Influence of Saharan dust on phytoplankton in the Eastern equatorial Atlantic

Abstract

The Saharan desert is the largest dust emitting region worldwide. Apart from its effect on albedo, Saharan dust may affect climate through stimulation of the export of organic matter to the deep ocean by 1. Ballasting of organic matter and 2. Fertilisation of phytoplankton by trace metals. In two sections, the current study set out to investigate these possible effects of Saharan dust in the equatorial Atlantic, where the majority of Saharan dust is deposited. Dust was sampled from 2013 to 2015 at the coast of Mauritania. The dust flux, grain size and chemistry were determined as well as possible source areas using air parcel backward trajectories. Dust fluxes were highest in spring, highlighting this season as possibly important for fertilisation and ballasting. During the summer monsoon distinctly finer grained and trace metal enriched dust was observed, possibly related to wet deposition. As wet deposition may also enhance bio-availability of trace metals (Gao et al., 2003), this season may also be of importance for phytoplankton fertilisation. During summer, dust mainly arrived with the coastal trade winds from the coastal plains of Western Sahara. During winter, Harmattan trades were also important, transporting dust from central northwest Africa. The second section of this thesis dealt with the effects of an actual dust event on phytoplankton. Settling organic matter was sampled using small sediment traps as well as gel traps. Indications were found of fertilisation during this dust event in the form of increased carbon and nitrogen settling fluxes as well as a changed carbon to nitrogen ratio of the organic matter. Although C/N ratios were indicative of ballasting, the evidence of ballasting during the dust event was not conclusive.