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The
Particulate Matter Energy Cycle in the Atoll Lagoons
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The
ATOLL and CYEL research programmes were carried out on the atoll
of Tikehau from 1982 to 1995. The organisations involved in
these programmes were IRD (formerly ORSTOM), CNRS, UFP (the
French Pacific University), EVAAM (Establishment for the
Application of Aquacultural and Maritime Research) and the
University of Sydney.
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The
objectives of the ATOLL programme were as follows:
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To
assess the natural pearl oyster stocks
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To
determine the natural growth patterns of pearl oysters
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To
assess the potential output of
a traditional oyster farm
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To
investigate how the ecosystems on the atolls function.
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atoll de Tikehau
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The
CYEL programme focused on four main themes, relating to the main factors
involved in the particulate matter energy cycle in the lagoons :
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The
General Pearl Oyster Research Programme
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The
General Pearl Oyster Research Programme (PGRN) was drawn
up by the Establishment for the Application of Aquacultural and
Maritime Research (EVAAM) and financed by the Territory,
the French State and the European Development Fund as the result
of the serious diseases which affected the oysters in
some of the lagoons between 1984 and 1989.
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In
order to carry out this research, it was necessary to acquire further
knowledge about pearl oysters as well as to study the diseases possibly
responsible for the mortality of these animals and to understand more
clearly how the ecosystems in which they live function.
The
group of scientists responsible for the CYEL programme therefore carried
out the following investigations on the natural environment at the
Takapoto lagoon: they assessed the stock of organic matter, the
production of primary plankton and benthic organisms, the production of
zooplankton and bacteria, the consumption of particulate organic matter
by the zooplankton and the activity of filtering agents other than pearl
oysters.
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TYPologie
of the ATOLLs
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Field
studies were carried out in the Tuamotu archipelago from
1994 to 1996. The main objective of the TYPATOLL programme
was to attempt to answer the following question:
"How does the environment affect the state of the
lagoons and the way the lagoon ecosystems function? "
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At
the scientific level, the aim was to model the way in which the
various types of lagoon ecosystems function.
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At
the economic level, the aim was to characterise the various
types of lagoons, with a view to providing a useful tool for
their management
In
these ecological studies, comparisons were made between the effects
of various constraining variables (such as the climatic conditions
and the oceanic, terrestrial and human environment) on the way in
which separate ecological units (atoll lagoon ecosystems) function.
The 3 main factors selected here were: (1) the area
of the lagoon, (2) the permeability of the coral ring to the
ocean waters, and (3) the orientation of any openings in the ring
with respect to the prevailing ocean currents. These 3 factors have
decisive effects on the way in which exchanges of water occur
between the lagoon and the open sea. They were used to classify the
lagoons on a relative confinement scale. The ratings obtained in
this way on as many of the Tuamotu atolls as possible by analysing
the literature and the SPOT satellite pictures were used to choose
the most typical atolls, which were studied in this project.
The
atolls studied in the framework of the TYPATOLL programme (Figure)
were selected on the basis of the ratings obtained using
the above 3 criteria (3 classes were defined in terms of the area, 2
in terms of the permeability of the coral ring and 2 in terms of the
orientation of the apertures in the ring). The atolls selected in
this way were Reka Reka, Southern Tepoto, Tekokota, Taiaro, Haraiki,
Hiti, Hikueru, Nihiru, Marokau, Kauehi, Rangiroa, Takapoto, Toau and
Tikehau (Table 2). The last 4 atolls were
not included in the Typatoll field studies, but the numerous data collected on these atolls in the
framework of other programmes (ATOLL, CYEL, PGRN) were included in
the final analysis.
The
internal variables relating to the physical and chemical composition of
the waters and sediments and the structure and production of the
trophic networks were used to deduce how the lagoon ecosystems
function. The mean values obtained differed between the various
types of lagoon: the masses, energies and fluxes, and therefore the
way the ecosystems function and their potential all differ
from one type of lagoon to another.
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Atoll_site_