Typology of atoll rims in Tuamotu Archipelago (French Polynesia) at landscape scale using SPOT HRV images

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.