In-situ holography microscopy of plankton and particles over the continental shelf of Senegal

We present the first results obtained by a newly developed submersible digital holography microscope (DHM), Holoflow@Sea, to enable continuous in-situ monitoring of ocean or fresh water bodies in a less intrusive manner. The microscope features an off-axis configuration with reduced-coherence illumination. The optics is designed to image plankton and particles in the size range 2 μm-200μm within a water volume of 1 mm × 1 mm × 2 mm. The prototype was successfully deployed for the first time over the continental shelf of Senegal during a fisheries survey carried out in March 2013. The objective was to combine several laboratory techniques used for plankton and particle identification (high-performance liquid chromatography, flow cytometry and optical microscopy) on discrete collected samples with DHM images taken in situ at locations with different environmental conditions. Hologram data were acquired inside an upwelling cell, i.e., new water, and along the coast, i.e., old water, as well as off the upwelling cell at the continental shelf border. Preliminary results of holographic reconstruction are encouraging, with the distinctive morphology of some phytoplankton species allowing easy identification to genera level. Challenges are recognised with the identification of small spheroid organisms. Analyses are underway to allow comparison with traditional methods of plankton identification and evaluate the benefit of additional in-situ observations obtained by holography microscopy. The preliminary results already demonstrate the potential of DHM for in-situ studies of plankton and particles.