Mois : juillet 2020

PhD thesis defense of Arnaud Louchart

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Title : Dynamique spatio-temporelle des communautés phytoplanctoniques côtières et de leurs caractéristiques intrinsèques, à partir d’une approche automatisée à haute résolution.

Date : 24 juillet 2020 à 9h30
Lieu : salle de conférence MREN LOG Wimereux

Jury :
Directeur de thèse : François G. Schmitt, LOG, CNRS
Co-encadrant de thèse : Luis Felipe Artigas, LOG, ULCO

Rapporteurs :
Nathalie Simon, SBR, Sorbonne Université
Philippe Grosjean, Laboartoire d’Ecologie Numérique, Université de Mons

Examinateurs :
Urania Christaki, LOG, ULCO
Mélilotus Thyssen, MIO, CNRS
Véronique Créach, CEFAS
Duncan Purdie, NOC, University of Southampton

Abstract :

In coastal ecosystems, phytoplankton composition, distribution and dynamics are strongly influenced by spatial and temporal variations of hydrological structures and biogeochemical parameters, consequences of natural and anthropogenic pressures. Reference monitoring, due to its low spatial and temporal resolution, may fail to detect key events as the initiation and end of phytoplankton outbursts or harmful algal blooms (HABs). By increasing the spatial and/or temporal resolution as well as completing taxonomical counting by investigating the phytoplankton whole size spectra, the use of automated sensors may allow contributing to a better understanding of the distribution and dynamics of this major player in biogeochemical cycles, at the basis of most food webs. This thesis consists in studying the characteristics of phytoplankton functional groups defined from their optical properties at the single-cell level, in relation to spatio-temporal variability encountered in contrasting marginal seas, applying the pulse shape-recording automated flow cytometry. This functional classification reflects the diversity of particles according to morphological and physiological properties. First of all, the distribution of phytoplankton groups and their traits where explored in the Western and Central English Channel during the summer period. Most groups formed patches of abundance and biomass near the Ushant front and were structured at the sub-mesoscale. Secondly, phytoplankton functional groups dynamics was characterized in the Eastern English Channel and Southern North Sea during the development period of diatoms and Phaeocystis globosa spring groups, by calculating LCBD and SCBD, which allowed the observation of spatial segregation between phytoplankton groups. Their distribution was explained by the niche parameters (marginality and tolerance). Finally, the vertical distribution of phytoplankton functional groups in a salinity gradient was addressed in the Baltic Sea, in relation to the biogeochemical properties of the water masses and the characteristics of each PFGs. The variations of the traits are thus stand out as the best predictors of the horizontal and vertical distribution of phytoplankton groups with respect to niche parameters and spatial descriptors (dispersion, physical and biological parameters). The functional approach, derived from phytoplankton optical properties addressed byt automated flow cytometry, coupled to the niche analysis, make it possible to better explain and predict community responses to environmental gradients, such responses being detected in parallel by diversity indices. This work benefited from the support of local (MARCO State-Region Plan Contract), national (CNRS-MTES convention) and international European H2020 JERICO-NEXT projects