Gildas Roudaut

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.

A new multifrequency acoustic method for the discrimination of biotic components in pelagic ecosystems: Application in a high diversity tropical ecosystem off Northeast Brazil

Underwater acoustics have an unrealized potential for multicomponent ecosystem characterization. A variety of methods (e.g., backscattering differences, clustering, Gaussian model) are currently used for multifrequency classification. In this study we propose and implement a new method based on the distribution of scatters on multifrequency spatial planes. Our approach is based on both the sum and the difference of backscattered energy of up to five bi-frequency pairs. This method was developed using four frequencies (38, 70, 120 and 200 kHz) data collected in the frame of the ABRAÇOS (Acoustics along the BRAzilian COaSt) project, around Archipelago of Fernando de Noronha and Atoll das Rocas; an area characterized by low productivity but high biodiversity. By applying the method we could discriminated six groups: Fish like, two types of High Resonant at 38 kHz associated to gelatinous, Fluid Like associated to crustacean macrozooplankton, High Resonant at 70 kHz associated to pelagic algae, and a group of Unclassified echoes. The algorithm was validated using in situ sampling performed by mesopelagic trawl and zooplankton nets. Results are coherent in terms of distribution pattern of each group allows for a 3D representation of organisms distribution around the oceanic islands. Among other, we reveal the importance of gelatinous are the dominant group in the vicinity of the islands where they form a dense layer above the thermocline. These results open new perspectives to improve knowledge on the patterns of distribution and the interaction of a variety of functional groups in tropical and other systems.