Geoscience Reference
In-Depth Information
extreme southeast of the North-American continent. They are
transported within the lower and middle atmosphere and reach the
ocean through dry deposits or humid deposits, during convective
rains. Their rate of spatio-temporal deposition is very variable, and it
is estimated that in certain regions, a single event can contribute to
almost half the natural annual supply. They certainly constitute an
important source of soluble, bioavailable iron, which is usable fairly
directly by plankton. They also contain a small amount of phosphorus
(0.08% of their mass, about 1/3 is bioavailable), and relatively few
nitrates (0.1% of their mass). They, therefore, have the potential to
enrich the oceanic surface layers in nutrients enabling the
development of certain groups of phytoplankton, in particular for tiny
phytoplankton (picophytoplankton), which constitute an important
part of the biomass in these regions. Their effect is more important on
the development of diazotrophes, which use nitrogen dissolved in
water, in place of nitrates, as a nutritive element, and will therefore be
less constrained by the weak concentrations in nitrates from these
aerosols. Experiments carried out during campaigns
in situ
, in
particular in the summer season, with tropical water to which is added
the Saharan aerosols, have shown an increase in the primary
production of diazotrophic organisms, suggesting that these are
colimited by the availability of iron and phosphorus of Saharan origin.
On a global scale, knowing the impact on the production of the
system is more complex, since stimulation by Saharan aerosol
supplies also affects the bacterial activity of the oligotrophic regions.
Thus, the accumulation of organic carbon dissolved in the surface
layers over the course of the summer season could be attributed to some
of these responses where organic carbon would ordinarily be consumed
by bacterial activity. The “biomass pull” effect of solid particles, when
these cross the column of water, has also been mentioned. The
question is to discover if the dissolving speed, total or partial, of the
solid particles is more rapid than the secondary “biomass pull” effects,
these probably being weak in the very oligotrophic regions of the
oceans. Moreover, it is necessary not to overstate the importance of
supplies of Saharan aerosols, knowing that currently the production in
these regions is probably caused more by the nitrogen input from
anthropic aerosols than by diazotrophic activity.
