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Control of the Antarctic pelagic ecosystem by higher trophic levels in relation to variations in environmental conditions

Antarctica phase III (1993-1997)

Control of the Antarctic pelagic ecosystem by higher trophic levels in relation to variations in environmental conditions

Promotor

Dr Jean-Henri Hecq
Université de Liège
Ecohydrodynamique
Sart Tilman - Physique B5
B-4000 LIEGE
Phone: +32 (0)4 366 36 46
Fax: +32 (0)4 366 23 55
E-mail: jh.hecq@ulg.ac.be

Topics

The productivity and biodiversity of the Ross Sea pelagic ecosystem were studied in relation with its specific environmental features. The variability which results both in spatial individualization of local sub-ecosystems and seasonal changes of those sub-ecosystems have been identified in collaboration with the Italian PNRA Programme.

Following the main objective which was to analyze and link physical and biological processes at different scales which govern the Southern Ocean global ecosystems and to determine the potential resilience to climatic and anthropogenic changes, we have developed and implemented a coupled physical/biological model able to simulate these multi-parametric constraints.

Data obtained during the Ross Sea cruises have confirmed that the most important factors regulating the Antarctic pelagic food chain are physical processes operating within the circumpolar marginal ice zone during the ice melting period. As a typical characteristic of the Ross Sea, the ice free surface of the polynia is propagating from the South to the North, with an increase of the water surface exposed to the sunlight. The diversity of biota assemblages in Ross Sea area seems attributable to a specific control by local constraints rather than to the presence of specific sub-ecosystem:

  • the ice edge melting is not simultaneous for the whole region,
  • the central and southern part opening sooner than the lateral part;
  • the areas of depth shallower than 500m seems to be inaccessible to krill which is strongly influencing the utilisation of primary production; and,
  • the ice algae content is more important in the western part.
    The concentration of phytoplankton pigments and their grazing-degradation products have been specifically determined by means of HPLC in the water column, in the ice, suspended sediments, zooplankton and krill digestive systems and in faecal pellets. Pigments values were used as algae tracers of fluxes across the food chain in the upper layers of the water column and as data for parameterizing, calibrating and validating ecological models.
    The major part of the Southern Ross Sea was dominated by Phaeocystis pigments (hexanoyl-oxy-fucoxanthin and chlorophyll C3). The diatoms pigment (fucoxanthin) dominated in high concentrations overall in coastal areas, in the ice and in the water column and in the central Ross Sea, at the northern limit of the polynia. In that area, the phaeophorbid high concentration assumes high krill grazing which reduce diatoms bloom. The results have completed the hypothesis of distribution of ice algae pigments in relation with ice retreat and emphasised the temporal succession of various algae groups like cryptophytes and prasinophytes and the control of primary production exportation by grazing.

Data acquired in the Ross Sea have been used to developed a coupled physical/biological one-dimensional model of the upper water column model with a k-1 turbulent closure. The model is used to simulate how the small scale physical processes of vertical stabilization of the water column within the Ross Sea marginal ice zones during ice melting period influence the physical and biological structure of upper layers and stimulate primary production, select the type of phyto- and zooplankton communities, control the vertical distribution of zooplankton and metabolic products (faecal pellets, NH4...) and finally control the upper trophic levels.

At the upper edge of the Southern Ocean ecosystem scale, changes correlated with whaling are relevant and the krill increase due to their exploitation seems to induce, allowing for faster growth, higher pregnancy rates and earlier sexual maturity. The hypothesis of food-limited by the whale stocks before whaling period, and the increase of the krill availability for others consumers: birds, seals and remaining whales related to industrial whale catching have been tested by a numerical model for the upper trophic levels of the Antarctic food chain.

Simulation without whaling activities emphasizes equilibrium in the system.

Simulation of the system with real fisheries statistics until 1990 and assuming constant whaling equal to actual levels after 1990 or no whaling activities after 1990 determined the influence of whaling on resilience of the whale stocks for the future.

The model validates the hypothesis of krill-limitation by the whale stocks before whaling period and also the control function of whaling on all upper trophic levels of the Antarctic food chain.

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