Assessing marine sedimentary carbon disturbance and seabed recovery in response to benthic trawling: a case study in the Celtic Sea

Bottom trawling is a widespread fishing practice that has consistently been established to result in harmful, enduring effects including physical modification of the seafloor, and impacts on biogeochemical cycling as well as benthic ecosystems. The mechanisms of the equipment used, such as trawl doors, penetrate the seafloor and disturb the sediment structure, resuspending particulate matter and altering the seafloor's organic matter composition and morphology. 

Previous studies have assessed the impacts of trawling on carbon storage in marine sediments, utilising sediment samples obtained from the seafloor and the water column to analyse the volume of sediment disturbed physically and geochemically. Additionally, Vessel Monitoring System (VMS) data has been employed to track fishing vessels and determine fishing intensity. These datasets are then widely used to determine the sedimentological and geochemical response to the trawl activity to demonstrate the potential global implications for disturbances in Blue Carbon environments regarding carbon remineralisation. However, post-trawl sampling intervals are irregular, differing in frequency and sampling methodology across studies. 

The Celtic Sea is an area of significant trawling activity, with a region known as “The Smalls”, a particular area of focus as a Nephrops fishing ground. The area comprises a predominantly muddy substrate and is intensely trawled using Otter trawl gear. A field experiment was conducted at two sites in the Celtic Sea in 2024 investigating the sediment and organic matter recovery following a trawl event. To do so, a benthic lander was deployed to measure seafloor community oxygen consumption rates. Sediment cores (30 cm) were recovered to calculate and qualify organic carbon stocks in addition to particle size analysis. Similarly, water samples were gathered near the seafloor to measure suspended sediment concentrations. Geophysical data was also gathered to measure seafloor sediment volumetric changes, persistence of trawl scars and sediment plume dynamics. The two sites examined in this study differed in their substrate type, with one site comprising muddy sediment and another slightly coarser silty sediment. A coordinated effort with the Irish Groundfish Survey (IGFS) allowed researchers to survey the two sites 12-24 hours in advance of a trawl occurring, establishing a baseline of environmental conditions. A geophysical survey was conducted in unison with the IGFS during trawling events and the area was then resurveyed over a time series of 1 hour to a week post trawl. Establishing pre-, during and post-trawl conditions at the two trawl sites and offering a thorough understanding of how the sedimentary carbon properties change following anthropogenic disturbance. 

Results indicate that oxygen consumption increased three times the original pre-trawled levels following a trawl event, with implications for carbon release. The use of ramped pyrolysis oxidation on core samples will elucidate changes in sedimentary organic carbon quality through carbon transformation/remineralisation. Different substrates at the two sites will allow for the analysis of differing sediment plume dynamics, with the finer substrate remaining in suspension for longer, potentially increasing transformation in the water column. The results of this study have implications for marine management in the area in the context of anthropogenic sediment disturbance.