Digital holography provides a unique perspective towards studying aquatic particles/organisms. The ability to sample particles in undisturbed conditions, coupled with the ability to generate 3-D spatial distributions is currently unmatched by any other technique. To leverage these advantages, field experiments with the goal of characterizing aquatic particle properties in situ, were conducted using a submersible holographic imaging system. Diverse aquatic environments were sampled over 3 separate deployments between 2014 and 2017. The areas included: (a) The Gulf of Mexico (GoM), in the vicinity of the Mississippi river plume; (b) Lake Erie; and (c) East Sound in the US Pacific Northwest. A database of more than two million different types of particles in the 10-10000 m size range, was created after processing > 100,000 holograms. Particle size distributions (PSDs) exhibited a Junge-type distribution when characterized by size grouping into logarithmically spaced bins. Particles/plankton were also classified into different groups (e.g. diatoms, copepods). Results presented will be broadly grouped into two parts: (a) PSDs at different depths within the water column during the occurrence of a Microcystis bloom at Lake Erie and individual cell counts within these colonies; and (b) Vertical structure of plankton in East Sound, specifically the presence of diatom thin layers. Finally, the rich diversity in species composition in the GoM and successful data collection towards creating a training set to implement automated classification routines will be briefly discussed.
Field experiments with the goal of characterizing aquatic particle properties, including size distributions and orientations in their natural environment, were conducted using a submersible holographic imaging system (HOLOCAM). Digital holography is a non-intrusive technique that allows particle fields to be mapped within a 3-D sampling volume at high resolution. The HOLOCAM was deployed at East Sound, a fjord in the US Pacific Northwest, and Lake Erie over three separate deployments from 2013 to 2015. A database of more than a million particles in the 100-10000 µm size range of varying shape and orientation was created after processing < 50,000 holograms. Furthermore, simultaneous, co-located acoustic Doppler velocimeter measurements of small-scale shear and turbulence structure were used to study the effects of the ambient flow field on particle orientation. Several interesting features presented themselves, with a Microcystis bloom dominating the surface layer of Lake Erie, while ‘thin layers’ of high particle concentrations dominated by colonial diatoms were seen in East Sound. Particle size distribution (PSD) slopes in the 50-250 µm size range were ~1.7-1.9, while for particles < 250 µm, the slopes were significantly higher. Clear evidence of ubiquitous particle alignment to the horizontal flow field in regions of low shear and turbulent dissipation was seen. This result, obtained under flow conditions representative of coastal and open oceans, can have significant consequences to ocean optics as random particle orientation is inherently assumed in theory and models. Preferential alignment can increase/decrease optical properties such as backscattering and attenuation relative to random distributions.
The link between suspended particle fields, particle dynamics and bulk optical properties in natural waters is poorly
known because adequate technology is lacking to fully characterize critical parameters and interactions, especially for
ephemeral bubbles and aggregates. This paper highlights the capabilities of digital holography to provide non-intrusive,
high-resolution 3-D imaging of particles and bubbles in their natural environment. As part of a NOPP project
(HOLOCAM) to commercialize an in-situ digital holographic microscope (DHM), field data with a prototype in-situ
DHM (the "Holosub") were collected in East Sound, WA. The Holosub, an in-line holography based submersible
platform, was deployed in two configurations: free-drifting mode for vertical profiling, and towed mode. In free-drifting
mode, vertical profiles of shear strain and dissipation rates, undisturbed size and spatial distributions of particles and
organisms, and the orientation of diatom chains were recorded using the holographic images. Hydrographic and optical
data, as well as discrete water samples to identify phytoplankton species were concurrently collected. In towed mode,
the size and spatial distributions of bubbles just below the surface were recorded to characterize the dissipation of a
wake generated by another ship, and compared to optical and acoustic scattering data recorded simultaneously. Tools to
extract the size distribution and concentration of bubbles from the holographic data were developed. A preliminary data
analysis indicated high concentrations of bubbles detected by all three instruments at the same locations, while
comparison of the bubble size distributions indicated some similarities in trends, as well as significant differences.
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