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Morphological
diversity
85
of the world's 425 atolls are in French Polynesia. Seventy-seven
of these atolls belong to the
Tuamotu archipelago. They show a wide range of
morphological features. They vary in size from a few square
kilometers to more than 1000 km2, and their lagoons,
if they have one at all, range between only a few per cent and
92% of the total area of the atoll and are anything from 2 to 60
m in depth. The apertures
on their coastlines correspond to 0.01
in the case of the lagoons with the smallest natural channels
leading to the sea, and 0.59 in the case of the widest natural
breach. Forty-five atolls do not have any natural passages of
this kind, whereas 10 atolls have several passages.
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Taiaro, an
atoll with a completely enclosed lagoon: the water from
the open sea reaches the lagoon only via a few narrow
channels ("hoas") in the coral ring, when the
seas are particularly high (photo from "Les atolls
des Tuamotu", Bonvallot et al., 1994, ed. ORSTOM).
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Anuanurunga,
an
atoll with an accessible lagoon: since the coral ring is
largely submerged, intensive exchanges take place
between the open sea and the lagoon waters (Photo :
B. Salvat)
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Mataiva
is
a reticulated lagoon with a whole network of
coral structures 30 to 300m in width forming 70
shallow pools with a mean area of approximately 20
hectares (photo from "Les atolls des Tuamotu ",
Bonvallot et al., 1994, ed. ORSTOM).
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The
great morphological diversity of the Tuamotu atolls has been
anaysed in detail using SPOT satellite teledetection methods (Andréfouët,
1998), resulting in the construction of a morphometric
database focusing on 49 atolls. The following figure, which
was drawn up in the framework of the latter "Typatoll"
project, shows 14 atolls on the same spatial scale. It shows the
great diversity of the atolls in terms of their shape, area, and
accessibility to the open sea, the relative size of the coral
ring and the lagoon, the area occupied by the "motus",
the development of the plant growth, etc.
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The 14
atolls chosen for the Typatoll project: the mosaïc-like
SPOT satellite pictures.
black : the
open sea and lagoon waters;
blue :
submerged reef ;
white : emergent
reef ;
red : island
occupied by plant growth
(based
on a SPOT picture processed by S. Andréfouët/Ifremer/Territoire de
Polynésie Française/Université Française du Pacifique)
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Attempts
at classification
Given
the great diversity of the atolls, the need has arisen to
classify them into groups depending on their characteistics:
this would make it possible to extrapolate what is known about
some of the atolls in a group to all the atolls in that group
The
classification of the Salvat atolls (1985)
has long served as a reference in this respect. Since the main
factor accounting for the diversity and the abundance of the
molluscs in the lagoons is the degree of confinement of the
lagoon waters, the main criterion on which the latter
classification was based was the accessibility of the lagoons.
The number of natural channels was taken here to play a decisive role. The atolls were
therefore described as "single-channel atolls",
", "two-channel atolls", etc., and the other
criteria used were
the presence or absence of an emergent coral ring, "hoas",
etc
In
his "Atlas of Polynesia" (1993), Morhange used another
system of classification involving the use of morphometric ratios
based on 3 criteria: the total area, the area of the coral ring
and the area of the "motus". Morhange deals with the
highest atolls separately (Makatea, Mataiva, Niau, Anaa). The
remaining atolls are then classified in order of size, regardless
of whether or not they have channels connecting the lagoon to the
sea.
Other
systems of classification have been proposed by Andréfouët
(1998). One of them is based on the composition of the coral
rings: the area occupied by plants, the inter-tidal area, the area
of the emergent lands and their exposure (north, east, etc…).
Another system is based on the breaches in the coral ring.
Comparative analyses on the way the lagoons on atolls of various
types function have shown, however, that the characteristics of
the apertures and the size of the atolls (the area and depth of
the lagoons), which both affect the rate of renewal of the lagoon
water, are decisive factors. A third parameter is the position of
the "hoas",
the natural channels and the immersed coral ring
with respect to the predominant currents, which modifies
the effects of the degree of aperture of the atolls on the rate of
renewal of the lagoon waters
(for further information, see...).
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Another
system of classification was based on the above three
parameters, as measured on the
49 available SPOT satellite pictures of Tuamotu
atolls (Andréfouët, 1998). In the case of the Tuamotu
archipelago, 6 groups of atolls were distinguished in
this way (Fig 4) :
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Morphometric
database
Socio-economic
diversity
According
to the 1996 census, the population of the Tuamotu archipelago consists of 14 283
permanent residents. This small population is unequally distributed. The atolls
with natural channels giving schooners easy access to their lagoon are mostly
inhabited. In 1996, only 31 atolls had more than 100 inhabitants 3 atolls more
than 1000 inhabitants (Rangiroa, Hao et Mururoa). 33 of the atolls are not
inhabited at all, or are only visited very occasionally.
References
Andréfouët S. (1998). Apport de la
télédétection pour une approche descriptive et fonctionnelle des systèmes
coralliens de Polynésie Française. Thèse Univ. Francaise du Pacifique,
243 p. + annexes.
Salvat B.
(1985). An integrated (geomorphological and economical)
classification of French polynesian atolls. Fifth Int. Coral reef
Congr. Tahiti, vol2 : 337 |
Atoll_site_