Variability in Export Production Documented by Downward Fluxes and Species Composition of Marine Planktic Diatoms: Observations from the Tropical and Equatorial Atlantic

Abstract

We analyzed the value of diatoms as a proxy of export production from the surface waters in the tropical and equatorial Atlantic. Species composition and downward flux of diatom assemblages was determined from time-series sediment traps off Cape Blanc in the Mauretanian upwelling zone; south of Cape Verde Island within the Guinea Dome; along a N-S transect in the high productivity belt of the eastern equatorial Atlantic in the Guinea Basin; in the less productive western equatorial Atlantic around 25°W; and at the Walvis Ridge within the Benguela upwelling system. Diatom fluxes were highest at the sites located adjacent to or influenced by coastal upwelling (off Cape Blanc and at the Walvis Ridge), south of Cape Verde Islands, and north of the equator in the Guinea Basin. Lowest values were recorded in the oligotrophic western Atlantic. Regardless of the percent contribution to the total flux, diatoms dominated (in numbers per m-2 d-1) the biogenic opal fraction at all sites and, in general, showed significant correlation with organic carbon flux. At all sites, downward fluxes revealed strong coupling with surface water production, and with the atmospheric and oceanic circulation systems related to the Trade Winds and the seasonal migration of the Intertropical Convergence Zone (ITCZ): Diatom flux maxima were observed in early spring and early summer off Cape Blanc; in fall and winter at Cape Verde; in spring and summer in the N-Guinea Basin, and only in spring in the S-Guinea Basin; and in austral fall and spring at the Walvis Ridge. No distinct seasonal pattern was seen in the western Atlantic. Diatom assemblages varied with the water mass characteristics influencing each trap location. Throughout the sampling periods, small specimens of Nitzschia bicapitata accompanied by other taxa typical of open-ocean conditions dominated the diatom assemblage in the eastern and western equatorial Atlantic, and at Cape Verde. At Cape Blanc and Walvis Ridge, on the other hand, a coastal upwelling assemblage was characterized by the colonial diatom Thalassionema nitzschioides var. nitzschioides and members of the chain-forming genus Chaetoceros, with a minor contribution of pelagic forms. In addition to the marine assemblage, freshwater diatoms and phytoliths (of continental origin) were present in the traps. Their fluxes coincided with the seasonal changes in Saharan dust transport patterns, and the geographical extension of the dust plume across the Atlantic. Flux values decreased as a function of distance from the African continent. Discrepancies between diatoms trapped and diatoms accumulating on the seafloor were seen in the eastern and western equatorial Atlantic, and at Walvis Ridge, and are attributed to strong dissolution at the sediment/water interface. At these sites, surface sediment assemblages are enriched in robust diatom species representative of the low productivity season of the surface waters. In contrast, diatoms in the trap off Cape Blanc, indicators of persistent offshore spreading of the coastal upwelling, possess moderately robust frustules readily preserved in the underlying sediments. Here, diatom species-specific selective dissolution was minor in sediment-trap samples. Despite this loss, the preserved diatom assemblage carries generalized information which can be related to the hydrographic conditions of the surface waters.