Continuous mapping of the main habitats of the Corsican littoral

Abstract

The availability of maps of the main habitats and types of bottom constitutes a priority for managers of the marine environment. For several years, a number of studies have been undertaken in order to meet this requirement, but they generally only cover limited areas and the methods used, as well as their accuracy, vary widely from one site to another. An original approach has been initiated for this purpose along the coast of Corsica (1 000 km of coastline); it has involved (i) surveying all the available data (293 identified maps), (ii) selecting and extracting the most relevant data (date and method of acquisition, accuracy), (iii) updating the data on surface habitats (e.g. upper limits of the Posidonia oceanica meadows) by means of remote sensing of recent images, (iv) homogenizing all the available data (typology of benthic biocenoses in the Mediterranean), (v) assembling the full set of data within an easily incrementable Geographical Information System (GIS), and (v) proposing a scale of reliability making it possible to associate a degree of confidence to each point on the map. This approach has resulted in the production of a map covering the whole of the Corsican coastline down to 50 to 150 m depth (depending on the sector considered), including, where there is an overlap, only the data with the highest degree of reliability. The surface area mapped is estimated at more than 299 700 ha, which makes it one of the largest recent maps ever produced for the Mediterranean. The P. oceanica meadow covers a surface area of about 57 500 ha, or 45.3% of the seabed between 0 and 50 m depth. In relation to the earlier data collected in the 1990s (potential surface area of seagrass beds), the extension of P. oceanica has remained relatively stable, and the differences observed may be explained mainly by the accuracy of this new map which possesses a greater degree of reliability.