Variability in Export Production Documented by Downward Fluxes
and Species Composition of Marine Planktic Diatoms:
Observations from the Tropical and Equatorial Atlantic
O. E. Romero!', C. B. Lange 2 , G. Fischer!, U. F. Treppke 3 and G. Wefer!
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
2 Scripps Institution o/Oceanography, Geosciences Research Division and
Marine Life Research Group, La Jolla, CA 92093-0215, USA
3 RichardWagnerStr. 31, D-30177 Hannover, Germany
*corresponding author (e-mail):oromero@uni-bremen.de
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 dol) 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 chainforming 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.
From FISCHER G, WEFER G (eds), !999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 365-392
and Species Composition of Marine Planktic Diatoms:
Observations from the Tropical and Equatorial Atlantic
O. E. Romero!', C. B. Lange 2 , G. Fischer!, U. F. Treppke 3 and G. Wefer!
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
2 Scripps Institution o/Oceanography, Geosciences Research Division and
Marine Life Research Group, La Jolla, CA 92093-0215, USA
3 RichardWagnerStr. 31, D-30177 Hannover, Germany
*corresponding author (e-mail):oromero@uni-bremen.de
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 dol) 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 chainforming 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.
From FISCHER G, WEFER G (eds), !999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 365-392