Sheet n°268 - May 2007
The ocean surface, a whole world in
motion
he quest for understanding
the greenhouse effect is rallying the scientific community
around the study of the ocean’s
upper sunlit layer (30 to 120
metres). This layer plays a role
in the absorption of atmospheric carbon dioxide by incorporating it in the form of organic
matter, which is then pulled
down towards the deep strata.
IRD researchers propose the
hypothesis that part of this flux
could rise up again and feed
the living organisms of the ecosystem that operates in the first
centimetres of the ocean. Little
is known about the way this
ecosystem works. The same
mechanisms could be involved also in carbon exchange
between the masses of water
and air. IRD oceanographers
used models to define more
accurately the distribution of
this surface fauna and of the
organic particles it absorbs.
These particles appear to be 10
times more concentrated in the
oceanic convergence zones
than in the other marine areas.
Such a large accumulation
would have consequences on
the distribution of certain fish
and could call into question the
calculations of the concentration of carbon absorbed by the
ocean.
© IRD/Loïc Charpy
T
Plankton in the deep waters of Clipperton Island, or Île de La Passion, its French name originally given to it
(French Polynesia)
Technical advances over the past 50 years
have allowed improved knowledge to be
gained of the properties of sea water at great
depths. Yet the first centimetres of the ocean
remain its least well known part. They are difficult to sample and study owing to the mixing
the oceanographic vessel provokes between
this superficial layer and the deeper strata
of water. Nevertheless, a whole ecosystem
exists within this layer, carrying numerous
living organisms like bacteria, zooplankton
and larger animals such as flying fish, which
feed and reproduce in it.
Research usually focuses rather on the
whole of the sunlit part of the ocean (the
first 30 to 120 metres) where the phytoplankton elaborates organic matter (in primary production) thanks to chlorophyll – its
green pigment - during photosynthesis.
Through this process, the ocean proves to
be capable of trapping the carbon dioxide
from the atmosphere by incorporating it
with the organic matter produced and storing it by sedimentation down towards the
deep waters. This downward movement
of carbon-carrying particles is the focus of
great attention among oceanographers.
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Sheet n°268 - May 2007
For further information:
CONTACTS :
YVES DANDONNEAU
IRD Laboratoire d'océanographie et du climat : expérimentations et approches
numériques (LOCEAN).
+33 (0)1 44 27 44 83
yves.dandonneau@loceanipsl.upmc.fr
PRESS OFFICE
+33 (0)1 48 03 75 19 ;
presse@ird.fr
INDIGO BASE, IRD PICTURE LIBRARY
+33 (0)1 48 03 78 99 ;
indigo@ird.fr
www.ird.fr/indigo
IRD AUDIOVISUEL
+33 (0)1 48 02 56 24 ;
audiovisuel@bondy.ird.fr
www.audiovisuel.ird.fr/
REFERENCES :
DANDONNNEAU YVES, MENKES
CHRISTOPHE, DUTEIL OLAF,
GORGUES THOMAS, Concentration
of floating biogenic material in
convergence zones, Journal of
Marine Systems (2007). Sous
presse.
doi:10.1016/j.jmarsys.2006.02.016
KEY-WORDS :
OCEAN SURFACE LAYER,
FLOATING MATERIAL,
© IRD/ Arnaud Bertrand
CONVERGENCE ZONES
Hauling the plankton net up on board the
oceanographic
ship Olaya during a survey campaign
In this theoretical context, IRD scientists (1) advanced the hypothesis that part
of this carbon flux would rise up instead of
descend, feeding the surface fauna. These
living beings and the organic particles they
absorb are still little known as a whole and
are designated as “floating biogenic material”. A physical ocean circulation model,
coupled with a model reproducing the
behaviour of the ecosystems, was used in
order to obtain more accurate notions of
the distribution of these particles present at
the ocean surface and of their relationships
with the food “hot spots”, first observed
several years ago (2).
The simulation run showed that the distribution of the biogenic material does not follow
that of the primary production that generates it. The surface-layer organisms with
their associated organic matter are under
the direct influence of the surface currents.
These currents carry this floating material
as far as the oceanic convergence zones,
places where two water masses meet.
These “fronts”, where the concentrations of
biogenic material are up to 10 times higher
than those of other marine regions, prove
to be no richer in phytoplankton and chlorophyll than the surrounding waters. The
surface layer of the convergence zones
is a site of accumulation only for floating
debris of marine life and the organisms that
feed on it.
Such concentrations of floating biomass in
the otherwise nutrient-poor ocean, with low
productivity and little organic-matter, is like
an oasis for fish in search of food. They
help explain in particular why tuna fishing
is carried out essentially near these fronts,
a factor that had not hitherto been clearly
elucidated.
Another finding was that the optical properties of this floating material were quite close
to those of chlorophyll. Consequently, this
biogenic material influences observation of
the colour of the ocean in the same way as
the green pigment. Its presence thus distorts calculation systems which use these
satellite colour data to estimate the chlorophyll concentration and therefore that
of the phytoplankton. The great quantities
of chlorophyll apparently detected in the
convergence zones could in reality cor-
respond to the presence of floating debris.
More accurate definition of the distribution
of the chlorophyll and biogenic material
that accumulate at the fronts could therefore lead to a better perception and use of
ocean colour as an indicator of oceanic circulation and of the ecosystem’s biological
and biogeochemical processes.
Further, the accumulation of microorganisms and floating debris might influence
carbon dioxide (CO2) absorption by the
ocean. In fact, the life forms of the ecosystem in the first centimetres of the water
breath and produce CO2. The existence
of an excess of carbon dioxide just under
the surface could therefore call into question the assessment methods used for
the quantifying the CO2 absorbed by the
ocean.
A surface sampling device is currently being
developed. This kind of tool is a prerequisite for studying this theme, highly important
for understanding climatic phenomena and
atmospheric carbon concentration, a key
parameter in global warming. (3)
(1) This research work was conducted
by the “Laboratoire d'océanographie et
du climat: expérimentations et approches numériques (LOCEAN)”, which
jointly involves scientists from the IRD,
the CNRS, the Université Paris VI and
MNHN.
(2) See scientific bulletin N° 190,
December 2003, accessible on www.ird.
fr/fr/actualites/fiches/2003/fiche190.htm
(3) This is the GRABISU programme,
in turn part of the national programme
LEFE-CYBER, one of the present priorities of which is to develop new techniques for sample-taking from the surface
film of the ocean.
Céline Bézy-IRD
Translation : Nicholas Flay
Marie Guillaume-Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@paris.ird.fr