Direct contribution of phytoplankton-sized particles to optical backscattering in the open ocean

Abstract

Abstract. Light scattering properties of oceanic particles have been suggested as an alternative index of phytoplankton biomass than chlorophyll-a concentration (chl-a), with the benefit of being less sensitive to physiological forcings (e.g., light and nutrients) that alter the intracellular pigment concentrations. The drawback of particulate scattering is that it is not unique to phytoplankton. Nevertheless, field studies have demonstrated that, to first order, the particulate beam-attenuation coefficient (cp) can track phytoplankton abundance. The relationship between cp and the particulate backscattering coefficient (bbp), a property retrievable from space, has not been fully evaluated, largely due to a lack of open-ocean field observations. Here, we present extensive data on inherent optical properties from the Equatorial Pacific surface waters and demonstrate a remarkable coherence in bbp and cp. Coincident measurements of particle size distributions (PSDs) and optical properties of size-fractionated samples indicate that this covariance is due to both the conserved nature of the PSD and a greater contribution of phytoplankton-sized particles to bbp than theoretically predicted. These findings suggest that satellite-derived bbp could provide similar information on phytoplankton biomass in the open ocean as cp.