Siliceous phytoplankton of the western equatorial Atlantic: sediment trapsand surface sediments

CaCO3, and biogenic opal in the western equatorial Atlantic from time-serie s 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 silic oflagellates were determined. Significant seasonal variations in export pro duction were observed between the equator and WA4 degrees S, and were subst antially diminished at WA7 degrees S (oligotrophic site). CaCO3 dominates s edimentation, followed by lithogenic particle and biogenic opal. Diatom and siliconflagellate maxima coincide with the southernmost position of the In tertropical Convergence Zone (January-March), and with the equatorial upwel ling season of July-September, mainly reflected at WA4 degrees S, where mea n diatom and silicoflagellate fluxes are highest. Interannual variability m ay be more common than generally assumed for oligotrophic central-ocean gyr es. On a yearly basis, total particle and CaCO3 fluxes covary and show oppo site latitudinal trends: high annual fluxes at WA4 degrees S in 1994 corres pond with low values at WA7 degrees S, and vice versa for 1993 and 1995. An nual diatom and silicoflagellate fluxes, observed only at the oligotrophic site WA7 degrees S, were one order of magnitude higher in 1993 than those o f 1994 and 1995. Lightly silicified, small diatoms of the Nitzschia bicapit ata group are the most abundant contributors to the diatom flux. Because th ese organisms are not preserved in the sedimentary record, significant disc repancies between trapped and preserved assemblages were seen. A substantia l enrichment of the moderately robust species, representative of the period of low diatom export, characterizes the assemblages of the surface sedimen ts. 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 sp ecies composition remained constant along the equatorial Atlantic. (C) 2000 Elsevier Science Ltd. All rights reserved.