Variability in bio-optical relationships from a global Bio-Argo database.

Characterizing phytoplankton distribution and dynamics requires in situ observations over a broad range of space and time scales. In addition to classic measurements of temperature and salinity, Bio-Argo profiling floats are capable of measuring at high frequency and at the basin scale a suite of bio-optical properties such as the chlorophyll fluorescence, a proxy of chlorophyll concentration (Chl), and the particulate backscattering coefficient (b_bp), a proxy of the stock of particulate organic carbon stock (POC). This study focuses on the analysis of more than 5715 profiles collected by 44 Bio-Argo floats in various hydrological and trophic conditions of the global open ocean. Our objective is two-fold: (i) we explore the relationships between the measured bio-optical properties at the regional, seasonal and vertical scales; (ii) we interpret the observed dynamics from a biogeochemical perspective. Our results indicate that, in the seasonally productive systems (i.e. Western Mediterranean Basin and the subpolar gyres), b_bp and Chl covary, suggesting that changes are mostly driven by those of phytoplankton carbon. In contrast, in the mixed layer of the permanently oligotrophic systems (i.e. in the Eastern Mediterranean Basin and the subtropical gyres), we did not observe any covariation between both bio-optical properties in the mixed layer. Near the deep chlorophyll maximum (DCM) in subtropical ecosystems, we distinguish situations of actual increase in phytoplankton biomass (i.e. concomitant increase in Chl and b_bp) and others of photoacclimatation (i.e. decoupling between Chl and b_bp). Hence, the DCM could actually represent a significant source of phytoplankton carbon biomass that is currently ignored by satellite ocean color sensors that only probe the ocean surface.