Characterization of biophysical interactions in the water column using in situ digital holography

A free-drifting submersible holography system, 'Holosub', was deployed in East Sound, Washington as part of a collaborative field study. Analysis of more than 20 000 holograms recorded during 2 slow profiling ascents (that avoided disturbance to the parameters measured) provided the size and spatial distributions of particles, the orientation of diatom chains, and the mean shear strain and turbulence dissipation rate profiles. This information was complemented by concurrently measured parameters including density, chlorophyll a concentration and optical scattering, and discrete water sampling for species characterization. Key findings were: (1) a prominent thin layer of mostly non-motile small particles, many of them chlorophyll a rich, formed in a region of near-zero shear and low dissipation rates, and high stratification; (2) multiple particle concentration peaks formed at several depths with low shear and dissipation rates; (3) Chaetoceros socialis colonies had several volume fraction peaks near the pycnocline, with the largest coinciding with the thin layer and some smaller peaks coinciding with small particle peaks; (4) zooplankton avoided many regions with elevated C. socialis and/or particle concentrations; (5) diatom chains had nearly horizontal alignment (zero angle) in the thin layer, with low angles also observed in other low shear/dissipation regions, and random orientations in several local shear/dissipation maxima regions; and (6) comparisons between particle counts from on-board cell cytometry, microscopic digital holography, and light microscopy of discrete samples showed similarities in trends but also discrepancies. In addition to these specific findings, this study demonstrates the great potential of digital holography for in situ studies of biophysical interactions and particle dynamics in oceanic flows.