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.
Related Projects