Of all the continental deserts, the Sahara is the world’s largest source of atmospheric soil dust. Huge amounts of Saharan-dust blown into and over the Atlantic every year are thought to supply nutrients for marine phytoplankton in the tropical North Atlantic, where N-fixation is co-limited by Fe and P. To gain knowledge on the spatiotemporal variability of phytoplankton communities thriving in this region, and the potential of Saharan dust as a nutrient fertilizer, we have investigated the export fluxes and seasonal dynamics of coccolithophore (calcifying phytoplankton) from a transatlantic transect of 4 sediment traps lying right underneath the largest dust plume originating from Africa (~12º N).
Results from 1 year of sediment trapping show large ecological contrasts across the transatlantic array. Wind-forced basin-scale variations of the thermocline/nutricline were clearly reflected on the distribution of distinct coccolith species fluxes. Increasing abundances of surface-dwelling opportunistic species E. huxleyi and G. oceanica from West to East and the concurrent increase of the deep-dwelling species F. profunda and G. flabellatus in the opposite direction, followed the geostrophic shoaling (deepening) of the thermocline/nutricline and the ML towards the eastern (western) tropical North Atlantic. We found that coccolith fluxes at the westernmost site (M4) were up to 3-5 times higher compared to the other sites, including the highly productive Cape Blanc upwelling region, and were mostly due to coccolith production in the poorly-illuminated lower photic zone. Finally, pulsed flux maxima of opportunistic species that were also observed at site M4 point to the occurrence of intermittent nutrient input promoted by sea surface cooling and wind-forced vertical mixing combined with dry dust deposition (during spring), and nutrient enrichment derived from Amazon River discharge combined with wet dust deposition (in the fall). The lack of a clear ecological response by the deep-dwelling flora during these events suggest that Amazon- and dust-born nutrients were mostly benefiting the species dwelling at the surface.
Our findings (i) provide relevant evidence to support the hypothesis of Saharan dust acting as a fertilizer for marine phytoplankton in the Atlantic Ocean, and (ii) highlight the importance of coccolithophore production in the lower photic zone, with implications for the global oceanic carbonate budget.
GUERREIRO, C. V., Baumann, K.-H., Brummer, G.-J. A., Fischer, G., Korte, L. F., Sá, C. and Stuut, J.-B. W.: Wind- forced transatlantic gradients in coccolithophore species fluxes, Submitted to Progress in Oceanography, 2018.
GUERREIRO, C. V., Baumann, K.-H., Brummer, G.-J. A., Fischer, G., Korte, L. F., Merkel, U., Sá, C., de Stigter, H., and Stuut, J.-B. W.: Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust, Biogeosciences 14, 4577-4599, https://doi.org/10.5194/bg-14- 4577-2017, 2017.
Jan-Berend Stuut
