The Continuous Plankton Recorder (CPR) survey has been deployed since 1931 to describe and analyze plankton variability in the North Atlantic and North Sea. This survey measures the presence and abundance of 437 phytoplankton and zooplankton taxa and provides an assessment of phytoplankton biomass, the Phytoplankton Colour Index (VCI). The diatoms and dinoflagellates are the two main phytoplankton groups identified by the CPR survey. The first part of this work provides insights into the space-time dynamics of phytoplankton communities through an analysis of diatom and dinoflagellate populations in the whole North Atlantic basin. Because the North Atlantic Ocean includes many different biotopes, the second part focuses on the mesoscale variability of phytoplankton species. The long-term fluctuations of the phytoplankton species are studied in the NW and in the NE Atlantic, the two best sampled areas of the CPR survey. In the NE Atlantic, the aim is to determine the contribution of the diatoms and dinoflagellates to the PCI, their fluctuations over 45 years of sampling and their geographical variations. Because local variability in environmental conditions is thought to play a dominant role in temporal fluctuations of phytoplankton biomass, the next part takes advantage to define small areas around the British Isles. This allowed me to study more precisely the processes influencing the long-term variation of phytoplankton assemblages. The North Atlantic Current transports water across the Northern basin of the Atlantic Ocean, along the shelf of Ireland and form the Norwegian current which corresponds to the inflow of oceanic waters into the Norwegian Sea and the North Sea. In this highly hydrodynamic region attention is focused on the fluctuations of plankton species in relation to the currents. The aim of this part is therefore to investigate the fluctuations of phytoplankton biomass, diatoms and dinoflagellates, their geographical distribution and abundance within the area and their relationship with physical processes. The intense hydrodynamic activity observed in the Northwestern Atlantic Shelves Province (NWCS) makes this region especially intriguing from the point of view of physical-biological interactions. The relationship between spatial and temporal structures of eddies (via Sea Surface Heights) and chlorophyll a (from the Sea-viewing Wide Field-of-view Sensor, Sea WiFS) was assessed along the Gulf Stream axis. In particular, the physical structures identified were followed and compared with phytoplankton distribution. In addition, the impact of the LSW changing flow along the Scotian Shelf and the influence of Gulf Stream rings along the George Bank is determined. This work demonstrated that changes are occurring in pelagic ecosystems at different temporal and spatial scales. These changes have been illustrated by the spatial variability induced by eddies and/or currents but also by the regional variability of the hydro-climatic processes, influencing in different ways Sea Surface Temperature, wind-regimes and mixing of local environments. Several different aspects of the North Atlantic Oscillations impact on pelagic ecosystems have been highlighted. In the northeast Atlantic, NA0 fluctuations imply changes in (i) SST in northern Europe, (ii) wind regimes, (iii) Atlantic Water inflow into the North Sea. In contrast, in the northwest Atlantic, the variations of NAO imply changes in (i) SST on the Scotian Shelf, (ii) coastal currents, and (iii) inflow of Labrador Sea Slope Water (LSSW) towards the Scotian Shelf and Georges Bank. These changes in environmental process impact phytoplankton production, abundance, spatial distribution, community structure phenology and ultimately would impact trophodynamics processes. It is, however, still difficult to explain unambiguously all the mechanisms that are involved in the control of the observed patterns.

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