Photo synthetic characteristics of microphytobenthos in Marennes-Oléron Bay, France: Preliminary results

Abstract

The photosynthetic characteristics of microphytobenthos inhabiting an intertidal mudflat in Marennes-Oléron Bay (France) have been studied on samples collected at the air-sediment interface during low tide. Photosynthesis-Irradiance (P-I) parameters were estimated at different times of low tide in March and in July. In summer, two different sediment 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 photoacclimation of microphytobenthos. At an hourly scale there was no change in I k. In March, there was no change in P-I parameters, but there was a significant change of α B and P m B in July with the highest 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 photosynthetic rhythm. There was no difference in the photosynthetic characteristics 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 microalgae did experience photoinhibition in experimental conditions, there was no indication of a photoinhibitory effect in situ. This suggests that microalgae were not exposed to strong illumination for extended periods of time during low tide in spite of the direct exposure of the mudflat to incident irradiance. We suggest that the strong attenuation of light within the sediment, selfshading within algal mats, the motility of microalgae, and bioturbation can explain this apparent paradox.