Ongoing PhD thesis led by Alice Vanhoutte-Brunier

Phytoplankton blooms in the English Channel and in the southern part of the North Sea :
development of an integrated physical-biogechemical model.

	Thesis Director : Paul Nival (University : Paris VI)
	Scientific leaders (IFREMER) : Alain Lefebvre (DEL/BL) and Alain Ménesguen (DEL/EC)
	Funding : French Nord-Pas de Calais Region and IFREMER.

Background :

The English Channel is provided major nutrients inputs by the Seine (and the Somme) rivers. The eastern Channel and the coastal areas
of the North sea are hence disturbed by large blooms of the microalga called Phaeocystis sp. which takes advantage of the nitrate
excesses in water during spring time.
 

Phaeocystis sp. bloom in Hardelot (beach situated in the Pas-de-Calais area, France) in June 1999 (B. Hitier),
and colony of Phaeocystis (x 400)

This colony forming species is particularly rich in mucus which is poured out in water in case of colony rupture. Under specific wind conditions,
a thick and smelly foam develops and totally covers beaches. Moreover, this proliferation may completely affect the way the whole
ecosystem functions (Lancelot, 1995). Studying the determinism of such HABs (Harmful Algal Bloom species) blooms by modelling is
hence part of the National Coastal Environmental Programme (PNEC) for the "Eastern Channel and southern part of the North Sea".

Furthermore, the English Channel is also affected by other HABs blooms creating "coloured" waters : Karenia mikimotoi. This
Dinoflagellate is producing cytotoxic haemolytic substances inducing high mortality rates for marine organisms such as fish, crustaceans
and molluscs. K. mikimotoi usually appears in the summertime is specific areas with strong stratifications, or in proximity to front zones.
Both these situations can be found in the western Channel. SeaWiFS satellite images enabled the detection of an extremely intense
bloom which developed in the western Channel in summer 2003. The CEFAS (Liam Fernand, pers. com. ) confirmed that
Karenia mikimotoi was guilty for this proliferation.

Concentration in chlorophyll a in the surface water, SeaWiFS image analysed with Gohin's algorithm, and Karenia mikimotoi (left) (x 300)

Modelling this event may lead to the evaluation of the processes which started this bloom and propagated it towards the French coasts.

Steps of the modelling process :

A first basic model was developed before two specific models for the HABs were added.

This basic model results of the integration of an hydrodynamic model (MARS 3D, Ifremer) with a simple biogeochemical model close
to the ones developed for tens of years by the DEL/EC Department ( e.g.: Bay of Seine model, Cugier, 1999).

Nitrogen, Phosphorus and Silicium cycles are simulated both in water and within the sediment. The phytoplankton is divided into two
classes according to its capacity to absorb silicates. A zooplankton class is closing the system.

Schemes of N, P and Si cycles in the nominal model.

The specific model for Phaeocystis sp. is developed in close collaboration with the laboratory of Marine Systems Ecology of the Free
University of Brussels (Pr. Christiane Lancelot and Nathalie Guypens), specialised in modelling this species (model of the Belgian
coastal ecosystem : MIRO).

The model describing Karenia mikimotoi was initially developed for the Bay of Biscay (Loyer et al., 2001). The formulation
of the mortality rate of this species induces its high sensitivity to the turbulence of the water masses, and also accounts for its
proliferation in stratified areas of the western Channel.

Size : 2331 Ko

Poster presented in June 2005 in Plymouth, "3D modelling of a planktonic ecosystem integrating the HABs species Phaeocystis globosa. Validation by satellital imagery / Tracking nitrogen's origin".
 

Contact :

IFREMER

BP 70 - 29280 Plouzané, France
Department : Dynamics of Coastal Environement
Laboratory : Benthic

Alice Vanhoutte-Brunier Email : Alice.Vanhoutte.Brunier@ifremer.fr_______________________________________________________________________________________