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Mapping of grass beds

Context

The Arcachon basin is a mesotidal lagoon of the south-west of France. Intertidal zones take up two thirds of this area (156 km² altogether). They are made of sand to muddy beaches, mostly colonised by wide grass beds of Zostera noltii (70 km²) and Zostera marina, an endangered species confined to channel banks.

These grass beds, the largest in Western Europe, give a strong ecological value to the basin. Their role of nursery, habitat and food and their function of primary producer justify the protection recommendations expressed within the Sea Development Scheme, which more particularly advises an accurate mapping of the Zostera marina grass beds.

If preliminary tests could be carried out several years ago, the LITEAU 2 and Cnes/Ifen programmes gave more recently strong impetus to the Earth observation based thematic mapping of the Arcachon basin. This activity is now supported by the Adour-Garonne Water Agency and Ifremer, which have co-funded an important vegetation mapping program, including the Arcachon basin as part of the implementation of the Water Framework Directive.

Activities currently carried out by scientific teams

Two approaches are jointly followed by the EPOC laboratory of the University Bordeaux 1 to achieve the thematic mapping of the Arcachon basin:

- The first one aims at characterising the reflectance of various surface types which can be met in the lagoon according to colour, granulometry and water content in the case of sediments or biomass and species in the case of plants. 120 collected reflectance spectra allowed to test several classification methods and develop a preliminary index for biomass quantification. The global optimisation classification process based upon the simulated annealing method offers the best results to classify these spectra.

- The second approach is directly linked with the classification of image pixels. The simulated annealing algorithm has been used to classify small areas where substrates are well known. This step makes it possible to define training classes for the supervised classification used to produce thematic maps. Six maps of this type have been developed for the timeframe 2003-2006, the validity of which has been tested using field measurements.

Relevance of a reference remote sensing database

The Kalideos Littoral database allows to use a large number of space data collected over a long period.

It offers to thematic experts on one hand the opportunity to improve methods for thematic classification and the assessment of biological indicators (more especially biomass), and on the other hand the possibility to better know and understand the current status and future development of the Arcachon basin.

At last, it enables local authorities to access accurate and regularly updated cartographic data.

Contact point

This activity is coordinated by Virginie LAFON from G.E.O. Transfert, the technology transfer cell of the CNRS Research Unit “Oceanic Environments and Paleoenvironments” (UMR EPOC).