Devenir des bactéries dans les réseaux trophiques des vasières intertidales : le cas de brouage (Baie de Marennes-Oléron)

Bacteria are highly abundant and productive in intertidal mudflat sediments. By analogy with pelagic system, it has been suggested that bacteria play an important role in benthic food web. Due to technical difficulties, numbers of studies dealing with benthic bacterivory are not numerous and give conflicting results. A method using community of 15N enriched bacteria as tracer was developed in order to measure grazing of meiofauna and macrofauna. Since enriched bacteria present characteristics of size, activity and diversity close to natural ones, bias due to selective bacterial ingestion by grazers are limited. Due to tidal cycles, intertidal mudflat constitutes highly variable environment at short time scale. Since those variations may influence the feeding behavior of grazers, grazing experiments were performed in order to evaluate effects of abiotic (temperature, salinity and luminosity) and biotic (bacterial and algal abundances) factors on rates of bacterial uptake. Studied grazers were the mudsnail Hydrobia ulvae, the foraminifera Ammonia tepida and a nematode community from Brouage mudflat (Marennes- Oléron Bay). Except A. tepida, grazers were poorly affected by abiotic factors. This result suggests that grazers are adapted to feed in the highly variable environment constituted by the air/sediment interface where algal biofilm appears during diurnal ebb. Algae were ingested at a higher rate than bacteria suggesting a lower importance of bacteria in grazer diets. In order to measure annual variations of bacterial ingestion, regular grazing experiments were performed in situ in Brouage mudflat. Nematode community and A. tepida presented a bacterivory negatively correlated with bacterial abundance suggesting that bacteria represent an alternative resource that is grazed when algal abundances are low. The grazing of studied grazer never represented more than 6% of bacterial production. As a result, grazing constitutes a limited fate of benthic bacteria in the Brouage mudflat.