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 and backscattering exponent bb ) are compared to conventional measurements. The and bb are different to the expected trend with Sauter mean diameter D s of suspended particles from low-to medium-salinity waters (LMW). D s increases in the LMW as does the 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; bb shows no trend in the LMW. Since bb is a proxy of proportion of fine particles versus large flocs, the variation of bb may be insensitive to changes in the medium-sized particles. The overall results demonstrate that bb is a reliable proxy for changes in particle size in a stratified environment.Plain Language Summary Suspended particles delivered by rivers affect water quality in estuaries, so effective and economical monitoring of the variability of particles is particularly important in environmental management. Direct measurements of suspended particle size, concentration, and composition from water samples require substantial time and labor, which has driven efforts to develop indirect, yet accurate, alternative measurements of particle properties. In this study, we used optical instruments to measure particle variability within the Columbia River Estuary. The observations demonstrate that the river water delivered smaller, organic particles to the estuary, where they aggregated with one another and deposited to the seafloor. Large tidal currents resuspended mineral-rich, larger aggregates from the seafloor, and these aggregates accumulated in the region where river waters encountered coastal ocean waters that flowed into the estuary. We observed that optical measurements related to particle size were different from the expected trend in the transition zone between fresh, river water and salty, coastal water. This unexpected trend requires more study, but we propose that it was caused by preferential aggregation of medium-sized organic-rich particles into large particles. This process would affect the processing of carbon, nutrients, and contaminants in estuaries.

Advantages and Limitations to the Use of Optical Measurements to Study Sediment Properties

“…The breakage of flocs near the bed decreased bulk estimated settling velocities from the profiles by transferring mass near the bed from faster-sinking large flocs (~1 mm s −1 ) into slower-sinking smaller flocs or single grains. Overall, however, the extensive packaging of small particles into large flocs during spring tides was associated with small values of γ bbp [15]. During the passage of Maria, we hypothesize that higher bottom shear stresses associated with the larger swell caused resuspension of fine sand typical of the site (D =~125 µm; [50]) and caused a greater degree of floc breakup.…”

“…These are rarely in suspension and, when they are in suspension, concentrations may saturate optical instruments. Optical proxies for size information include size distributions inverted from measurements of near forward scattering at several angles [12], the exponents of power-law fits of the particulate attenuation or backscattering spectrum [13][14][15] (but see [16]), and the fluctuation in optical signals, which can be used to obtain the average size of suspended particles [17]. Images of particles have also been used to derive size distributions, particularly of larger flocs and aggregates (e.g., [1,3]).…”

“…Resuspended sand also can account for the broadened peak in the particle size distributions. Greater floc breakup can account for smaller values of γ cp during passage of Maria, because particle mass was transferred out of large flocs that were not sensed by the AC-9 into smaller flocs or single grains that were [15]. It can also account for large values of γ bbp [15].…”

“…Lower values are associated with larger size averaged particles. γ cp is biased towards the smaller (0.5 to 10 µm) particles in the population [9], and γ bbp may be more sensitive to larger particles [15].…”

Correlation of Remotely Sensed Surface Reflectance With Forcing Variables in Six Different Estuaries

“…Remote sensing is capable of providing a synoptic view of surface and near‐surface water conditions at a large spatial scale over the coastal area. However, the important estuarine processes occur in the subsurface layer, such as vertical mixing between freshwater and saline water (Wolanski, ), sediment resuspension, trapping, and flocculation (Burchard et al, ; Tao et al, ; Wolanski, ). Hudson, Talke, and Jay () quantitatively explored the link between physical processes and surface reflectance measured by the MODIS (Moderate Resolution Imaging Spectroradiometer).…”