Mesozooplankton size-fractionated metabolism and feeding off NW Spain during autumn: effects of a poleward current

Two consecutive cruises were carried out off NW Spain in October 1999 in an attempt to obtain information on the poleward current that affects the hydrodynamics of this area during autumn and winter. The stations sampled were divided into three zones: coastal (C), between the saline intrusion and the coast; poleward current (P), where salinity at 100-m depth was >35.85; and oceanic (O), outside the poleward current. Phytoplankton biomass, integrated through the photic zone, was similar for the three zones. Conversely, mesozooplankton biomass showed high spatial variation, with the lowest values associated with the high-salinity current. There was a decreasing trend in the relative importance of the biomass of the largest size class (>1000 μm) from coastal to oceanic zones. The lowest percentages of both Chl a and primary production ingested daily by copepods were measured at the poleward current stations. Specific metabolic rates were not significantly different between zones. The total amount of carbon respired and the NH 4 and PO 4 released were highest in coastal areas and lowest within the poleward current, in agreement with the pattern observed for mesozooplankton biomass. Diel vertical migrations were more marked in the coastal areas, decreasing gradually towards the oceanic stations. The role of mesozooplankton in promoting the flux of carbon, nitrogen, and phosphorus followed the same spatial distribution. Overall, the contribution of mesozooplankton to biogeochemical cycles seems to be minor in the poleward current compared to the other two zones, possibly as a consequence of the low biomass associated with this current.