Offshore influence of coastal upwelling off Mauritania, NW Africa, as recorded by diatoms in sediment traps at 2195 m water depth

Abstract

Downward flux and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples off Cap Blanc (CB1, NW Africa) over a 1-year period (March1988–March1989) and compared to the assemblages in the underlying surface sedi ment. Trap content was composed of biogenic and lithogenic material. The dominant constituents of the biogenic fraction were calcium carbonate, biogenic opal, and organic matter. Coccolithophorids, planktonic foraminifera and pteropods contributed to the CaCO3 flux; opal derived mainly from diatoms. During the sampling year prominent total flux maxima occurred in spring and summer. Highest diatom flux values were reached in March–April (20.1×105 valvesm-2d-1) and July–August (22.9×105valvesm-2d-1) in coincidence with the total flux pattern. Radiolaria showed three distinct peaks: in autumn (16×104shellsm-2d-1), spring (∼11×104shellsm-2d-1) and summer (10×104shellsm-2d-1). The flux pattern of silicoflagellates differed greatly from those of the other groups and from the total particle flux, with highest values between October 1988 and March 1989. For all microorganisms surveyed, September was the month of lowest production. We identified a total of 166 diatom taxa and 3 silicoflagellate species. Off Cap Blanc, upwelling occurs year-round, and “giant filaments” of relatively high pigment concentration develop and persist throughout the year with substantial seasonal and interannual variations. We show that the shifts in siliceous phytoplankton do reflect both the seaward displacement of the coastal upwelling center and the periods of relaxation when oligotrophic waters prevail. Accumulation rates in the surface sediment were calculated to be 1.36×108valvesm-2yr-1 for diatoms and 1.05×106skeletonsm-2yr-1 for silicoflagellates. Most of the diatoms found in the water column at 2195m also occurred in the sediment surface, with a slight enrichement of robust taxa in the sediments. In an attempt to contrast coastal vs. oceanic upwelling, we compared the Cap Blanc trap results (a coastal/open-ocean transition site) with the patterns recorded previously in sediment traps from the Guinea Basin (GBN3; an open-ocean equatorial upwelling site). Enhanced fluxes at both sites corresponded in time with the occurrence of upwelling (i.e. spring and early summer for CB1, and early spring and summer for GBN3). Total, opal and lithogenic mean daily fluxes were 2.4, 1.6 and 6.3 times higher at CB1 than at GBN3. Diatom and silicoflagellate fluxes were 8.9 and 1.6 times higher at GBN3. On a yearly basis, the diatom flora for CB1 can be characterized as “coastal with oceanic influence”, and for GBN3 as “open ocean with moderate coastal influence”. Chain-forming and colonial diatoms with individual cell diameters of >5μm dominated the coastal upwelling site (Thalassionema nitzschioides and Chaetoceros), while small (<5μm) solitary diatoms (Nitzschia bicapitata) dominated the open ocean equatorial upwelling regime. Comparable relative abundances of freshwater diatoms were noted at both trap sites; their seasonal distribution within each geographical area was attributed to the Saharan dust transport patterns involved. While diatom indicators of coastal upwelling were readily preser ved in the surface sediments off Cap Blanc, the assemblage in the Guinea Basin sediments differed greatly from that in the traps with the summer signal for equatorial upwelling being removed from the sediment. We conclude that significant differences between the assemblages trapped at both sites support the usefulness of these data as sensitive indicators of dissimilar oceanographic settings.