Simultaneous in-line and off-axis subsea holographic recording of plankton and other marine particles

Optical holography provides non-intrusive, non-destructive recording of living, motile organisms and particles in their natural environment. High-resolution, three-dimensionality and `optical sectioning' of the image allow identification of species and enumeration of size, orientation and spatial distribution. We have developed a submersible holographic camera to simultaneously expose in-line and, uniquely, off-axis holograms to record organisms from a few micrometres upwards (in-line), in concentrations down to a few tens of particles per cubic metre (off-axis). It can record up to 50 000 cm3 of the water column in a single off-axis hologram and 3500 cm3 in an in-line hologram. We describe the first sea deployment of the camera and the successful recording of holograms of plankton and marine snow to a depth of 100 m water.