Downward particle fluxes within different productivity regimes off the Mauritanian upwelling zone (EUMELI program)

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of similar to 71-78% biogenic origin (i.e. consisting Of CaCO3, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18 degrees 30'N, 21 degrees 00'W) to the remote, open-ocean, oligotrophic site (21 degrees 00'N, 31 degrees 00'W). This decrease largely reflects the difference in total primary production between the two sites, from similar to 260 to similar to 110 g organic C m(-2) yr(-1). At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site. (C) 2001 Elsevier Science Ltd. All rights reserved.