Zooplankton biomass and abundance in the Coastal Transition Zone off Northwest Africa

Abstract

The influence of mesoscale activity on zooplankton biomass, abundance, size-fraction distribution and taxonomical composition was studied along a transect from the coastal waters off the NW African upwelling to the offshore waters of the Canary Islands. The transect crossed an upwelling filament and an island-induced anticyclonic eddy southward off Gran Canaria. A second transect was carried out in order to study the three-dimensional structure of the eddy. Samples were scanned and analyzed using digital image processing (ZooImage). Our results confirm the influence of the mesoscale structures on the zooplankton distribution. The upwelling filament enriched the anticyclonic eddy located offshore, promoting an increase in biomass and abundance of zooplankton. Due to the clockwise rotation of the eddy, phyto-and zooplankton concentrated at its edges and core, both experiencing a decrease as the eddy rotated. Medium size organisms were dominant while copepods were the most abundant group, with predominance of small and intermediate size-fractions. The largest individuals (>1000 µm) were observed near the upwelling region and the filament, while small organisms (200-500 µm) were dominant in oceanic waters. Strikingly, medium size zooplankton (mainly copepods in the 500-1000 µm size fraction) was observed near the core of the anticyclonic eddy, suggesting size selectivity inside this mesoscale structure.