Research project A3/03/001 (Research action A3)
THEME
It is becoming increasingly clear that the Antarctic plays a decisive role in the problem of "global change" through substantial interactions between atmosphere, ice, ocean and biota. The biological transformation of carbon dioxide into particulate biogenic matter is an important process in this respect.
Within this context, the present study is aimed at determining the origin, evolution and fate of particulate biogenic matter in the Southern Ocean. Special attention will be given to the nitrogen system in the upper layers of the water column and to the relationship with the flux of production towards the deep layers.
OBJECTIVES
The problems examined in this study primarily concern two topics:
- the factors determining the regime of nitrogen assimilation by the phytoplankton in the upper layers of the water column;
- the link between this regime and the production exportation flux.
Seasonal variations in nitrogen assimilation are determined according to the profile of nutrients in the water masses and the rates of assimilation of the different nitrogen compounds. The nitrogen assimilation regime is expressed in terms of variations in the "fratio" (measurement of the relative contribution to nitrate assimilation in primary production) and the RPI. At the same time, plankton composition and trophic transfers are also examined, including herbivorous activity and the mineralization of nutrients (by bacteria and zooplankton).
On the other hand, when the biogenic matter in the euphotic layer is not transferred to the higher trophic levels, it is deposited as sediment in the deep layers and possibly on the sea floor. The quantitative and qualitative exportation fluxes must be considered in terms of seasonal variations in primary production, specific assimilation of nitrogen and plankton composition. They are determined from vertical distributions of particles and the flux of the particulate matter sampled with the use of sediment traps.
To assess the role played by the Southern Ocean in the cycle of biogenic compounds, it is important to link up the period of the assimilation system with the production exportation flux. In addition, it is necessary to know the inter-regional differences which exist between the productive areas of the sea ice margin and the oligotrophic areas, such as the open ocean and the regions with a permanent ice cover. The relative contribution of each area to the exportation flux is variable: higher productivity in the marginal sea ice area does not necessarily lead to increased exportation but rather to a substantial recycling of organic matter. This is not the case in the oligotrophic areas.
