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Abstract: Experimental and field studies were carried out to quantify the hydrodynamic mobility under swells of a circalittoral epifauna species. The spatial distribution and structure of one population (NW Mediterranean) of the serpulid polychaete Ditrupa arietina was assessed before and after a moderate swell event in late winter 2006 with the two readings separated by a seven-week interval. The overall density in the bay did not change significantly. However, in certain locations, significant but heterogeneous changes occurred: at some stations, D. arietina disappeared, at others it appeared with high densities while at others, densities remained fairly constant, although in this third case, the population size structure was different. In addition, no trend was observed along a cross-shore gradient. Experimental studies were conducted to determine bed friction velocity thresholds for incipient motion, bed load transport and suspension transport for different sizes of D. arietina. The minimum friction velocity required to transport D. arietina as bed load over a smooth rigid bed ranged from 1.5 to for tube lengths ranging from 6 to 25mm. Thus, calcified worms (more than 6mm long) cannot be destabilized by ordinary currents. Conversely, during the moderate swell event, the maximum bed friction was large enough to mobilize D. arietina, making probably all sizes available for bed load transport, while only worms with a tube length of less than 12mm were transported in suspension at 27m water depth. Wave statistics in Banyuls Bay indicate that D. arietina transport under moderate swell events should not be a rare phenomenon (28 days per year at 20m water depth). Thus, the spatial scale for population dynamics studies should account for the extent of D. arietina spatial redistribution by the hydrodynamics. It is also recommended D. arietina be discarded from biotic quality index computations as this species is sensitive to natural hydrodynamic conditions. [Copyright &y& Elsevier]

Abstract: In the framework of the European project Eurostrataform, an array of six near-bottom mooring lines was deployed in the heads of the major submarine canyons incising the continental slope of the Gulf of Lions (NW Mediterranean). All moorings were equipped with sediment traps, current meters and turbidity Optical Backscatters Sensors (OBS) situated at few meters above the bottom. Particulate mass fluxes were recorded weekly by the sediment traps between November 2003 and May 2004 and compared with horizontal export fluxes obtained from the turbidity and current meters. Results show that near-bottom vertical particulate mass fluxes generally increase along slope between the northeastern-most and the southwestern region of the Gulf of Lions. Horizontal export fluxes also show some increase southwestward with flux in the southwestern-most canyon up to two-orders of magnitude higher than in northeast canyons. Furthermore, most of the export flux is driven by energetic and short events such as marine storms and dense shelf water cascading. Although the consequence of storms and cascading was obvious from traps and OBS measurements, substantial differences between vertical and horizontal fluxes appeared. While sediment traps recorded the largest vertical flux during the autumn storm (4 December) with contributions from river discharge and resuspended material, horizontal flux as obtained from OBS data was highest in winter (21–24 February). We believe that such discrepancies are partly due to measurement artifacts originating from the particle size effects and/or the different height above the bottom occupied by the traps and OBS which complicate comparisons between vertical and horizontal fluxes. However, different water-column conditions (stratified versus unstratified) together with the hydrodynamic regime during the two periods appear as a likely explanation for the flux differences between fall and winter storms. [Copyright &y& Elsevier]