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Abstract: A monitoring effort to address the physical effects of bottom trawling was conducted on the Ebro prodeltaic mud belt during the RESPONSE project. The monitoring was carried out for 14 months covering periods of different trawling intensities and a close season for the trawling fleet. The seabed morphology was studied by side-scan sonar and sediment texture and organic carbon content were analysed. Suspended sediment vertical distribution was recorded by CTD+turbidity hydrographic profiles and sediment transport was computed using time series from moored turbidimeters and current meters. The results show that the seabed of the fishing ground is strongly affected by scraping and ploughing induced by bottom trawling. Part of the finer fraction of the prodeltaic mud resuspended by trawling is winnowed, increasing the silt content of the settling sediment and also near-bottom turbidity. Sediment resuspended by trawling is incorporated in the bottom nepheloid layer and transported across- and along-shelf, increasing sediment fluxes. Trawling also induces an increase in the organic carbon content in the bottom sediment. All these effects induced by trawling have occurred during the last few decades, changing natural conditions in the fishing ground. [Copyright &y& Elsevier]

Abstract: To investigate the processes by which sediment is transported through a submarine canyon incised in a continental margin affected by recurrent dense shelf water cascading events, several instrumented moorings were deployed in the Cap de Creus Canyon from September 2004 to September 2005. This was done as part of the EuroSTRATAFORM Program that investigated sediment transport and accumulation processes in the Gulf of Lions. Results obtained in this observational study confirm that major cascading events can effectively contribute to the rapid export of sediment from the shelf and upper slope to deeper environments, and suggest that the associated strong currents carrying coarse particles are able to erode the canyon floor and generate sedimentary furrows. During winter 2004–2005, persistent northerly winds and the absence of river floods contributed to decrease the buoyancy of coastal waters and to dramatically enhance the intensity of dense shelf water cascades in the Gulf of Lions. Under such conditions, cascading continuously affected the entire Cap de Creus upper canyon section for more than a month and sustained cold temperatures and down-canyon steady currents >60cm/s (up to 100cm/s), showing periodic fluctuations that lasted between 3 and 6 days. Increases in suspended sediment concentrations were associated with dense shelf water cascading outbursts, but the magnitude of the concentration peaks decreased with time, suggesting a progressive exhaustion of the resuspendable sediments from the shelf and canyon floor. Grain size analyses of the particles caught by a near-bottom sediment trap show that dense shelf water cascades are able to transport coarse sediments (up to 65% sand) in suspension (and presumably as bed load), which have the potential to abrade the seafloor and generate erosive bed forms. The orientation of a large field of “wide” (i.e., widths about 1/2 spacing indicative of erosive formation) sedimentary furrows recently observed in the Cap de Creus Canyon clearly coincides with the preferential direction of highest velocities measured by the moored current meters, indicating a causative relationship between contemporary dense shelf water cascades and furrow formation. [Copyright &y& Elsevier]

Abstract: A 1-year survey of sediment dynamics on the Têt inner-shelf in the south-western part of the Gulf of Lions was conducted as part of the EUROSTRATAFORM program (European Margin Strata Formation) from October 2004 to November 2005. Several bottom instruments (ADCP, wave gauge and altimeters) were deployed at 28m water depth on the Têt prodelta to measure forcing responsible for sediment erosion and transport on the inner-shelf. The survey period was characterized by low inputs from the coastal rivers and the absence of severe E–SE marine storms, but intense cold dense shelf water formation along the inner-shelf due to strong and persistent N–NW continental winds. The strong (>20cm/s) and prolonged (4 months) southwards alongshore currents during the preconditioning and formation phases of dense shelf water generated continuous sediment erosion of several centimetres on the Têt prodelta and significant suspended-sediment transport. Hydrodynamical modelling shows that the core of the flow predominantly affects the inner-shelf, but occasionally spreads over the mid-shelf mud belt and the outer-shelf, due to the variable intensity of the NW wind (Tramontane). The impact of this N–NW wind-driven process in terms of resuspension rates and suspended-sediment transport clearly differs from that of extreme E–SE storms observed during the previous winter at the same site [Guillén, J., Bourrin, F., Palanques, A., Durrieu de Madron, X., Puig, P., Buscail, R., 2006. Sediment dynamics during wet and dry storm events on the Têt inner shelf (SW Gulf of Lions). Marine Geology 234 (1–4), 129–142.]. Bottom shear stresses and resuspension rates due to strong dense shelf water flow peaked at 0.7–1Pa and 0.3–0.4g/m2/s, respectively, while they reached values of 6–9Pa and 1.6–1.8g/m2/s, respectively, during the strong E–SE storms events. However, due to differing durations, both led to significant (few centimetres) erosion of the inner-shelf sediment. [Copyright &y& Elsevier]