Assessing the effects of particle size and composition on light scattering through measurements of size-fractionated seawater samples

Measurements of the particulate volume scattering function, beta(p)(psi), at light wavelength of 532 nm, particle size distribution, PSD, and several metrics of particulate concentration and composition were made on eight contrasting seawater samples from nearshore and coastal oceanic environments including river estuary and offshore locations. Both beta(p)(psi) and PSDs were measured on original (unfiltered) samples and particle size-fractionated samples obtained through filtration using mesh filters with pore sizes of 5 and 20 mu m. 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 mu m in size) consistently produced a major or dominant contribution (similar to 50-80%) to the particulate backscattering coefficient, b(bp), in organic, either phytoplankton or nonalgal, dominated samples regardless of significant variations in PSD between these samples. The notable exception was a sample dominated by large-celled diatoms from microphytoplankton size range, which exemplifies a scenario when large particles (> 20 mu m) can produce a considerable contribution (similar to 40%) to b(bp). We also observed a trend for inorganic-dominated samples exhibiting consistently lower contributions (similar to 30-40%) of small particles to b(bp). The particle size-based budget for the particulate scattering coefficient, b(p), indicates a significant decrease in the role of small particles accompanied by an increase in the role of larger particles compared to the b(bp) budget.