Relationship between hydrology and seasonal distribution of suspended sediments on the continental shelf of the Bay of Biscay

Numerous oceanographic cruises (with hydrology, water sampling, drift current measurements) carried out since 1980 on the continental shelf of the Bay of Biscay, together with available NOAA/AVHRR infra-red images, form the basis of a proposed explanation for the processes responsible for the distribution of suspended sediments on the shelf. The seasonal hydrographic structure of continental shelf waters is of paramount importance in sediment distribution. In summer, there is an horizontal stratification of water masses, and suspended sediment distribution is closely related to the thermo-haline structure. A fresher water mass with less suspended material lies on a thicker and more turbid homogeneous layer. During winter, when sediment discharge from the rivers often reaches its annual maximum, an oceanic thermo-haline wedge occurs on the shelf at around −100 m. As a result, winter turbidity values on the outer continental shelf are low (comparable to summer values), and a permanent nepheloı̈d layer is never observed. The wedge, which lasts for several months, may act as a filter, preventing transport to the slope. High turbidities on the external shelf and the continental slope are only measured in spring, when the thermo-haline wedge disappears. It seems possible that during winter time, suspended materials brought by rivers are deposited in the “Grande Vasière” (the “large mud patch”). It is postulated that the position of this mud patch is linked to the long-term stable location of the thermo-haline front that separates oceanic waters from the colder and less salty coastal waters.