New insight into particulate mineral and organic matter in coastal ocean waters through optical inversion

Abstract

Suspended particulate inorganic matter (PIM) and particulate organic matter (POM) often exhibit significant variation both spatially and temporally in coastal oceans. The size distributions and optical properties of these particles are poorly known. Utilizing a newly developed inversion technique from the measured angular scattering pattern, we were able to examine POM and PIM in terms of detailed particle size distributions (PSD) and optical volume scattering functions (VSF), gaining further insights and knowledge of particles that will greatly improve biogeochemical investigations and remote-sensing algorithms. We report the results on two extremes or end-members of possible coastal environments, sediment-laden, turbid Mobile Bay, Alabama, USA and biologically productive, clear water Monterey Bay, California, USA. The optically inferred mass concentrations of PIM and POM, when accounting for the fractal nature of suspended particles, agreed well with the respective gravimetric determinations within the analysis and inversion uncertainty. Despite intra- and inter-site variability, the inferred PSDs in both coastal regions commonly showed an apparent background population of PIM at radii <0.6–1 μm overlaid by POM of radii between 2 and 20 μm. The PSDs also saw increased contribution by PIM at radii >50 μm. The clearly distinctive PSDs between PIM and POM provide evidence to support the Risović two-component model for suspended particulates. The shape of the VSFs, i.e., the scattering phase functions, for POM are similar between the two sites (backscattering ratio ≈ 0.0015), but the PIM in Monterey Bay exhibited a higher backscattering ratio than in Mobile Bay (backscattering ratios 0.012 vs. 0.008, respectively). At both sites, the mass-specific scattering cross section values for PIM (σ[PIM]) are about 70–80% lower than σ[POM], while the mass-specific backscattering cross section values for PIM (σb[PIM]) are 10–25% greater than σb[POM].