Bottom trawling contribution to the spatio-temporal variability of sediment fluxes on the continental shelf of the Bay of Biscay (France)

Abstract

Abstract Based on experiments, a trawling-induced erosion dynamics was included in a 3D realistic and pre-validated hydro-sedimentary model so as to assess its contribution to the overall regional sediment dynamics in the specific case of the Bay of Biscay continental shelf. The effect of trawling was investigated throughout a 5-year period in terms of horizontal/vertical fluxes and changes in seabed mud content, and compared to the natural variability of sediment fluxes at different spatio-temporal scales. Trawling is shown in the model to significantly contribute to sediment re-suspension, and changes in the seabed at the scale of the shelf. The contribution of trawling to re-suspension is >80% during the high fishing season at many locations on the outer shelf and over the Grande-Vasiere area (a large and intensively trawled muddy zone located in the middle of the shelf). The contribution of trawling is significantly weaker in winter and at annual scale, particularly over zones with (muddy-) sand facies (between 1‰ and 1%), but remains high, and reaches 20% of the annual erosion over muddy areas (e.g. northern part of the Grande-Vasiere). This significant trawling-induced erosion dynamics results in high sediment fluxes which increase natural poleward dynamics from a few percent up to 40%, and contributes to the natural offshore export of sediment on the outer shelf (e.g. +35% of mud fluxes along the 130\u202fm isobath). It is noteworthy that trawling-induced fluxes are larger in winter although the corresponding re-suspension is lower than in summer: this is due to stronger near-bottom flows occurring at this period (largely influenced by energetic wind conditions). In contrast, on the inner shelf, trawling-induced sediment fluxes are rather oriented towards the coast. Finally, simulation results reveal a trawling signature on changes in the seabed. A significant decrease (between 10% and >50%) in mud content is simulated over trawling hotspots, especially over the internal part of the Grande-Vasiere and on mud patches located in front of the Gironde estuary. Conversely, trawling causes or intensifies mud deposition on both sides of these preferentially trawled areas.