THE ROLE OF PLANKTON IN PARTICLE-FLUX - 2 CASE-STUDIES FROM THE NORTHEAST ATLANTIC

The relationship between the vertical flux of microplankton and its standing stock in the upper ocean was determined in the subtropical (33-degrees-N, 21-degrees-W) and tropical (18-degrees-N, 30-degrees-W) northeast Atlantic in spring 1989 as part of the North Atlantic Bloom Experiment. In the subtropical area specific sedimentation rates at all depths were low (0.1% of standing stock) and 100-20% of settled particulate organic carbon (POC) was viable diatoms. The high contribution of viable diatoms, their empty frustules and tintinnid loricae to settled material characterized a system in transition between a diatom bloom sedimentation event and an oligotrophic summer situation. In the tropical area specific sedimentation rates were similar, but absolute rates (3 mg C m-2 day-1) were only about a third of those in the subtropical area. Microplankton carbon contributed only 2-6% to POC. Hard parts of heterotrophs found embedded in amorphous detrital matter suggest that particles had passed through a complex food web prior to sedimentation. Coccolithophorids, not diatoms dominated the autotrophic fraction in traps, and a shift in the composition of autotrophs may indicate a perturbation of the oligotrophic system.