atoll lagoons are separated from the open sea by a barrier reef,
which is covered in some places by islets called
"motu" in Tahitian. On most of the atolls, there are
openings which enable exchanges of water to take place between
the lagoon and the open sea: these passages are called "hoas"
in the case of the shallowest channels, and
"passes" when they are navigable. The lagoon
waters flow in and out of these openings, depending on the tides
and on the sea swell. Many species of fish swim up these
connecting channels into the lagoons, where they spend most of
their lives, The extent to which the physical and chemical
characteristics of a lagoon differ from those of the open sea
depend on its degree of aperture.
lagoon sediments consist of fine sand resulting from the
degradation of coral structures, limestone, limestone algae and
the shells of the organisms inhabiting the lagoon beds. On the
sandy surface, microscopic autotrophic
producers (algae and cyanobacteria) are to be found, as well
as macroscopic algae and marine plants (phanerogamia). These
primary producers are consumed by the heterotrophic
animals living either on top of the sediments or buried in
the sand. A few centimeters under the surface of the sand, the
organic matter is mineralised by various bacterial processes.
the lagoon waters, autotrophic (primary) production is carried
out by the phytoplankton. The latter organisms are ingested by heterotrophic organisms (zooplancton). The bacteria
present in the water column remineralise the organic matter and
take an active part in the nitrogen and phosphorus cycles.
order to study the fluxes of particulate matter
occurring in the lagoons, it is necessary to assess the
biomasses of the various ecological compartments (the
phytobenthos, phytoplankton, zoobenthos, etc.), to
measure their production, and in the case of the
heterotrophic producers, to determine
the quantities ingested and excreted and those
involved in the respiratory processes. This will make it
possible to develop a “box” model for the
various compartments and the exchanges of matter
occurring between the compartments.