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Ecological modelling of the planktonic microbial food-web
Lancelot, C.; Becquevort, S.; Menon, P.; Mathot, S.; Dandois, J.-M. (1997). Ecological modelling of the planktonic microbial food-web, in: Caschetto, S. (Ed.) Belgian research programme on the Antarctic: scientific results of phase III (1992-1996): 1. Marine biochemistry and ecodynamics. pp. A3/11/001/1-78
In: Caschetto, S. (Ed.) (1997). Belgian research programme on the Antarctic: scientific results of phase III (1992-1996): 1. Marine biochemistry and ecodynamics. Federal Office for Scientific, Technical and Cultural Affairs: Brussel. 220 pp., more

Available in  Authors 
    VLIZ: Expedition Reports [8936]

Keyword
    Marine

Authors  Top 
  • Lancelot, C., more
  • Becquevort, S., more
  • Menon, P.
  • Mathot, S.
  • Dandois, J.-M.

Abstract
    Physical, chemical and biological conditions governing phytoplankton bioom development and food chain structure in the Southern Ocean were investigated, based on field observations and mathematical modelling. Particular attention was given to sea-ice dynamics and wind stress in triggering phytoplankton bloom induction; and iron and krill as vector of food chain structure and related surface carbon retention versus exportation. Within this framework, research was focusing on conditions determining the development of diatoms- versus nanoflagellates- dominated phytoplankton communities as well as on the dynamics of the microbial food web. Sampled sites were crossing areas with contrasting meteorological conditions and sea-ice dynamics, dissolved iron availability and krill inhabitence: the EPOS site in the iron-rich marginal ice zone of the northwestern Weddell Sea, crossed by krill swarms and experiencing in spring-summer 1988 extremely favourable meteorological conditions (average wind: 7m.s-1); the ANTX/6 site crossing at 6°W the iron-rich Polar Frontal Jet and the sea-ice-associated irondeficient Antarctic Circumpolar Current while submitted in early spring 1992 to severe meteorological conditions (average wind: 11 m.s-1); the iron-suffisant ANTARES 2 site in the Indian sector of the Soutern Ocean and experiencing in late summer 1994 auspicious meteorological conditions. Biomass of auto- (diatoms and nanoflagellates) and hetero- (bacteria, bactivorous and herbivorous protozoa) trophic microorganisms were spatiotemporaly measured and their mutual interactions were assessed. Process-oriented studies were conducted for determining the physiological characteristics of diatoms and nanoflagellates growth and the feeding functional properties of the protozoan community. It is shown that diatoms and nanoflagellates growth differs by the only iron biochemistry and affinity for iron concentration, photosynthetic properties being identical for both phytoplankton groups. The protozoan community can be regarded as composed of two groups characterized by their own diet and feeding characteristics: the strictly bactivorous heterotrophic nanoflagellates and the protistovorous protozoa feeding on almost exclusively the auto- and heterotrophic flagellates. Furthermore ingestion rates of these two communities can be described by a specific food saturation function above a threshold food concentration below which feeding does not occurr. Based on these results, the numerical code of the 1 D mechanistic SWAMCO model, describing carbon, nitrogen and iron cycling through the microbial food chain closed by copepod grazing pressure was established. The SWAMCO model was calibrated and validated on the ANTX/6 site. Observational ecosystem analysis and mathematical simulations with the 1 D-coupled physical-biological SWAMCO model forced by the chemical and meteorological conditions during the EPOS and ANTX/6 expeditions show the tight coupling between atmospheric forcing -most notable in frequency, duration and strenght of storm events -and phytoplankton blooms occurrence. In particular it is demonstrated that the sustained windy meteorological conditions prevailing duringthe ANTX/6 expedition was the majn factor preventing blooms from developping at the receeding ice-edge. Furthermore it is demonstrated that under events of favourable meteorological conditions for phytoplankton bloom initiation, the structure of the developing phytoplankton community is determined by iron availabjlity with nanoflagellates outcompeting diatoms at iron subnanomolar concentration. Hence it can be savely concluded that the general HNLC (High Nutrient Low Chlorophyll) conditions of the Southern Ocean are resulting from the sucessfull development of grazer-controlled nanophytoplanktonic communities in a low-iron environment. Superimposing this active microbial food web, episodic blooms of diatoms are well developping in iron-enriched areas experiencing favourable meteorological conditions like near-shore neretic areas supplied with iron from shelf sediments, the rapidly eastward flowing Polar Frontal Jet retaining a significant signal of iron from shelf source and to a less extent some sea-ice covered areas having cumulated minor annual aerosol inputs. When optimal light conditions are maintained diatom growth is however limited by iron avalaibility and/or krill grazing pressure and the phytoplankton community structure shifts towards a nanoflagellates dominance. Further research development would sake for coupling the generic SWAMCO mechanistic model with a 3D coupled sea-ice-ocean model of the global Southern Ocean to properly address the role of this remote ocean in global carbon cycle.

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