The research consists of a multidisciplinary study of the biodiversity and productivity of the global ecosystem of the Southern Ocean in a context of variation of the environmental and climatic conditions. It develops an approach based on the analysis of sub-ecosystems at specific time and space scales and on the interconnection of these systems by means of a proper modelling. The sub-ecosystems studied are : 1) the planktonic ecosystem, which is the subject of the study of the processes relating to biodiversity, dynamics and phyto-zooplanktonic interactions (specific scale : the water column and ice layer); 2) the pelagic ecosystem, with which is associated the macrozooplankton and in particular the krill (specific scales : Ross Sea and decade); 3) the ecosystem of the higher trophic levels (specific scales : Southern Ocean and century). It bears on the conceptualisation, the parameterisation and the validation of a coupled 3D numerical model integrating the whole of the physical and biological processes which govern the dynamics and the variability of the global ecosystem of the Southern Ocean. The biological module of this model will be primarily centred on the zooplankton (in particular the krill) and the higher predators, but will take into account phytoplanktonic variables limiting the feeding of the herbivores. In a second stage, the coupled 3D model will be applied to evaluate the response of this ecosystem according to various forcing scenarios taking account of both natural and anthropogenic factors. OBJECTIVES The objective of the INTEGRATED PROJECT is twofold : A) To understand how the physical and biological processes coupled at various scales control the functioning and the evolution of the global ecosystem of the Southern Ocean. More specifically, the goal is to quantitatively determine how this ecosystem reacts to the whole set of the physical constraints, whether directly - under the influence of the hydrodynamics, the vertical structure of the water column, the ice cover and the penetration of light into the water, or indirectly - via the trophic conditions, both qualitative (the favoured species or group of species) and quantitative (the quantity and productivity of these species), which are imposed by the physical constraints. B) To determine, in order to aid the decision-making process regarding environmental protection, how this system would respond to environmental and climatic changes, both natural and anthropogenic. These objectives are pursued within the framework of an international programme on the global ecosystem of the Southern Ocean in the Ross Sea in collaboration with the Italian National Research Programme on the Antarctic (PNRA) and, in particular, during campaigns in the Ross Sea planned for 1997-98 and 1998-99.