MIDWATER TUNICATES - ARE THEY RELATED TO THE PERMANENT FRONT OF THE LIGURIAN SEA (NW MEDITERRANEAN)

Midwater filter feeders may play an important role in the downward carbon flux by aggregating particulate organic matter into large, slowly sinking mucoid aggregates or rapidly sinking fecal material. Successive submersible cruises made in the frontal zone of the Ligurian Sea showed the presence of midwater (400 to 800 m) populations of a not yet described larvacean species of the genus Oikopleura and of the thaliacean Salpa fusiformis Cuvier. The vertical stratification of autotrophic and heterotrophic organisms and the occurrence of autotrophic microphytoplankton at aphotic depths suggested that food supply for midwater filter feeders was actively transported by the frontal circulation from the subsurface layers along the inclined isopycnals. Minimum flow velocity for renewal of organic carbon at the midwater stratum was estimated to be 20 m d-1. The vertical distribution of the midwater filter-feeder community corresponded to the deep origins of peripheral (near the coast) and central divergences and may indicate the deep junction of convection cells.