Biogeochemical origins of particles obtained from the inversion of the volume scattering function and spectral absorption in coastal waters

X Zhang,W J Rhea,Y Huot,A Weidemann,D J Gray

doi:10.5194/bg-10-6029-2013

Abstract

Abstract. In the aquatic environment, particles can be broadly separated into phytoplankton (PHY), non-algal particle (NAP) and dissolved (or very small particle, VSP) fractions. Typically, absorption spectra are inverted to quantify these fractions, but volume scattering functions (VSFs) can also be used. Both absorption spectra and VSFs were used to estimate particle fractions for an experiment in the Chesapeake Bay. A complete set of water inherent optical properties was measured using a suite of commercial instruments and a prototype Multispectral Volume Scattering Meter (MVSM); the chlorophyll concentration, [Chl] was determined using the HPLC method. The total scattering coefficient measured by an ac-s and the VSF at a few backward angles measured by a HydroScat-6 and an ECO-VSF agreed with the LISST and MVSM data within 5%, thus indicating inter-instrument consistency. The size distribution and scattering parameters for PHY, NAP and VSP were inverted from measured VSFs. For the absorption inversion, the "dissolved" absorption spectra were measured for filtrate passing through a 0.2 μm filter, whereas [Chl] and NAP absorption spectra were inverted from the particulate fraction. Even though the total scattering coefficient showed no correlation with [Chl], estimates of [Chl] from the VSF-inversion agreed well with the HPLC measurements (r = 0.68, mean relative errors = −20%). The scattering associated with NAP and VSP both correlated well with the NAP and "dissolved" absorption coefficients, respectively. While NAP dominated forward, and hence total, scattering, our results also suggest that the scattering by VSP was far from negligible and dominated backscattering. Since the sizes of VSP range from 0.02 to 0.2 μm, covering (a portion of) the operationally defined "dissolved" matter, the typical assumption that colored dissolved organic matter (i.e., CDOM) does not scatter may not hold, particularly in a coastal or estuarine environment.

Highlights

Ocean ScienceThe interaction of light with aquatic particles alters the spectral and angular characteristics of the incident light field
We have provided the first experimental evidence that it is possible to invert the volume scattering functions (VSFs) to obtain the abundance of “dissolved” matter (VSP and/or colored dissolved organic matter (CDOM)), non-algal particles and phytoplankton populations, which have been traditionally derived from spectral absorption
All comparisons between the concentrations of dissolved matter, non-algal particles (NAP) and phytoplankton that have been made using absorptionbased methods and those made by inverting the VSF have showed that the estimates were well correlated

Summary

Introduction

Ocean ScienceThe interaction of light with aquatic particles alters the spectral and angular characteristics of the incident light field. Inversion approaches in ocean optics are based on our understanding of these interactions, which allow water constituent properties to be inferred froSmomlideasEuraemrtehnts of the light field. X. Zhang et al.: Inversion of the volume scattering function tween the biogeochemical and optical properties of particle populations. In contrast to apparent optical properties, which depend on the angular distribution of the incident light field, the IOPs depend only on the nature and concentration of the particles, and require fewer assumptions or ancillary measurements to solve the inverse problems. There are two fundamental inherent optical properties: the absorption coefficient (a, m−1) and the volume scattering function (VSF, m−1 sr−1). The sum of the absorption and scattering coefficients is the attenuation coefficient (c, m−1)

Methods

Results

Conclusion