PHOTOSYNTHETIC CHARACTERISTICS OF MICROPHYTOBENTHOS IN MARENNES-OLERON BAY, FRANCE - PRELIMINARY-RESULTS

The photosynthetic characteristics of microphytobenthos inhabiting an intertidal mudflat in Marennes-Oleron Bay (France) have been studied o n samples collected at the air-sediment interface during low tide, Pho tosynthesis-Irradiance (P-I) parameters were estimated at different ti mes of low tide in March and in July. In summer, two different sedimen t depths were also compared. The results point out different kinds of changes in the photophysiological characteristics of microphytobenthos , according to the time scale under consideration. At a seasonal scale , there was an increase of I-k from March to July, thus indicating pho toacclimation of microphytobenthos. At an hourly scale there was no ch ange in I-k. In March, there was no change in P-I parameters, but ther e was a significant change of alpha(B) and P-m(B) in July with the hig hest values at T-3 (middle of low tide) at the surface of the sediment . It is speculated that this change could be due to an endogenous phot osynthetic rhythm. There was no difference in the photosynthetic chara cteristics of microphytobenthos from the photic and the aphotic layers , suggesting sediment mixing by physical and/or biological processes ( or microalgal migration) at a rate compatible with the persistence of the photosynthetic response in the aphotic layer. Although benthic mic roalgae did experience photoinhibition in experimental conditions, the re was no indication of a photoinhibitory effect in situ. This suggest s that microalgae were not exposed to strong illumination for extended periods of time during low tide in spite of the direct exposure of th e mudflat to incident irradiance, We suggest that the strong attenuati on of light within the sediment, self-shading within algal mats, the m otility of microalgae, and bioturbation can explain this apparent para dox.