Variability of Suspended Particle Properties Using Optical Measurements Within the Columbia River Estuary

Abstract

Optical properties are used to understand the spatial and temporal variability of particle properties and distribution within the Columbia River Estuary, especially in the salinity transition zone and in the estuarine turbidity maximum region. Observations of optical properties in the Columbia River Estuary are consistent with the established model that the river water brings more organic, smaller particles into the estuary, where they flocculate and settle into the salt wedge seaward of the density front. Large tidal currents resuspend mineral-rich, larger aggregates from the seabed, which accumulate at the density front. Optical proxies for particle size (beam attenuation exponent γ and backscattering exponent γbb) are compared to conventional measurements. The γ and γbb are different to the expected trend with Sauter mean diameter Ds of suspended particles from low- to medium-salinity waters (LMW). Ds increases in the LMW as does the γ derived from a WET Labs ac-9, which indicates that the particle population dominating the ac-9 is decreasing in size. The most likely explanation is that flocculation acting at LMW transfers mass preferentially from medium-sized particles to large-sized particles that are out of the size range to which the ac-9 is most sensitive; γbb shows no trend in the LMW. Since γbb is a proxy of proportion of fine particles versus large flocs, the variation of γbb may be insensitive to changes in the medium-sized particles. The overall results demonstrate that γbb is a reliable proxy for changes in particle size in a stratified environment.