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Diel variations of in situ bioluminescence. Anne-Sophie Cussatlegras* IRENav - BP 600 - Lanveoc Poulmic, 29240 Brest Naval, France      *(cussat@ecole-navale.fr) Bioluminescence measurements were made in coastal Atlantic waters and in the south-west Mediterranean Sea. Light emissions were measured with an on-board bathyphotometer composed of a dark chamber and a photomultiplier tube. Results shows that temporal variations are of great importance and can be higher than spatial ones. In Mediterranean sea, a 3 days-long sampling of bioluminescence every 2 hours shows that flashes are more numerous and of greater intensity at night between 23 h and 6 h. These differences are maximum in the first 70 m depth and do not occur below 150 m. In a shallow zone of the Iroise sea, the same station was sampled during 24 h up to 80 m depth. Light emissions followed a clear day-night cycle at all depths. Considering that only organisms smaller than 2 mm penetrate the instrument and that the whole column is sampled, the results suggest that enhanced bioluminescence at night in this case is rather due to a release of dinoflagellates photoinhibition than vertical migrations of zooplankton towards the surface. [Talk: cussatle.anneso.81551]

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