The role of seawater constituents in light backscattering in the ocean

The significance of light backscattering in the ocean is wide ranging, especially in optical remote sensing. However, the complexity of natural seawater as an optical medium often obscures the measured optical signals to the point that our present-day interpretation and detailed understanding of major sources of backscattering and its variability in the ocean are uncertain and controversial. Here we review the roles played by various seawater constituents in light backscattering and we address a question of'missing' backscattering. Historically, this question has resulted from a hypothesis that under non-bloom conditions in the open ocean, phytoplankton make a significantly smaller contribution to the particulate backscattering coefficient than to the particulate (total) scattering coefficient. By discussing the backscattering properties and potential contributions of the various water constituents (colloids, bacteria, phytoplankton, biogenic detritus, minerogenic particles, bubbles), we show that due to substantial variability in water composition, different types of constituents can explain the 'missing' backscattering. Under typical non-bloom conditions in the open ocean, the small-sized non-living particles appear to be the most important because of their high abundance relative to other particle types. These particles are believed to be primarily of organic origin but an important role of minerogenic particles cannot be excluded. Still, in the very clear ocean water the backscattering by water molecules themselves can contribute as much as 80% to the total backscattering coefficient in the blue spectral region. The general scenario of the dominance of molecules and small-sized particles can, however, be readily perturbed due to changes in local conditions. For example, bubbles entrained by breaking waves can intermittently dominate the backscattering at shallow depths below the sea surface, the calcifying phytoplankton (coccoli thophores) producing calcite scales of high refractive index can dominate if present in sufficient concentration, and other plankton species can dominate during blooms. The role of phytoplankton could be generally greater than commonly assumed given the fact that real cells backscatter more light than predicted from homogeneous sphere models. In addition, high refractive index mineral particles can dominate in many coastal areas, and perhaps also in some open ocean areas during events of atmospheric dust deposition. It is likely that the different scenarios are quite widespread and frequent. Further improvements in quantitative understanding of the variability in light backscattering and its sources require an increased effort in basic research to better characterize the optical properties of the various seawater constituents and the variability in the detailed composition of seawater. Seawater is a complex optical medium containing a great variety of particle types and soluble species that vary in concentration and composition with time and location in the ocean, so ocean optics science must progress beyond the traditional overly simplified description, which has been based only on a few constituent categories defined broadly as molecular water, suspended particles (phytoplankton and non-algal particles), and dissolved organic matter. (C) 2004 Elsevier Ltd. All rights reserved.