Spatiotemporal bioturbation patterns in a tidal freshwater marsh

Bioturbation has been hypothesized to exhibit different forms of sediment mixing in aquatic systems, but few in situ tests have been conducted in estuaries, and anyone along a flooding gradient which is the main feature characterizing intertidal areas. The relationships between bioturbation and macroinvertebrate communities were studied as part of a restoration project in the tidal freshwater zone of the Schelde estuary, and highlighted specific sediment mixing patterns along a tidal gradient. Three permanent sites, evenly distributed along the flooding gradient, were monitored over a period of one year. Tidal influence engendered a clear gradient opposing newly-established aquatic communities (low elevation and strong disturbance) to remnant terrestrial communities (high elevation and low disturbance). Different bioturbative modes were identified along this gradient. Biodiffusion (random spreading of sediment particles) was the dominant mode at high and mid elevations. Low elevation was characterized by bioadvection (vertical movement of sediment particles) and higher bioturbative intensities. Maximum bioturbative intensities were observed in summer. This is the first bioturbation study, conducted along a flooding gradient, and which characterizes the bioturbative modes and intensities among tidal habitats and confirms the key role of disturbance. These findings underline the significance of the multiplicity of bioturbation modes in estuarine habitats, and the potential implications in estuarine biogeochemistry in general.