WATER MASSES AND PHYTOPLANKTON BIOMASS DISTRIBUTION DURING SUMMER IN THE WEDDELL SEA MARGINAL ICE-ZONE

This study - carried out between the Elephant and the Orkney islands - showed that spatial variability in the marginal ice-edge zone (MIZ) w as greater than that found in open sea. Salinity and silicate contents point to the existence of two fronts: the first - near Elephant Islan d - separates the Surface Antarctic Water coming from Drake Passage fr om the winter water originating in the Weddell Sea. This water is char acterized by its high concentrations of nitrate (> 30 mu M) and silica tes (> 80 mu M). The second front - near the South Orkney Islands unde rgoes the influence by a less saline water lens (< 34 %), poor in nutr ients (N-NO3 < 24 mu M) and rich in chlorophyll (> 3.5 mg Chl a 1(-1)) . Water masses circulation reveals the existence of a series of conver gences and divergences which alternate along MIZ, together with differ ent haline fronts. These fronts limit three differents zones, taking i nto account the presence of chlorophyll. In zones where the surface la yer presents both low salinity and stability with a thickness of about 40 m, chlorophyll contents are rather high - between 1.5 and 5 mg Chl a. m(-3). On the other hand in the mixing vertical zone, these chloro phyll contents are low (< 0.3 mg Chl a.m(-3)) while the frontal region separating these two zones presents intermediate values and the exist ence of deep subsurface maximum of chlorophyll. Horizontal variability of integrated Values (on the whole of the euphotic zone) is clearly i nferior to the one present in surface concentrations. PON/Chl a and PO C/PON relations show their minimum levels in the pack-ice fusion water , as an effect of the high photosynthetic activity.