Development and deployment of a point‐source digital inline holographic microscope for the study of plankton and particles to a depth of 6000 m

Abstract

A point‐source digital inline holographic microscope (DIHM) was designed for the imaging of particles from 50 µm to several millimeters in size. The DIHM operates autonomously without connection to external recording devices or power sources and delivers 4.2 megapixel images at a rate of approximately 7 images s −1 , each image representing a snapshot of 1.8 mL seawater. Reliance on largely off‐the‐shelf components, and simplification in its construction makes this camera system adaptable to various particle size ranges and environments, and easy‐to‐operate for nonexpert users. The DIHM produced sharp images of protists with skeletal structures (e.g., acantharians, tintinnids, dinoflagellates), mesoplankton (e.g., copepods, appendicularians, medusae), Trichodesmium colonies and marine snow particles while descending in the water column at 1 m s −1 , a typical velocity for deployments of tethered instruments and samplers from oceanographic vessels. To validate the usefulness of the new instrument in an oceanographic context, data are presented of the surface distribution of Trichodesmium spp., and of the vertical frequency distribution of fecal pellets and other particles in the deep sea. The point‐source DIHM has the potential to become a standard instrument on the CTD rosette (i.e., on the basic oceanographic instrument and sampling frame) in the future providing a permanent archival record of the water column that can be mined for specific target particles in the future.