Modelling of the control of the pelagic ecosystem by the higher trophic levels in the marginal ice zone and application of the model to simulation of the effects of potential anthropic disturbances.
OBJECTIVES
- Determination of the interaction mechanisms between, on the one hand, the dynamics of the secondary planktonic trophic levels and, on the other hand, the dynamics of the lower and higher planktonic trophic levels, in accordance with the main environmental variables.
- Modelling of the functioning of the pelagic ecosystem based on these interaction mechanisms.
- Application of the model to simulation of the evolution of the pelagic ecosystem as a reaction to external constraints of climatic or anthropic origin.
A series of phenomena characteristic of the dynamics of the Southern Ocean's marine ecosystem are taken into consideration in pursuance of these objectives:
- The stabilization of the surface layers of the water column, which appears seasonally through the combined action of hydrodynamic and meteorological factors, stimulates primary production in the marginal area of the circumpolar ice by providing phytoplankton with optimum development conditions. Apart from its hydrographic stabilization action, the seasonal melting of the sea ice also releases algae populations from which the development of phytoplankton begins in the water column (seeding process).
The primary production generated in this way, which is vertically structured, is gradually consumed by the higher trophic levels and thus contributes towards the determination of their spatial distribution. The process appears to be continuous and depends on how the sea ice retreats.
- Given their abundance in the Ross Sea in spring, herbivorous copepods and krill put strong grazing pressure on the diatoms and control their biomass. The quantities of ammonia excreted by these organisms stimulates the productivity of smaller phytoplanktonic forms which develop in summer.
- Depending on its specific composition, the zooplankton directs the organic matter of phytoplanktonic origin both towards different trophic compartments and towards different masses of water. Furthermore, depending on its specific migratory behaviour, the zooplankton recycles the organic matter in the euphotic area or in the deeper layers.
The measurements made during oceanographic campaigns are aimed at parameterizing, calibrating and validating the numerical model simulating phytoplankton/zooplankton interactions along the water column. The strategy of these campaigns will be oriented towards long-term stations characteristic of a given temporal state.
