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Plankton bioluminescence measurements were made in the Almeria-Oran frontal zone during December 1997 and January 1998. Vertical profiles of bioluminescence, chlorophyll fluorescence, temperature and salinity were obtained using a bathyphotometer associated with a CTD (conductivity-temperature-depth) probe on a rosette. The first leg of the cruise was a regular sampling along a cross section of the area. The second leg consisted of a repetitive sampling of twelve stations at each one of the eight sites located in different water masses. Hydrological data allowed a distinction from north to south of three different water masses: Mediterranean, frontal and Atlantic. The continuous sampling indicated an increased bioluminescence in the frontal zone, with high values in the surface water and numerous light emissions as deep as 200 m. Mediterranean waters are characterized by an intense bioluminescence in the first 50 m with a maximum just above the thermocline and a few bioluminescent flashes above, whereas bioluminescence in Atlantic waters is evenly distributed from the surface to 130 m depth with no strong maxima. Inter-site bioluminescence variability is much greater than intra-site, demonstrating that bioluminescence profiles clearly reflect differences between hydrological areas. Bioluminescence is correlated with fluorescence at three out of the eight sites, suggesting a relative importance of chlorophyllian bioluminescent organisms. No direct correlation with temperature and salinity has been demonstrated, however, the thermocline is nearly always accompanied by an increased bioluminescent activity.

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Abstract. Bioluminescence of plankton organisms induced by water movements has long been observed and is still under investigations because of its great complexity. In particular, the exact mechanism occurring at the level of the cell has not been yet fully understood. This work is devoted to the study of the bioluminescence of the dinoflagellates plankton species Pyrocystis noctiluca in response to mechanical stimuli generated by water flows. Several experiments were performed with different types of flows in a Couette shearing apparatus. All of them converge to the conclusion that stationary homogeneous laminar shear does not trigger massive bioluminescence, but that acceleration and shear are both necessary to stimulate together an intense bioluminescence response. The distribution of the experimental bioluminescence thresholds is finally calculated from the light emission response for the Pyrocystis noctiluca species.

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