Microstructure of microphytobenthic biofilm and its spatio-temporal dynamics in an intertidal mudflat (Aiguillon Bay, France)

The spatio-temporal dynamics of the microphytobenthic biofilm was analysed at microscale over a complete day-time emersion period in an intertidal mudflat. The structures of the biofilm were monitored by analysing the variability of the epipelic diatom cover in the horizontal dimension and the kinetics of biomass dispersion in the vertical dimension. Colonisation of the sediment surface by diatoms was rapid (within 15 min); 75% of the biomass contained in the top 1 mm was concentrated in the upper 200 mum, but the cover never reached 100%. This biomass had a higher chlorophyll a/pheopigment ratio than deeper in the sediment, suggesting a better physiological state and a higher photosynthetic potential. The dynamics of the biomass in the upper 200 mum turned out to be independent of that deeper in the sediment, supporting the concept of an independent surface compartment, mainly responsible for primary production. In the second part of emersion, the horizontal dispersion of the microphytobenthic biofilm was concomitant with the onset of downward migration. The process was slow, showing that the diatoms were not phased in their downward movement. The migration process appeared to be asymmetrical: rapid upward migration and biofilm formation versus slow downward movement and dispersion of the biofilm. To take into account these structural characteristics of the biofilm and to simulate its dynamics, we developed a new mathematical model that provides simulations consistent with our observations. We have also shown for the first time that the constitution of the biofilm at the sediment surface (0 to 200 mum) is connected to an increase of biomass in the top 1 cm.