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

Optical holography provides non-intrusive, non-destructive recording of liv ing, motile organisms and particles in their natural environment. High-reso lution, three-dimensionality and 'optical sectioning' of the image allow id entification of species and enumeration of size, orientation and spatial di stribution. We have developed a submersible holographic camera to simultane ously expose in-line and, uniquely, off-axis holograms to record organisms from a few micrometres upwards (in-line), in concentrations down to a few t ens of particles per cubic metre (off-axis). It can record up to 50000 cm(3 ) of the water column in a single off-axis hologram and 3500 cm(3) 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.