More field data points are needed in order to improve the accuracy of modeled fluxes and the precision of the scaling relationship. This study shows how the relative contributions of episodic sedimentation events on the longer-term morphology may be quantified.