GROSS AND NET PRIMARY PRODUCTION IN THE SCOTIA-WEDDELL SEA SECTOR OF THE SOUTHERN-OCEAN DURING SPRING 1988

Daily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC-m-2.day-1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC.m-2.day-1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC.m-2.day-1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC.m-2.day-1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the "well illuminated" OOZ area.