Relevance of macrozoobenthic grazers to understand the dynamic behaviour of sediment erodibility and microphytobenthos resuspension in sunny summer conditions

The quantification of overall microphytobenthos productivity should include the export of biomass from the intertidal zone during high tides, which implies refined estimates and concepts of erosion parameters. For the first time, the export of microphytobenthic cells was assessed over an intertidal mudflat in the Marennes-Oléron Bay, France, during a complete spring/neap tide modulation. In the summer of 2008, resuspension rates of chl-a exported only reached 2.5% of the standing stock of benthic diatoms on each day. Sedimentary factors failed to explain any variation regarding bed and microphytobenthos erodibility. During the early fluff layer erosion phase, there were negative effects of grazing activities exerted by motile infauna (Peringia ulvae) on erosion fluxes of chl-a, while there was a related positive correlation with pheopigment proportion. The erosion process plays an important role in this vegetal–herbivore interaction by reinforcing the decline of the microphytobenthic biomass and provoking a catastrophic shift to mass erosion after a sequence of several days of co-occurring intense grazing by snails and chl-a decline. During mass erosion, the biofilm decline explained the variations of sediment erodibility, with a marked negative correlation between bound extracellular polymeric substance (EPS) proteins and critical threshold for bed erosion, in contrast with the commonly observed positive influence of EPS secretion on bed resistance. The complex nature of the effects of EPS by microphytobenthos must be further investigated to unravel their complex role in bioengineering sediments. The increase of protein proportion in EPS could provide specific properties related to hydrophilic features. Nevertheless, the level of grazing pressure by P. ulvae should be so intense that the top-down control must explain this original finding, since there was a positive correlation of proteins in EPS and snail density that could be related to mucus secretion (as a constitutive part of the EPS pool).