Abstract
In situ measurements were undertaken to characterize particle fields in undisturbed oceanic environments. Simultaneous, co‐located depth profiles of particle fields and flow characteristics were recorded using a submersible holographic imaging system and an acoustic Doppler velocimeter, under different flow conditions and varying particle concentration loads, typical of those found in coastal oceans and lakes. Nearly one million particles with major axis lengths ranging from ∼14 μm to 11.6 mm, representing diverse shapes, sizes, and aspect ratios were characterized as part of this study. The particle field consisted of marine snow, detrital matter, and phytoplankton, including colonial diatoms, which sometimes formed “thin layers” of high particle abundance. Clear evidence of preferential alignment of particles was seen at all sampling stations, where the orientation probability density function (PDF) peaked at near horizontal angles and coincided with regions of low velocity shear and weak turbulent dissipation rates. Furthermore, PDF values increased with increasing particle aspect ratios, in excellent agreement with models of spheroidal particle motion in simple shear flows. To the best of our knowledge, although preferential particle orientation in the ocean has been reported in two prior cases, our findings represent the first comprehensive field study examining this phenomenon. Evidence of nonrandom particle alignment in aquatic systems has significant consequences to aquatic optics theory and remote sensing, where perfectly random particle orientation and thus isotropic symmetry in optical parameters is assumed. Ecologically, chain‐forming phytoplankton may have evolved to form large aspect ratio chains as a strategy to optimize light harvesting.
Highlights
In situ measurements were undertaken to characterize particle fields in undisturbed oceanic environments
Particle statistics, including particle size distributions (PSDs), orientation as a function of depth and aspect ratios, and associated correlations to the simultaneously quantified mean velocity shear and turbulence structure are discussed in detail
A unique instrumentation suite was deployed in a field study at East Sound, Washington, to obtain simultaneous distributions of particle fields, shear and turbulence measurements, and optical data including absorption
Summary
In situ measurements were undertaken to characterize particle fields in undisturbed oceanic environments. Oceanic particles important for biogeochemical and optical studies consist of a variety of inorganic material, organic detritus, and living organisms, that encompass a diverse range of shapes and vary in size from sub-microns (e.g., picoplankton) to a few cm (e.g., colonial diatom chains) (Lal 1977) Due to their ubiquitous presence, they influence areas of interest spanning aspects of ocean sciences as diverse as sediment transport, marine ecology, climate change, remote sensing, and ocean optics. The inherent optical properties (IOPs), which control the propagation of light in water are strongly influenced by the composition of local particulate matter (Bohren and Huffman 1983; Jonasz and Fournier 2007) Both active and passive remote sensing rely on interpreting the signature of backscattered light from the ocean to detect algal biomass, “thin layers” with high phytoplankton concentration, suspended particle mass, and to quantify biological primary productivity (Platt and Satyendranath 1988; Stramski and Kiefer 1991; Twardowski et al 2007; Churnside et al 2014). Characterizing oceanic particulates based on type, size, and shape has been the focus of a multitude of studies over the past several decades (Jackson et al 1997; Boss et al 2001; Twardowski et al 2001; Groundwater et al 2012; Twardowski et al 2012)
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