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Sea trials were performed on two zones with different fishing efforts on the continental shelf of the Bay of Biscay ('Grande-Vasière' area of muddy sand) in order to assess particulate matter resuspension and seabed disturbances (i.e., penetration, reworking, grain size changes) induced by different types of trawls. Optical and acoustic measurements made in the water column indicate a significant trawling-induced resuspension mainly due to the scraping action of doors. It manifests as a highly dynamic turbid plume confined near the seabed, where suspended sediment concentrations can reach 200 mg l. Concentration levels measured behind an 'alternative' configuration (trawls with jumper doors instead of classical doors penetrating the sediment) are significantly lower (around 10-20 mg l), which indicates a potential limiting impact regarding the seabed. Grain size analyses of the surficial sediment led to highlight a potential reworking influence of bottom trawling. On the intensively trawled zone, this reworking manifests as an upward coarsening trend in the first 5 cm of the cores. A significant decrease in mud content (30 %) has been also witnessed on this zone between 1967 and 2014, which suggests an influence on the seabed evolution. The geometric analysis of bottom tracks (4-5-cm depth, 20-cm width) observed with a benthic video sledge was used to compute an experimental trawling-induced erosion rate of 0.13 kg m. This erosion rate was combined with fishing effort data, in order to estimate trawling-induced erosion fluxes which were then compared to natural erosion fluxes over the Grande-Vasière at monthly, seasonal and annual scales. Winter storms control the annual resuspended load and trawling contribution to annual resuspension is in the order of 1 %. However, results show that trawling resuspension can become dominant during the fishing high season (i.e., until several times the natural one in summer). In addition, the contribution of trawling-induced resuspension is shown to increase with water depth, because of the rapid decay of wave effects. Finally, the seasonal evolution of the respective contributions for erosion (mainly trawling and waves) could be mapped for the whole study area. [ABSTRACT FROM AUTHOR]

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-Vasière 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-Vasière). 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 m 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-Vasière 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. • Trawling-induced sediment dynamics is significant over mid and outer shelf regions. • Trawling increases natural off-shelf export of fine sediment by 30%. • Trawling induces seabed nature changes over and around fishing zones. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the seabed morphodynamics of the south Aquitaine inner shelf in the area known as “La Salie” (150 km 2 , Atlantic ocean, west coast of France), through a descriptive and comparative analysis (time lapse of 29 years) of geophysical and sedimentological datasets. At a water depth of 24–50 m, four orders of sedimentary body types were observed at different scales. The first order are large cross-shore “morphological ridges,” corresponding with the properties of very large sorted bedforms. The second order consisted in patchy sorted bedforms, composed of alternately medium to fine sand patches (0.5–2 m in thickness), cut by smaller, elongated coarse sediment depressions. In particular, the data from the sub-bottom profiler revealed that sand patches predominantly overlayed the coarse-grained blankets on the eastern (shoreward) extremities, while coarse-grained blanket wedges were found in front of the sand patches (southwestward) or locally overlying them on the southwestern extremities. The third order of bedforms involved groups of dune-like features (fine/medium sand), lying in wide areas of coarse-grained sediment. Finally, in the fouth order, the entire inner shelf was covered with wave-generated ripples, oriented N15°, that were larger where sediments were coarse (wavelengths of 2.2 m) than where sediments were fine (wavelengths of 0.3 m). Over the past 29 years, at a large scale of observation, patchy sorted bedforms have remained remarkably persistent, as has their overall appearance. However, at a smaller scale, weak but constant movements were observed. The coarse depressions have become elongated at their extremities (by a maximum of 300 m over 15 years), and certain coarse/fine sediment boundaries have moved toward the northeast and southeast (by a maximum of 75 m over 12 years). The general movement has been shoreward as has the migration of third-order submarine dune-like features. The persistence of sorted bedforms thus appears to be the consequence of sediment sorting feedback and recurrent storm events. [ABSTRACT FROM AUTHOR]