Benthic biodiversity evolution of subtidal mudflats of the Seine estuary in response to the sedimentary dynamic. From descriptive to functional approach.

The temporal changes of benthic macrofauna in relation with the dynamic of sediments has been studied through a temporal hierarchical approach. The three selected temporal scales corresponded to three sedimentary events: (i) the pluri-annual silting-up of the Seine estuary, (ii) the seasonal sedimentary cover, and (iii) the short-term sediment reshuffling processes induced by the macrotidal regime. First, this study displayed a strong spatial variability between the upstream and downstream parts of the Seine estuary. The downstream part was characterized by rich, highly diversified and stable benthic communities, in contrast to upstream where benthic communities were less diverse. Indeed, the upstream species composition underlined the ecotonal characteristic of this area, and its unclear and fluctuating boundaries revealed a highly temporal variability in the structure of these communities. In addition, results showed a weak influence of the sediment dynamic on the variability of benthic communities and highlighted the difficulty to estimate a relationship between macrofauna and sediment when the different sedimentary types are defined solely on particle sizes.<br />To assess the role of benthic communities on the geochemical ecosystem functioning, the contribution of these organisms to biogeochemical processes at the water-sediment interface has been studied through the estimation of irrigation fluxes. The strong correlation found between oxygen fluxes and macrofauna descriptors indicate that oxygen fluxes appeared to be a better indicator of benthic macrofauna activities than the measured ammonia fluxes. Nevertheless, depending on macrofauna descriptors (i.e. species richness, trophic guilds, bioturbation groups), three distinct « diversity-functioning » hypotheses could be proposed for the Seine estuary: (i) idiosyncrasy, (ii) rivet-pop, and (iii) diversity-stability. It is therefore difficult to establish a single law in the functioning of the macrobenthic compartment of the estuary.<br />Surprisingly, experimental studies carried out in mesocosms indicate a variable temporal response of macrofauna to sediment deposits. Indeed, whatever the nature, the thichness, the stability and residency time of the deposits were, the disturbance of the habitat significantly induced mortality in winter, whereas the survival of macrofauna was not threatened in spring-summer. The frequency of the sedimentary events undergone by the macrofauna at the mouth of the estuary might be responsible for its fitness and resilicence.