Author(s): Alain SOURNIA, JeanMichel BRYLINSKI, Serge DALLOT, Pierre LE CORRE, Michel LEVEAU
Investigations on the. hydrological ·fronts have been undertaken in various coastal laboratories of France for a varying number of years and are coordinàted. within a national program since 1987. A comparative state-of-the-art is presented here from North to South. · In the narrow passage of the Straits of Dover, between the English Channel and the North Sea, an unstable, recently discovered coastal front is forced by a northward flow of Jess saline water of continental origin, and it is modulated in time and space by the tides. The continental shelf off Brittany (Atlantic Ocean and entrance of the English Channel) exhibits three typical examples of frontal structures: at the mouth of the bays of Brest and Douarnenez, the interna! tidal front of the Iroise Sea; at about the middle of the shelf, the Ushant tidal front; and above the slope, the shelf front of the Gulf of Gascogne and Ce/tic Sea. The two former structures are governed by variations of the energy dissipated by tidal currents whereas the shelf front is produced by the interactions between wind and internai tide. Altogether, the fronts off Brittany have been the more intensively studied as regards physical modelling, nutrient cycling, primary production, trophic webs, and the respective effects of tidal periodicities on the ecosystem. The plume front of the River Rhône (Gulf of Lions, Mediterranean Sea) bas recently led to a radar-based physical modelling of the plume and to studies of the fate of the organic matter which 1s accumulated at the frontal interface. Last, the Ligurian Sea between France and Corsica (Mediterranean Sea) provides the typical case of a meso-scale, geostrophic front. The recognition of the latter, together with the associated convection ails and oblique downward transport . of matter towards depth, proved to be a fruitful tool for the understanding of the pelagie ecosystem in this area. These diverse fronts provide a mutually complementary set of experimental sites which can be easily reached from the coastal laboratories. The knowledge and experience gained there may be extrapolated in the future to fronts of larger of world-wide scale. Each of these fronts bas revealed, in a way of its own, the originality and interest of frontal processes, namely: privileged interactions between physics and biology, introduction of high amounts of auxilliary energy into biological systems, interface effects, dominance of meso-scale (if not fine-scale) phenomena, and frequently increased biological productivity. Future progress will heavily rely on the availability of sampling and measuring techniques specifically adapted to the frontal scales, such as: acoustic Doppler current profiling, continuous in situ measurements for as many variables as possible, stratified sampling of the zooplankton, acoustic localization of the planktonic and nektonic biomasses.