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Fields eelgrass, which fall into seagrass beds in sandy sediments, or in estuaries, are one of the most productive ecosystems on Earth and play a very important role in the functioning of marine ecosystems. The large eelgrass beds are indeed very important in terms of food and habitat especially for the fry of many species. They also play an essential role in stabilizing the sediment as well as the water quality. They should be followed over the long term in order to achieve good management of these sites.

The study was conducted on six images acquired by the Spot satellite between 1991 and 2005, during the summer at low tide and cloudless (20m resolution (1991 images, 1993, 1996, 1997, 1998) and 10m (SPOT 2005)).

The images were converted into ground reflectance with the method known as Empiric Line Correction (Méléder, 2003). A geographical mask, based on bathymetry, rocky areas and oyster beds, was applied to images in order to take into consideration that the area in intertidal sandy mud sediment where seagrasses develop.

Spectral signatures of measurements were obtained in the field with a spectroradiometer FieldSpec ASD. The spectral responses of the main macrophytes and sediments found in the intertidal area were recorded in the near infrared visible between 400 and 900nm. Meanwhile, a working spectroradiometric measures was conducted in the laboratory to test the ability of NDVI (Normalized Difference Vegetation Index) to estimate the biomass of eelgrass leaves in g / m².

The NDVI is calculated as follows from the tapes Near Infrared and Red SPOT images converted into reflectance (NIR - R) / (NIR + R).

Vegetation indices are presented in class: low index (0.2 to 0.4) and high index (0.4 to 0.7) for 6 dates. Biomass card is obtained after the work of field and laboratory with the following relationship:

Biomass = 610.61 (NDVI) 1.88 (n = 31, r ² = 0.97)