The
Main Oceanographic Characteristics of Polynesia
Marie
José Langlade , IRD
(a
review from F. Rougerie and B. Wauthy
in Atlas de
Polynésie Française)
Hydro-climatic
features of the southern central Pacific: eddies (or "gyres")
At
inter-tropical latitudes, the trade winds generally cause the
water masses in the Pacific Ocean to flow from east to west.
These movements are known as the Equatorial Currents (EC). The
water accumulating in the west then tends to flow back towards
the east, taking routes where the winds are light or variable,
and these reflux movements are known as the Equatorial
Counter-Currents (ECC). South of
the latitude 30° S, the prevalent westerly winds result
in an eastward flow of the tropical and sub-tropical waters, via
the South Pacific Current (SPC), which is an extension of the
Eastern Australian and Tasmanian currents. Farther south,
starting at latitudes below 45° S, is the region containing the
most powerful ocean current of all, the Circumpolar Antarctic Current,
(CAC), which transports an enormous volume of water.
As this current approaches the South American continent, part of
the water mass is deviated northwards and goes to form the
Humbolt Current, which enters the equatorial zone after making a
detour to the west. This is how the easterly trade winds and the
prevailing
westerlies form an indomitable partnership causing the waters in
the tropical and sub-tropical regions to form a swirling anti-cyclonic
stream. The process ("Gyre") which takes place between
the American continent on the east and the largely southward
streams arising in the region of the Tuamotu-Gambier archipelago
is called the South Pacific Eddy.
The
surface feature of the Polynesian ocean
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The
thermo-haline profile observed in the South Pacific
during the Austral summer and the Austral winter differs
between the eastern and western regions. By mapping the
salinity levels, it was possible to exactly determine
the region
occupied by the South Pacific Tropical Waters to the
east of the Tuamotu atolls, where salinity levels above
36.5 % have been measured.
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In
the tropical and sub-tropical regions (15°S to 30°S),
the ocean absorbs large amounts of CO2, which
results in high pH
levels (> 8,3). These conditions favour the
construction of coral complexes by madreporarians. |
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The
measured levels of dissolved mineral nitrate, an element
which is necessary to the autotrophic production of
organic matter by phytoplankton and algae, provide an
excellent index to the potential productivity of
a mass of water. The Tuamotu waters are
oligotrophic waters, which means that they have a very
low nutrient salt content.
The
very low levels of chlorophyll pigments, and hence of
phyto-plankton, present in the Tuamotu waters are
responsible for the great transparency and the colour
for which the South Seas are renowned, but which are
also synonymous with a biologically barren marine
landscape. |
The
Pacific ocean around Polynesia and farther away
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The
physico-chemical cross-sections given below describe the
depths from 0 to 400m
at latitudes of 150° - 140° W, and at longitudes
ranging from the Equator to 35° S. |
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Temperature |
Salinity |
Dissolved
Oxygen |
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Chlorophyll |
Dissolved
Nitrate |
Quantity
of particles |
Conclusions
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The
central region of the Pacific Ocean in which the Polynesian
archipelagos are scattered is almost entirely swept by the
immense South Pacific Gyre, the circular motion of which
results from the combined effects of the tropical trade
winds and the westerly winds prevailing in the sub-tropical
regions. The convergent nature of this circular pattern of
flow leads to the surface waters accumulating in the centre
of the Gyre. The extremely strong sunshine which occurs
under tropical climatic conditions contributes to
maintaining the high temperature and high salinity levels,
and the relatively homogeneous water mass thus formed
behaves like a floating disk some 200 metres thick,
supported by the denser, deeper surrounding water. Owing to
the great vertical stability conferred by this process of
stratification, the tropical waters are isolated from the
surrounding ocean.
In
the surface layer, where the water spends the longest time,
any phyto-plankton produced quickly exhaust the available
supply of nutrient salts. In addition, any living organic particles and residues with
little buoyancy tend to descend in response to the pull of
gravity, and thus gradually disappear from the euphotic
layer, forming a layer of sediment at the bottom of the
basin, at depths of 5000 m or so. This insidious process of exportation results in
a net loss for the pelagic ecosystem, and at the same
time explains why the waters in the centre of the Gyre are
so transparent: they are almost entirely devoid of nutrient
salts, particles and plankton, and therefore contain no
organisms belonging to the higher trophic levels,
cephalopods or fish. This
is in fact a vast ocean desert, the western limits of which
easily encompass the whole of the French Polynesian
Exclusive Economic Zone (EEZ), apart from the Marquesas
archipelago.
Apart
from these overall features, the barren ocean landscape we
have just described contains some little oases, the lagoons
on the higher atolls, and some atolls which are particularly
productive, especially those in the centre of the coral
ecosystem. The
fact that these reef and lagoon complexes are productive on
the whole despite the fact that they are surrounded by such
desolate waters is rather puzzling. Several explanations
have been put forward for this contradictory state of
affairs. Those who wish to pursue this point further can
refer to the analysis presented elsewhere under the heading
"Exchanges between the
atolls and the open sea".
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References
Rougerie
F, Wauty B, 1993, l'océanographie du Pacifique Central Sud in Atlas
de Polynésie Française, ORSTOM Editions, 20-21 |
Atoll_site_