atoll lagoons are shallow basins only a few metres deep,
separated from the open sea by an emergent ring of coral barely
a few centimetres high, which is usually planted with coconut
trees. The bottom of the basin is usually covered with sand (or
sediment) composed of coral particles, the size of which (granulometry)
depends on the hydro-dynamic specificities of the lagoon. A few
pinnacles formed by biological accretions sometimes emerge above
the surface of the lagoons.
lagoon waters are either free waters, in the case of the water column,
or interstitial waters located between the grains of
lagoon waters can be characterised in terms of their
pH level and salt content.
of the salts they contain, the nutrient salts, are particularly
important because they constitute one of the main sources of
nitrogen and phosphorus: these elements are required for the
photo-synthetic production of phytoplankton and the phytobenthos. In addition, the lagoon waters and sediments
contain dissolved and particulate organic matter originating
from the various products of the lagoon: these are often used in
turn by heterotrophic organisms.