Retrieval of the particle size distribution from satellite ocean color observations

Abstract

The particle size distribution (PSD) provides important information about pelagic ocean ecosystem structure and function. Knowledge of the PSD and its changes in time can be used to assess the contributions made by phytoplankton functional groups to primary production, particle sinking, and carbon sequestration by the ocean. However, few field measurements of the PSD have been made in the pelagic ocean, and little is known about its space‐time variation. Here, a novel bio‐optical algorithm is introduced to retrieve the parameters of a power law particle size spectrum from satellite ocean color observations. First, the particle backscattering coefficient spectrum, bbp(λ), is retrieved from monthly Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) normalized water‐leaving radiance observations following Loisel et al. (2006). Mie modeling is then used to estimate the parameters of a power law PSD (the PSD slope and the particle differential number concentration for a given reference diameter) as a function of the particulate backscattering spectrum. Algorithm uncertainties are greater when bbp(λ) slopes are low, which occurs in high‐productivity areas. Satellite‐based retrievals of PSD parameters are reasonably consistent with available field observations. As an example, the algorithm was applied to monthly SeaWiFS global imagery from August 2007. Global spatial distributions show subtropical oligotrophic gyres characterized by higher PSD slopes and smaller particle number concentrations, as compared with coastal and other high‐productivity areas. Partitioning particle number and volume concentrations into picophytoplankton‐, nanophytoplankton‐, and microphytoplankton‐sized classes indicates that the abundance of picoplankton‐sized particles is roughly constant spatially and that they dominate the particle volume concentrations in oligotrophic regions. On the other hand, abundances of microplankton‐sized particles vary over many orders of magnitude, and they contribute to volume concentration only in the highest‐productivity areas. These results are consistent with current understanding of particle dynamics of pelagic ecosystems and provide new tools for biogeochemical modeling and assessment of the global ocean ecosystem.