Inherent optical properties and particle characteristics of the sea-surface microlayer

Abstract

The sea-surface microlayer (SML) is known to have physical, chemical, and biological properties that are distinctly different from the underlying subsurface water (USW). However, only a few studies in the past reported on measurements of the optical properties of the SML and were limited to light absorption. In this study we present results for the main inherent optical properties (IOPs), the spectral absorption coefficients and the volume scattering function, as well as particle size distribution (PSD), from measurements of the SML and USW in contrasting ocean environments with near-surface chlorophyll-a concentration ranging from 0.06 mg m−3 in waters off Hawaiian Islands to 1 mg m−3 in the Santa Barbara Channel. Our observations also included prominent surface slick conditions associated with a dense bloom of Trichodesmium. Significant and highly variable enhancements of the optical properties and particle concentration, including significant changes in the shape of PSD, were observed in the SML compared with USW at all investigated sites. In clear oligotrophic waters the total concentration of particles larger than 0.7 μm in size was enriched in the SML more than 8-fold. In all examined waters the enrichment was consistently higher for larger particles (>10 μm) than smaller particles. The highest enhancement of light absorption coefficients, >100-fold for particulate absorption and >200-fold for phytoplankton absorption in the near-UV and red spectral regions, was observed during the Trichodesmium bloom. In clear oligotrophic waters the particulate absorption coefficient was enhanced by as much as 45-fold in the green spectral region and the non-algal component exhibited consistently higher enhancement than phytoplankton component across the examined spectrum. In contrast to absorption, the volume scattering function was enhanced more in clear oligotrophic waters (>15-fold at scattering angles ψ between about 65° and 80°) than in the situation of Trichodesmium bloom. With the exception of Trichodesmium bloom, we consistently observed significantly lower values of the degree of linear polarization of light scattered by suspended particles and whole seawater samples (by as much as 30% in the vicinity of ψ = 90°) in the SML compared with USW. This result indicates that the SML can have important effect on the state of polarization of downwelling light entering the ocean and upwelling light leaving the ocean across the air-sea interface. The determinations of IOPs in the SML can extend the capabilities for characterizing constituents of microlayer and provide useful information for radiative transfer and remote-sensing related studies.