Marine planktonic diatoms from the tropical and equatorial Atlantic: temporal flux patterns and the sediment record.

The value of siliccous phytoplankton as a proxy of export production from the surface waters to the sea floor in the eastern tropical (off Cape Blanc and south of Cape Verde Islands) and equatorial Atlantic was analyzed. Downward fluxes of diatoms and silicoflagellates determined from long-term sediment trap experiments revealed strong coupling with surface water production, and atmospheric and oceanographic circulation related to the Trade Winds and the seasonal migration of the Intertropical Convergence Zone (ITCZ). Fluxes were highest at sites influenced by coastal upwelling (strong influence: off Cape Blanc; moderate: N-Guinea Basin; minor: south of Cape Verde) where total primary production of the surface waters is high to moderate (Berger, 1989; Longhurst et al., 1995; Table 1-1). Lowest values were recorded south of the equator in the Guinea Basin ancl in the oligotrophic western equatorial Atlantic, where productivity values were moderate to low (Berger, 1989; Longhurst et al., 1995; Table 1-1). In the western equatorial Atlantic, the diatom fluxes reflected the transition from oligotrophic open ocean conditions (around 7°S) of the eastern subtropical gyre to an area (between the equator anel 4°S) influenced to a larger degree by pelagic blooms of the equatorial Atlantic (Longhurst, 1993).