Assessing the Effects of Particle Size and Composition on Light Scattering Through Measurements of Size-Fractionated Seawater Samples

Daniel Koestner, Dariusz Stramski and Rick A Reynolds, University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States
Understanding the sources of light backscattering in seawater has relevance to the interpretation and development of optical applications in oceanography and is the intent of the present study. Measurements of the particulate volume scattering function, βp(ψ), at light wavelength of 532 nm, particle size distribution, PSD, and several metrics of particulate concentration and composition were made on fifteen contrasting seawater samples from nearshore and coastal oceanic environments including river estuary and offshore locations. Both βp(ψ) and PSDs were measured on unfiltered samples and particle size-fractionated samples obtained through filtration using mesh filters with pore sizes of 5 and 20 μm. We present results based on direct size-fractionated measurements and data adjusted for imperfect fractionation, which provide insights into the roles played by particle size and composition in angle-resolved light scattering produced by highly variable natural assemblages of aquatic particles. Despite intricate interplay between the effects of particle size and composition, small particles (< 5 μm in size) consistently produced a major or dominant contribution (~ 50–80%) to the particulate backscattering coefficient, bbp, in organic-dominated samples regardless of significant variations in PSD between these samples. The notable exception was a sample dominated by large-celled diatoms, which exemplifies a scenario when large particles (> 20 μm) can produce a considerable contribution (~ 40%) to bbp. We also observed a trend for inorganic-dominated samples exhibiting consistently lower contributions (~ 30–40%) of small particles to bbp.