Apports des hautes résolutions spatiales pour l'étude et la cartographie des algues vertes. Application à la grève de St-Michel(22)

The increasing amounts of agricultural effluents associated with particular hydrodynamical locations are resulting in strong proliferations of suspended green algae Ulva lactuca. During ebb tide, the algae deposit on beaches, creating a nuisance for both tourism and fishermen. Biomass estimates are hence needed both for understanding the phenomenon and for measuring its magnitude. During the spring tide period of September 1993, two high resolutions digital sensors were run jointly on the French A.R.A.T (Remote Sensing and Atmospheric Research Aircraft). A pushbroom instrument called RAMI (run by C.N.E.S.) with spectral bands identical to those of S.P.O.T. and the Canadian C.A.S.I (which possesses a lot of spectral bands) acquired images over the sandy beach of St- Michel-en-Grève (Northern Brittany, France). The main topic of this investigation was the biomass assessment of green algae proliferations using high resolution airborne images. Spectral handset positioning of C.A.S.I resulted from field measurement made a month earlier with a SPECTRON portable spectrometer. Geometric and radiometric corrections were carried out on the images and the Normalised Vegetation Index (N.D.V.I) was used to detect deposited seaweeds. A relation between the biomass and the reflectance was established using the spectra acquired in August. This relation lead to an estimate of the deposited seaweeds. A thematic map was also used to study the biomass distribution of green algae and very thin structures like the shapes of hydrographical network. In the discussion a few operational issues, such as the choice of better resolution, sensors aircraft availability, and the feasibility of yearly monitoring, are examined.