Biological response to mesoscale eddies in the Algerian Basin

[1] The Eddies and Leddies Interdisciplinary Study in the Algerian Basin ( ELISA) experiment ( 1997 - 1998, MAST-3/MTP2/MATER program) was a multidisciplinary and multiplatform experiment designed to study the anticyclonic Algerian Eddies (AEs) generated by the instability of the Algerian Current and their influence on the general circulation and biological phenomena. This paper presents preliminary results of the data obtained over the four year-round cruises ELISA-1 to 4. Two AEs ( called 96-1 and 97-1) were tracked with satellite images over their counterclockwise circuit in the eastern Algerian Basin, from the Algerian to the Sardinian slope, and then to the open sea. They have been sampled over different periods and positions. Associated biological response was analyzed considering the hydrodynamical structure and the distribution of chlorophyll and nitrate concentration in the upper 300 m. In summer both AEs (96-1 located along slope and 97-1 located offshore) corresponded to highly oligotrophic areas. The deep chlorophyll maximum was similar to 90 - 110 m, with concentrations up to similar to1 mg m(-3), the nitrate concentrations were low down to similar to 250 m in the AEs' central zone. The downward entrainment of chlorophyll along isopycnals in the AEs' peripheral zone was well observed down to similar to 250 m. In spring the maximum integrated chlorophyll concentrations were found offshore, in 96-1, where the upper similar to 150 m were quasi-homogeneous. Lower integrated chlorophyll concentrations were found inshore in 97-1, which was embedded in an Algerian Current meander and remained stratified throughout wintertime. AEs generate secondary phenomena such as small-scale cyclonic shear eddies, where the highest chlorophyll concentrations (similar to 4 mg m(-3)) were found. We show that through the AEs it generates, the Algerian Current can be responsible for producing areas in the coastal zone that are at least as oligotrophic as the eastern Mediterranean and, alternately, for productive areas offshore. As AEs generally follow a counterclockwise circuit in the Algerian Basin, it is inferred that they play an important role in the redistribution of matter on a basin scale. However, the biological response associated with AEs varies according to their history, a combination of trajectory, location, and season, in ways that are not yet clear.