Spectral dependency of optical backscattering by marine particles from satellite remote sensing of the global ocean

Knowledge of the relative proportion between small‐sized and larger particles in the surface ocean is essential to understand the ocean ecology and biogeochemistry, including particle dynamics and carbon cycling. We show that this information may be assessed qualitatively from satellite observations of ocean color. Such capability is based on the estimation of spectral dependence, γ, of particulate backscattering coefficient, bbp, which is sensitive to particle size distribution. Our results obtained from satellite observations of the global ocean are supported by in situ measurements, and they demonstrate a general decrease of the spectral slope γ from oligotrophic to eutrophic regimes, although significant regional differences are observed in the relationship between γ and the chlorophyll a concentration, Chl. To first approximation, such a decrease in γ is expected to be accompanied by an increased role of larger particles. This is consistent with our field data that show relatively high concentrations of submicron particles in very clear oceanic waters. Different seasonal patterns are also observed depending on the oceanic regions. The seasonal amplitude of γ is generally higher than that of Chl and bbp in equatorial and tropical regions, and it is much lower at temperate latitudes. These spatio‐temporal patterns are interpreted in terms of processes that modify the composition of particulate assemblages and physiology of phytoplankton in response to environmental forcing. The changes in γ are clearly related to variations in the mixed layer depth and photosynthetic available radiation.