Coastal countries of North West Africa are highly dependent on their ocean which supplies food locally and abroad (Europe and Asia mostly) and represents an important source of employment for local people. The area south of the Canary Current Upwelling System (SCCUS) is one of the most productive marine ecosystems in which small pelagic fish (SPF: sardine, sardinella, anchovy) represent an abundant resource. The spatial and temporal variability of SPF distribution is strongly constrained by the distribution of plankton communities that constitute the main diet of these planktivorous fish. Together biogeochemistry and hydrodynamics therefore define the habitats of these SPFs and influence their growth, reproduction, mortality and distribution. Despite the social issues, the SCCUS has been very little studied the last 20-30 years. The separation of the southern Senegalese coast from the northern part of the system by the Cape Verde peninsula, the width and regularity of the continental shelf and the forcing of upwelling by trade winds make SCCUS an area of particular interest to study the dynamics of upwelling and the coupling between SPFs and environmental conditions. All these elements lead to a relatively stable cross-shore structure during the upwelling season which is characterized by: an inner-shelf band (20-30 km from the coast) warm and rich; a plume of cold upwelling over the central part (40- 70 km from the coast); and an outer area of the shelf and continental slope where upwelling can also occur but where the water is generally getting warmer, more stratified and better mixed by meso and submesoscale structures or inertia-gravity wave activity. Based on existing studies and the analyses of observations acquired these past few years, it is clear that this zonation of the system has strong implications for the organization of the ecosystem. These sub-areas have indeed different macro-nutrient ratios and also different levels of production associated with distinct planktonic assemblages. These differences result in biogeochemical cycles specific to each sub-area, although the degree of exchange between these sub-areas has yet to be quantified.

During SCOPES, we acquired new information on the plankton communities in relation to the dynamics of these different sub-regions. We sampled the region to study the plankton biodiversity in response to physical and chemical conditions across their characteristic size spectrum and through various approaches (cytometry, HPLC, taxonomy, DNA sequencing, image processing, acoustic), and we will determine the particle size spectra (~ 0.5 µm to a few millimeters for macrozooplankton ). Combined with fatty acids measurements, stoichiometry (C: N: P: Si), primary and secondary productions, all these observations will inform us about interactions within the plankton ecosystem and allow us to move forward on this key aspect of the SPF diet.

The multidisciplinary nature of the cruise, the variety of planned operations, and the number of scientific needed guided us to request FRV Thalassa to carry out this cruise. The involvement of LOPS, LOCEAN, LEMAR, LOG, MIO and IMAGO labs provides us with an ideal setting that led to the design of a singular experiment plan (including a Lagrangian component well suited to study the ecosystem), ensures the availability of a wide range of state-of-the-art equipment operated by experienced scientists, and will favor a rich post-cruise treatment/analysis phase of collected observations. The team onboard has all the expertise needed for different types of planned observations. The work will be carried out in collaboration with our Senegalese partners who work closely on these studies through researcher mobility, master/doctorate training, data sharing or research projects. The data acquired will also feed the research work of PhD students.

This cruise was coupled with the SCOPES (YAOUANK) cruise carried out on a ship.