Evaluating the Consequences of Bottom Trawling on Regional Biogeochemical Cycles and Benthic–Pelagic Ecosystem Functioning

Abstract

Bottom trawling is a fishing practice that has long been a source of political contention and scientific debate. While it remains a cornerstone of demersal fisheries, growing attention has focused on its broader ecological and climatic consequences. Recent global-scale studies have argued that bottom trawling disturbs sedimentary carbon stocks and accelerates CO2 release to the atmosphere, positioning it as a significant contributor to climate change. These claims have rapidly entered policy discourse, prompting calls for trawling bans in carbon-rich regions and the inclusion of seabed disturbance in national greenhouse gas inventories. However, these narratives often rest on simplified assumptions: they treat organic carbon loss as inevitable upon disturbance, neglect vertical and lateral carbon cycling, and exclude the role of biological feedback. As a result, much of the scientific and political urgency has preceded a detailed understanding of the system-scale dynamics underpinning carbon exchange in shelf seas. This thesis addresses that gap by presenting an integrated, systematic model-based assessment of how bottom trawling amends biogeochemical cycling, ecosystem productivity, and air–sea CO2 exchange in the North Sea. Through three interconnected studies, I show that the effects of trawling extend beyond immediate sediment disturbance and are far more complex than previously assumed. In the first study, I simulate the effects of trawling-induced sediment resuspension on primary and secondary productivity using the coupled 3D physical–biogeochemical model SCHISM-ECOSMO. The findings reveal strong regional responses. While increased turbidity suppresses productivity in the shallow Southern North Sea, enhanced nutrient fluxes from resuspended and remineralized organic matter increase productivity in stratified regions such as the Norwegian Trench. These outcomes are not only spatially heterogeneous but also seasonally dynamic, shaped by the interplay of light availability, nutrient limitation, and mixing regimes. In the second study, I extend this model to include a full carbonate chemistry module (SCHISM- ECOSMO-CARBON) to quantify changes in air–sea CO2 flux. Contrary to prevailing assumptions of net CO2 outgassing from trawled sediments, my results show that under certain ecological and hydrographic conditions, trawling can enhance CO2 uptake in the short-term. This is driven not by direct carbon burial, but by biologically mediated drawdown of dissolved inorganic carbon in sur- face waters. At the same time, the redistribution of remineralization from the benthic to the pelagic zone leads to enhanced respiration at depth, suppressing CO2 uptake in shallow mixed areas. These dual pathways of enhancement and suppression demonstrate that trawling does not produce a uniform biogeochemical signal and that its climatic impact depends critically on where and when it occurs. The third component of this work synthesizes observational data and long term end-to-end (E2E) ecosystem simulations to evaluate the distribution, transformation, and fate of organic carbon across trophic levels. I demonstrate that current carbon models consistently underestimate the role of zooplankton, fish, and benthic invertebrates in carbon export and cycling. Moreover, regions with high carbon accumulation potential such as depositional zones in the Norwegian Trench, Skagerrak and Helgoland Mud Area coincide with areas of high trawling intensity, suggesting that the potential for long term carbon storage is being undermined in precisely the areas where it matters most; neglecting not only the effects of anthropogenic activities like bottom trawling but also other activities like wind farms. Together, these studies challenge the notion that trawling-induced carbon disturbance always leads to an outgassing of CO2 to the atmosphere. They show that the biogeochemical response of shelf systems to physical disturbance is non-linear, localized, and dependent on ecological structure. By coupling hydrodynamics, ecosystem processes, air–sea gas exchange, and human impacts in a unified modeling framework, this thesis opens a new layer of understanding that has been overlooked in previous carbon accounting efforts and management frameworks. In doing so, it repositions bottom trawling not simply as a fisheries issue, but as a dynamic interface between ocean ecology and climate policy.