Siliceous phytoplankton of the western equatorial Atlantic: sediment traps and surface sediments

CaCO3, and biogenic opal in the western equatorial Atlantic from time-series sediment traps deployed between 550 and 850 m water depth at the equator (WAO degrees), 4 degrees S (WAL4 degrees S) and 7 degrees S (WA7 degrees S). In addition, species composition and downward fluxes of diatoms and silicoflagellates were determined. Significant seasonal variations in export production were observed between the equator and WA4 degrees S, and were substantially diminished at WA7 degrees S (oligotrophic site). CaCO3 dominates sedimentation, followed by lithogenic particle and biogenic opal. Diatom and siliconflagellate maxima coincide with the southernmost position of the Intertropical Convergence Zone (January-March), and with the equatorial upwelling season of July-September, mainly reflected at WA4 degrees S, where mean diatom and silicoflagellate fluxes are highest. Interannual variability may be more common than generally assumed for oligotrophic central-ocean gyres. On a yearly basis, total particle and CaCO3 fluxes covary and show opposite latitudinal trends: high annual fluxes at WA4 degrees S in 1994 correspond with low values at WA7 degrees S, and vice versa for 1993 and 1995. Annual diatom and silicoflagellate fluxes, observed only at the oligotrophic site WA7 degrees S, were one order of magnitude higher in 1993 than those of 1994 and 1995. Lightly silicified, small diatoms of the Nitzschia bicapitata group are the most abundant contributors to the diatom flux. Because these organisms are not preserved in the sedimentary record, significant discrepancies between trapped and preserved assemblages were seen. A substantial enrichment of the moderately robust species, representative of the period of low diatom export, characterizes the assemblages of the surface sediments. Differences in total particle and diatom fluxes between the western and eastern basins of the equatorial Atlantic reflect basin to basin asymmetry with W-E increase in primary productivity estimates, organic carbon fluxes, and silicate content in the subsurface waters, and the W-E shallowing of the thermocline depth, Despite flux differences, siliceous phytoplankton species composition remained constant along the equatorial Atlantic. (C) 2000 Elsevier Science Ltd. All rights reserved.