Ecoregions in the Mediterranean Sea through the reanalysis of phytoplankton functional types and carbon fluxes

We show that the simulation of the phytoplankton community structure and carbon fluxes in marine ecosystems can be improved by assimilating phytoplankton functional types (PFTs) derived from satellite observations. A regional ocean-colour product of four PFTs (diatoms, dinoflagellates, nanophytoplankton, picophytoplankton) was assimilated into a coupled physical-biogeochemical model of the Mediterranean Sea (POLCOMS-ERSEM) by using a 100-member Ensemble Kalman filter, in a reanalysis simulation for years 1998-2014. The reanalysis outperformed the reference simulation in representing the assimilated ocean-colour PFT fractions to total chlorophyll, although the skill for the ocean-colour PFT concentrations was not improved significantly. The reanalysis did not impact noticeably the reference simulation of not-assimilated in situ observations, with the exception of a slight bias reduction for the situ PFT concentrations, and a deterioration of the phosphate simulation. We found that the Mediterranean Sea can be subdivided in three PFT-based ecoregions, founded on the spatial variability of the PFT fraction dominance or relevance. Picophytoplankton dominates the largest part of open-ocean waters; microphytoplankton dominates in a few, highly-productive coastal spots near large-river mouths; nanophytoplankton is relevant in intermediate-productive coastal and Atlantic-influenced waters. The trophic and carbon sedimentation efficiencies are highest in the microphytoplankton ecoregion and lowest in the picophytoplankton and nanophytoplankton ecoregions. The reanalysis and regionalization offer new perspectives on the variability of the structure and functioning of the phytoplankton community and related biogeochemical fluxes, with foreseeable applications in Blue Growth of the Mediterranean Sea.