Zooplankton spatial and temporal distribution in a tropical oceanic area off West Africa

Tropical instability vortices are believed to modify the trophic food web by affecting plankton production. Such instabilities have been evidenced in a tuna fishing area of the Atlantic (10 degrees-20 degrees W and 2 degrees-5 degrees N). However, the origin of the tuna abundance remains uncertain as no data on zooplankton communities on which tuna preys are supposed to fed upon are available. This study was focused on short term spatial and diel variations of mesozooplankton communities sampled at 2 degrees N during and after the passage of an instability wave evidenced from satellite imagery, at 4 degrees N and at 0 degrees, i.e. in the Equatorial divergence. Samples were collected with an opening-closing multisampler and with a WP2 net (200 mu m). Copepods prevailed (86-92% of the total zooplankton) with Clausocalanidae, Oncaeidae, Corycaeidae, Calanidae, Eucalanidae as dominant families. At 0 degrees and at 2 degrees N during the wave, large-sized organisms constituted over 50% of total dry weight (DW). These percentages decreased after the wave and at 4 degrees N. Zooplankton DW and densities were highest at 0 degrees and lowest at 4 degrees N. At 2 degrees N after the wave, we observed a strong increase in the abundance of zooplankton which surpassed that at 0 degrees. This increase mainly resulted from an increase in small copepods and copepodites. Our results suggest that the high production linked to the upwelling of nutrients caused by the geostrophic divergence is partly advected northward by the instability wave. The advection did not reach 4 degrees N which displayed the characteristics of a typical tropical structure.