The Spectral Backscattering Properties of Marine Particles

The inherent and apparent optical properties of different ocean regimes are the basis for all optical remote sensing of the ocean. Ecological information derived from remote sensors therefore relies on having a detailed understanding of how particulate backscattering and absorption contribute to the bulk optical signal. The absorption characteristics of oceanic particles, e.g. phytoplankton and marine bacteria, organic detritus, and minerogenic particles, have been well characterized, and there are several ways to determine their contribution to bulk signals. In contrast, the backscattering properties of marine particles are not well understood, and indeed there is still some uncertainty regarding the dominant sources of backscattering in the ocean. Recent advances in optical instrumentation now permit laboratory and in situ examination of the spectral backscattering properties of marine particles, and we use these new tools to improve the characterization of backscattering in the ocean. We first investigated the ratio of backscattering to total scattering across a wide range of oceanic environments and particle types. The spectral dependency of the particulate backscattering ratio (backscattering/scattering in all directions) is relevant in the fields of ocean color inversion, light field modeling, and inferring particle properties from optical measurements. Aside from theoretical predictions for spherical, homogeneous particles, we have had very limited data showing the actual in situ spectral variability of the particulate backscattering ratio. Our analysis of five data sets from different ocean regimes revealed no spectral dependence of the particulate backscattering ratio within our measurement certainty. We did find however, that different particle populations demonstrated qualitative differences in the backscattering ratio. In an effort to better understand the variability that we observed in in situ backscattering, we investigated the spectral backscattering properties of thirteen species of marine phytoplankton using laboratory cultures. Theoretical analysis has shown that the backscattering coefficient and backscattering ratio may be influenced by particle size, shape, composition, and internal structure. We found species-specific relationships between backscattering and photosynthetic pigment concentration, and distinct differences between species in the backscattering ratio. These differences were related to cell size and were likely influenced by internal cell structure and composition. Of particular importance is our finding that backscattering by phytoplankton cells is higher than predicted by model studies. Finally, we used the backscattering coefficient and the backscattering ratio to aid in the discrimination of non-algal particle populations and major phytoplankton taxonomic groups in a complex coastal environment. We combined information from multiple in situ measurements, including chlorophyll concentration, hyperspectral absorption and attenuation, as well as backscattering, to discriminate and track phytoplankton groups and colored detrital matter in an optically complex, nearshore environment. We applied these approaches to interpret a time-series of hyperspectral optical observations from a coastal mooring.