Downscaling of Physat-Med to detect phytoplankton groups in the oligotrophic waters of Corsica (NW Mediterranean Sea)

The aim of two European Union’s ambitious directives (Water Framework Directive and Marine Strategy Framework Directive) is to protect more effectively the marine environment across Europe. Both directives include phytoplankton biomass and composition as indicators of ecological and eutrophication status of upper water masses. In the Northwestern Mediterranean Sea, generally characterized by clear sky conditions, satellite-derived chlorophyll a is largely used as a proxy for surface phytoplankton biomass. More recently, several remote sensing algorithms have been developed to characterize the dominant size classes or phytoplankton functional groups. A regionalized version of the PHYSAT method (Alvain et al. 08, 12) has been developed by Navarro et al. (14, 17) to characterize the peculiarities of phytoplankton assemblages in this basin. We present an adaptation of the PHYSAT-Med method to small scale Mediterranean coastal areas (downscaling). The adapted method, referred as OC5-PHYSAT-Med, was validated with a pluriannual data set of in situ pigment concentrations measured at two sites in the oligotrophic coastal waters of Corsica (site 1 : Calvi, Ligurian Sea, 5 years of weekly data; site 2 : Bastia, Tyrrhenian Sea, 4 years of monthly data). The algorithm reproduces very satisfactorily the weekly and monthly variations of the total phytoplankton biomass (Tchl a ) and of the dominant phytoplankton groups (prymnesiophytes, cyanobacteria, diatoms), identified by their pigment signature. Quantification of « other eucaryotes » need to be improved. This suggests that OC5 PHYSAT-Med will increase our capacities to monitor phytoplankton in the Mediterranean Sea and to track potential signs of changes.