Managing shelf sea carbon cycles and greenhouse gas release from physical disturbance of the seafloor (C-floor)

The ocean is the largest carbon sink in the biosphere but has been neglected by research and policy relative to terrestrial carbon stores. Carbon deposits on shallow continental shelves are globally significant but unlike terrestrial carbon stores, such as forests or peat bogs, seabed carbon has not been properly mapped, quantified or assessed for risks from human activities. Recent studies suggest that anthropogenic seabed disturbance, principally by bottom fishing using mobile trawls that stir up sediment, could resuspend organic carbon into the water column where it is remineralised, thereby impairing the ocean's ability to mitigate the impacts of greenhouse gas emissions, in turn accelerating climate change. The science is contested due to the complexity of the processes involved and serious knowledge gaps relating to seabed and water column carbon cycling. For example, disturbance also mobilises nutrients, which may stimulate primary production and CO2, and enhance microbial production of nitrous oxide, a potent greenhouse gas. If we are to achieve net zero, we urgently need a holistic understanding of how fishing disturbance affects the processes of carbon uptake, burial and re-release from seabed sediments, taking into account the complex interactive effects of nutrient resuspension. The proposed project, C-FLOOR, will quantify on regional scales the effects of trawling on sediment particulate organic carbon and inorganic carbon and nutrients and the resulting release of greenhouse gases to the water column and atmosphere. To achieve these objectives, C-FLOOR will take advantage of extensive data archives that span environmental conditions for UK shelf seas, combined with ship-based field campaigns to measure active changes in carbon and nutrient stoichiometry in resuspended sediment plumes. The project takes a highly integrated interdisciplinary approach to answer the following questions: 1. How do trawling frequency and sediment type affect the potential for marine sediments to act as a net source of greenhouse gases in different seasons? 2: What is the fate of resuspended sediment and how does this modulate local seawater chemistry? 3. How do vertical mixing, water column production, and respiration affect the potential for trawl-driven biogeochemical change to result in measurable impact on air-sea exchanges? 4. What is the potential for recovery of seabed sediment carbon stocks and for additional carbon storage, under different spatial management regimes? C-FLOOR will identify patterns and causal relationships, using state-of-the-art autonomous observation technologies, combined with machine learning, to aid a holistic understanding of how bottom-trawl fishing modifies carbon cycles in seabed sediments and influences greenhouse gas exchanges (GHG). We will combine these observations with extensive manipulative laboratory and field experiments, informed by ongoing collaborations in related projects, that will link changes in carbon, nutrient stoichiometry and GHG dynamics (CO2, CH4, N2O) with the effects of different trawl gears, fishing intensity, historic fishing pressure, sediment types, species composition, and seasonal timing. Modelling will enable inferences on the larger scale and longer-term dynamics and fate of resuspended carbon and nutrients. Our science will provide confidence bound evidence needs to estimate the role of seabed disturbing fishing activities in climate change. With direct support from key partners, we will broker transformative knowledge exchange about the marine environment, and co-create, deliver and implement a natural capital-based decision support system for UK marine waters to identify and inform management options to sustain productive fisheries, while also helping achieve net zero.