Diversity of phytoplankton communities in relation to environmental factors in the Iroise Sea: 3D modeling approach.

Summary Environmental factors (physical, chemical and biological) influence the composition of marine phytoplankton communities. In addition, dynamic transport can also impact the relative abundance of organisms within these communities. Consequently, phytoplankton biomasses, as well as the nature of the organisms that compose them, show significant variability both spatially (bioregionalization) and temporally (successions). Coastal regions are particularly contrasted areas in which environmental gradients are generally marked. Thus, the Iroise Sea is characterized by the presence of a seasonal tidal front (Ushant front), particularly productive, which separates two distinct regimes. To the east of the front, the waters of the continental shelf are regularly made homogeneous by the presence of strong tidal currents, whereas the offshore area is subject to a seasonal cycle marked by a summer vertical stratification. This is therefore a suitable region for a more general study of the mechanisms of interaction between frontal structures and phytoplankton diversity. The more specific aim of this thesis is to characterize, with the help of 3D modelling, the nature and diversity of phytoplankton in the Iroise Sea, both in terms of functional groups and phenotypic diversity, during a seasonal cycle in general and more particularly during the summer period at the frontal zone.The first results obtained showed that the composition in functional groups of phytoplankton presents a marked seasonal cycle, mainly influenced by the depth of the mixing layer. During winter, picoplankton dominate throughout the study area. Stratification, which begins in April, leads to a phytoplankton bloom dominated by microphytoplankton (mainly diatoms). The summer period then corresponds to the establishment of a bio-regionalization of environmental conditions in the Iroise Sea with (i) the mixed coastal zone which remains highly productive and dominated by diatoms and (ii) the offshore zone, in which autotrophic growth is limited by surface nutrients, which favors the coexistence of microphytoplankton and picophytoplankton. The results show a zone of high diversity at the surface, slightly shifted towards the west with respect to the frontal zone (in which the biomass is maximum). At this diversity maximum, the importance of vertical exchanges (upwelling and mixing) on the warm (stratified) side of the front was highlighted. Thus, a mixture between ubiquitous phenotypes present in the mixed zone east of the front and picoplankton, originating from both the subsurface chlorophyll maximum and the oligotrophic surface to the west, is observed in the diversity maximum.Finally, a last study on the effect of the spring/neap tidal cycle allowed us to understand, for the first time, the processes that explain the impact of this cycle on the modification of phytoplankton biomass and on the composition of the community in terms of phenotypic diversity in the homogeneous coastal system. The results show an increase in the total biomass as well as in the proportion of diatoms and a decrease in diversity during the stratification periods associated with neap tides.