Abstract

In attempt to study the spatial differences in autumnal equatorial upwelling intensity relative to bio-productivity in the eastern equatorial Atlantic, oceanographic in situ data along regions 10°W, 0°E, 2.5°E and 6°E, collected in the Gulf of Guinea during autumn cruise carried out in September 2007 were analyzed. The surface mean values for temperature along 10°W, 0°E, 2.5°E and 6°E are 25.02, 26.15, 26.88 and 25.60°C, respectively. There was eastward weakening of the equatorial undercurrent from 10°W until its complete disappearance at 6°E. The highest concentration of nitrate recorded at the surface at 10°W was attributed to the shoaling pycnocline observed at this region. The surface mean values for nitrate along 10°W, 0°E, 2.5°E and 6°E are 0.37, 0.09, 0.04 and 0.04 μmol.kg-1, respectively. In response to the shoaling pycnocline at 10°W, corresponding to the highest concentration of chlorophyll fluorescence was recorded at this region. The surface mean values for chlorophyll fluorescence along 10°W, 0°E, 2.5°E and 6°E are 0.34, 0.05, 0.07 and 0.08 mg/m3, respectively. Contributions to equatorial upwelling by the equatorial undercurrent were the strongest and mostly expressed along 10°W. Profiles for apparent oxygen utilization and chlorophyll fluorescence gave indications that biological response to surface enrichments within the equatorial bands was highest at region 10°W. Vertical sections for studied parameters were unsuggestive of westward advection from 6°E to 10°W within the equatorial band, and this signifies the important role of vertical processes in equatorial enrichment at 10°W during boreal autumn.

Key words: Vertical mixing, zonal advection, equatorial undercurrent, equatorial upwelling, Gulf of Guinea, nutrients, phytoplankton biomass, surface enrichment.