Significant contribution of large particles to optical backscattering in the open ocean

Abstract

Abstract. The 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 (c p ) can track phytoplankton biomass. The relationship between c p and the particulate backscattering coefficient (b bp ), 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 b bp and c p . Coincident measurements of particle size distributions (PSDs) and optical properties of sizefractionated samples indicate that this covariance is due to both the conserved nature of the PSD and a greater contribution of phytoplankton-sized particles to b bp than theoretically predicted. These findings suggest that satellite-derived b bp could provide similar information on phytoplankton biomass in the open ocean as c p .

Variability in optical particle backscattering in contrasting bio‐optical oceanic regimes

“…In addition, although the role of phytoplankton is generally considered minor in determining b bp (according to Mie theory; Stramski and Kiefer 1991;Stramski et al 2004), parameterizations of b bp often derive from observations on pure phytoplankton cultures that have little to do with the real environment in terms of composition of the overall particle pool (Ahn et al 1992). New evidence indicates that phytoplankton-sized particles may contribute more to backscattering than has been previously thought (Dall'Olmo et al 2009).…”

“…Instead, what is shown here indicates that biooptical relationships might be described as a succession of distinct regimes with rapid transitions among states. The same comment probably applies to spatial distributions as well (Dall'Olmo et al 2009;Westberry et al 2010). The 'continuum view' stems from forming global relationships by aggregating data collected around the World Ocean in varied environments.…”

“…The data assembled in Fig. 6 offer one of the most comprehensive views of this relationship (see also the recent work by Dall'Olmo et al 2009 andWestberry et al 2010). The correspondence of b bp with c p is tighter than with [Chl], with lower RMSE of the two power-law fits (Fig.…”

In situ variability of mass‐specific beam attenuation and backscattering of marine particles with respect to particle size, density, and composition

“…Coverage of submicron particles is of special concern, given that under typical conditions in the open ocean and at 550 nm Mie theory predicts dominant contributions (70-90%) to b bp and significant contributions (up to 50%) to c p (Stramski and Kiefer 1991). Fractionation experiments in open ocean and coastal waters, however, suggest contributions from submicron particles to b bp of only 10-30% and a much greater importance of particles larger than 3 mm of 53% 6 7% (at 470 nm, Dall'Olmo et al 2009) and about 70% (at 440 nm, Roesler and Boss 2008), respectively.…”

An optical model for deriving the spectral particulate backscattering coefficients in oceanic waters

“…The particulate backscattering depends at first order on the particulate concentration, and at second order on the chemical composition (index of refraction), particle size distribution (PSD), and structure of the bulk particulate matter (Stramski et al, 2004). The recent field measurements showed that particles smaller than three microns contribute to about 50 % of b bp , in contrast to the results based on the Mie scattering theory (Antoine et el., 2011;Dall'Olmo et al, 2009;Loisel et al, 2011;Morel and Ahn, 1991;Stramski and Kiefer, 1991). The fraction of b bp affects the ocean colour, determined by the relative contribution of living and nonliving particles (such as inorganic minerals, phytoplankton, and organic detritus) (Gordon et al, 1975).…”