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Ecological modelling of the planktonic microbial food web

Antarctica phase III (1993-1997)

Ecological modelling of the planktonic microbial food web

Promotor

Dr Christiane Lancelot
Université Libre de Bruxelles - C.P. 221
Microbiologie des Milieux Aquatiques
Campus de la Plaine
Bd. du Triomphe
B-1050 BRUSSELS
Phone: +32 (0)2 650 59 88
Fax: +32 (0)2 650 59 93
E-mail: lancelot@ulb.ac.be

Topics

Physical, chemical and biological conditions governing phytoplankton bloom development and food chain structure in the Southern Ocean were investigated, based on field observations and mathematical modelling. Particular attention was paid to sea-ice dynamics and wind stress in triggering phytoplankton bloom induction, and to iron and krill as vector of food chain structure and related surface carbon retention vs exportation. Research focused on conditions determining the development of diatoms- vs 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 inhabitance, namely:

  • 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 iron-deficient Antarctic Circumpolar Current while submitted in early spring 1992 to severe meteorological conditions (average wind: 11m.s-1); and,
  • the iron-sufficient ANTARES 2 site in the Indian sector of the Southern Ocean and experiencing in late summer 1994 auspicious meteorological conditions.
    Biomass of autotrophic (diatoms and nanoflagellates) and heterotrophic (bacteria, bactivorous and herbivorous protozoa) microorganisms were spatio-temporally 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 occur. Based on these results, the numerical code of the 1D 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 1D 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 strength of storm events - and phytoplankton blooms occurrence.

It was demonstrated that:

  • the sustained windy meteorological conditions prevailing during the ANTX/6 expedition was the main factor preventing blooms from developing at the receding ice-edge; and,
  • under events of favourable meteorological conditions for phytoplankton bloom initiation, the structure of the developing phytoplankton community is determined by iron availability with nanoflagellates outcompeting diatoms at iron sub-nanomolar concentration.
    Hence it can be safely concluded that the general HNLC (High Nutrient Low Chlorophyll) conditions of the Southern Ocean are resulting from the successful development of grazer-controlled nanophytoplanktonic communities in a low-iron environment. Superimposing this active microbial food web, episodic blooms of diatoms are well developing in iron-enriched areas experiencing favourable meteorological conditions like near-shore neritic 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 availability and/or krill grazing pressure and the phytoplankton community structure shifts towards a nanoflagellates dominance.

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