Evidence for ubiquitous preferential particle orientation in representative oceanic shear flows

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.

Optical Modeling of Spectral Backscattering and Remote Sensing Reflectance From Emiliania huxleyi Blooms

“…We have assumed throughout this work that the detached coccoliths are randomly oriented with respect to incident light, which allows us to apply the theorem of Cauchy (1850) to calculate the average geometric cross-section via Equation 3. However, it has recently been shown that large phytoplankton cells and marine aggregates have a tendency to orient horizontally in natural waters (Nayak et al, 2018). If coccoliths, even though much smaller in size, have just a slight tendency to orient horizontally with respect to incident light, the influence on backscattering will be significant, because light is scattered off a larger surface.…”

Bio-optical Properties of Cyanobacteria Blooms in Western Lake Erie

“…Phytoplankton populations were identified and counted using traditional microscopy methods and an in situ holographic microscope system Zamankhan et al, 2016;Nayak et al, 2017). The in situ digital holographic camera system (HOLOCAM) uses in-line holographic illumination techniques (e.g., Katz and Sheng, 2010) to create images that can be further used with pattern recognition software to identify and quantify the size properties of phytoplankton/particles within a size range of ∼1 µm to ∼10 mm.…”

Phytoplankton Orientation in a Turbulent Ocean: A Microscale Perspective

“…Talapatra et al (2013), using a submersible holographic imaging system, analyzed diatom chain orientation and its relationship with mean shear rate in East Sound (WA, EEUU). More recently, also using holography, Nayak et al (2018) observed preferential cell alignment at horizontal angles coinciding with regions of low velocity shear and weak turbulent dissipation rates. A different process related with preferential cell orientation of diatoms was inferred from in situ laser transmissometry (LISST-100×) by Font-Muñoz et al (2019).…”