Particle flux in the Subtropical Atlantic near the Azores:: influence of mesozooplankton

The amount and composition of sinking material were studied at two depths (200 m and base of the photic layer) in the Subtropical Atlantic Ocean near the Azores during the AZORES I (August 1998) and II (April-May 1999) cruises. Particulate carbon and nitrogen fluxes collected in sediment traps decreased with depth, and presented maximum values near the Azores Front. However, this frontal system represents an increase of only 4.5% in the overall regional exportation during summer. Sedimentation rates outside the front were higher during spring. Particulate organic carbon exported at 200 m always represented <1.5% of water column (photic zone) standing stock. Mesozooplankton faecal pellets contributed significantly to carbon flux. On average, carbon in the form of faeces represented 31% (spring) and 65% (summer) of total carbon collected at 200 m. Composition of the copepod community seems to be related to the pattern of faecal pellet sedimentation, with omnivorous copepods (Oithona and Oncaea) being more abundant at stations where faecal flux decreased with depth. Phytoplankton sedimented at 200 m were dominated by diatoms and dinoflagellates. Phytoplankton exported at 200 m represented <0.5% of water column (photic zone) standing stock.