Spatial and temporal patterns of downward particle fluxes on the continental slope of the Bay of Biscay (northeastern Atlantic)

An array of six time series sediment traps and current meters was deployed in water depths of 2300 and 3000 m on two bottom-mounted moorings along the axis of the Cap-Ferret Canyon, a large depression in the southeastern part of the Bay of Biscay (northeastern Atlantic), as part of the ECOMARGE Atlantic experiment ECOFER. The 14-month experiment consisted of three successive deployments during which particle fluxes were collected over sampling periods of 16–27 d. Two large-aperture traps also were deployed to measure fluxes at a faster sampling rate (4–7 d). The recovered samples were analyzed for total mass, concentration of major constituents (organic and inorganic carbon, opal), activity of 210Pb (Radakovitch O., Heussner S., 1999. Fluxes and budget of 210Pb on the continental margin of the Bay of Biscay (northeastern Atlantic). Deep-Sea Research II 46, 2175–2203) and coccoliths (Beaufort L., Heussner S., 1999. Coccolithophorids on the continental slope of the Bay of Biscay – production, transport and contribution to mass fluxes. Deep-Sea Research II 46, 2147–2174). The temporal pattern of the major constituent fluxes essentially matched that of the total mass, suggesting that, in general, particle transfer to the slope is largely driven by events acting on the same time scale. The temporal variability of the measured concentrations and fluxes lacked in any obvious connection with large-scale driving forces, either external (wind, river discharge) or internal (biological production), which all exhibited a marked seasonal signal. The rapid changes of particle transfer, largely non-seasonal, is in contrast with most other ocean margin trap experiments. On the other hand, indications were found that the variability of mass fluxes could be controlled by the along-slope current, and particularly by its short-term fluctuations. These dynamical processes could be responsible for significant changes in the dispersion of the settling particles and hence of the resultant flux changes. The spatial distribution of annual mean fluxes – from 498 (380 m) to 1477 mg m−2 d−1 (2250 m) at the shallower site, and from 328 (1900 m) to 459 mg m−2 d−1 (2950 m) at the deeper site – exhibited a linear increase with increasing depth and a seaward decrease at equivalent depth or distance from the bottom. Such a distribution, typical of continental slope environments, requires inputs of material through lateral advection. The increased homogeneity of the bulk chemical composition of settling particles with increasing water depth and total mass flux indicates that such inputs are largely provided by resuspended material with quite constant characteristics, largely originating from the Bay of Biscayʹs shelf and upper slope. Due to the general northward, along-slope current, the sources are preferentially located south of the Cap-Ferret Canyon. A particle transfer scenario for this region is proposed that, in particular, calls for a basic mechanism of upstream–downstream concentration gradients in the sources of settling particles to explain the increase in flux with depth.