Zooplankton in an upwelling area off northwest Africa: composition, distribution and ecology

Abstract

Zooplankton were collected in an upwelling area off northwest Africa from March to May 1974, in vertical hauls with nets of mesh 0.1 mm. Biomass in gm −2 was studied according to its composition by sizes, major taxa, and its distribution by areas and periods. Areas were inner shelf, outer shelf and slope, along a line at 21°40′N, which was traversed at six periods. The zooplankton consisted predominantly of copepods on the shelf and euphausiaceans and thaliaceans on the slope. Total biomass was lower on the inner shelf than elsewhere. The main difference by size was that animals from 0.1 to 0.5 mm had a higher biomass on the outer shelf whereas those over 1.0 mm had a higher biomass on the slope. Statistically significant changes in biomass with time were observed for total zooplankton, Copepoda, and Thaliacea. There were no such changes on the outer shelf during most of the study period. The zooplankton in the small area studied were more affected by advection than by local biological events. In an area of 3.9 × 10 8 m 2 across the outer shelf, where flow was almost always southward at all depths, daily change in biomass was estimated as follows: (100 to 500) × 10 8 g entering and leaving by advection, and (1 to 30) × 10 8 g entering and leaving as a result of local production and local predation by fish.