Coastal mesoscale processes and their effect in phytoplankton distribution and community composition in the SE Bay of Biscay

Abstract. Mesoscale dynamics play a major role in several ocean processes, not only in the transport of momentum, heat, mass, particles and microorganisms but also in the provisioning of nutrients into the euphotic zone. Mesoscale processes can define niches where specific phytoplankton species flourish. However, this effect is not straightforward in coastal areas, which are submitted to a more complex interplay between different oceanic processes. In this context, the ETOILE campaign surveyed the CapBreton canyon area in the South-East of Bay of Biscay in early August 2017. The main objective of this study was to link the occurrence and distribution of phytoplankton with the mesoscale ocean processes. On top of the remote sensing data available for this area, such as High Frequency radar or satellite data, in situ discrete hydrographic measurements were carried out by a CTD and a Moving Vessel Profiler. Likewise, multi-spectral fluorescence casts were performed in selected stations. Other parameters such as temperature, conductivity and in vivo multi-spectral fluorescence were also continuously recorded at surface. From our observations, we discuss on the distinct effect and importance of different factors affecting the phytoplankton distribution. Overall, salinity is the most important parameter modulating not only algae distribution but also the composition of the community in terms of spectral groups. Although below the mixed layer salinity still impacts significantly phytoplankton, vorticity comes into play and becomes the dominant factor determining both distribution and composition. The present study brings into consideration the relevance of the hydrodynamical variables in the study of phytoplankton.