The lagoon of an atoll is separated from the ocean by a rim. As the rim
controls the flux of water between ocean and lagoon, its structure is
one of the major forcing factors of the biological processes that
depend on the renewal rate of lagoonal water. Characterizing rim
structure and its degree of hydrodynamic aperture is mandatory for
comparing the functioning of different atoll lagoons. This paper
characterizes at landscape scale the different types of rims of the
atolls of the Tuamotu Archipelago (French Polynesia) using SPOT HRV
multi-spectral images. The classification of 117 segments of rims
highlights nine different rims. They differ in the relative importance
of vegetated, submerged, intertidal and emerged domains. These classes
are recognized with accuracy greater than 85% using a simple
statistical supervised algorithm. A gradient of hydrodynamic aperture
is described, from 0.02%-very closed rim exposed to the north, to
0.65%-wide open rim exposed to dominant southern swell. We show that
most of these nine rims have a preferential exposure. According to the
direction of the dominant swell in the Tuamotu region, such exposure
may explain the structure of the rims and their degree of hydrodynamic
aperture. We discuss the implications of these results for research and
management.
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