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Abstract: Biogenic bottom features, animal burrows and biological activities interact with the hydrodynamics of the sediment–water interface to produce altered patterns of sediment erosion, transport and deposition which have consequences for large-scale geomorphologic features. It has been suggested that depending on the hydrodynamic status of the habitat, the biological activity on the bottom may have a variety of effects. In some cases, different bioturbation activities by the same organism can result in different consequences. The burrowing crab Neohelice granulata is the most important bioturbator at SW Atlantic saltmarshes and tidal plains. Because of the great variety of habitats that this species may inhabit, it is possible to compare its bioturbation effects between zones dominated by different hydrodynamic conditions. Internal marsh microhabitats, tidal creeks bottoms and basins, and open mudflats were selected as contrasting zones for the comparison on a large saltmarsh at Bahía Blanca Estuary (Argentina). Crab burrows act as passive traps of sediment in all zones, because their entrances remain open during inundation periods at high tide. Mounds are generated when crabs remove sediments from the burrows to the surface and become distinctive features in all the zones. Two different mechanisms of sediment transport utilizing mounds as sediment sources were registered. In the first one, parts of fresh mound sediments were transported when exposed to water flow during flooding and ebbing tide, with higher mound erosion where currents were higher as compared to internal marsh habitats and open mudflats. In the second mechanism, mounds exposed to atmospheric influence during low tide became desiccated and cracked forming ellipsoidal blocks, which were then transported by currents in zones of intense water flow in the saltmarsh edge. Sedimentary dynamics varied between zones; crabs were promoting trapping of sediments in the internal saltmarsh (380gm−2 day−1) and open mudflats (1.2kgm−2 day−1), but were enhancing sediment removal in the saltmarsh edge (between 10 and 500gm−2 day−1 in summer). The implication is that biologically mediated sedimentological changes could be different among microhabitats, potentially leading to contrasting geomorphologic effects within a particular ecosystem. [Copyright &y& Elsevier]

Bahía Blanca Estuary is a complex network of tidal channels, extensive tidal flats, low marshes areas and islands. The lack of a large source of sediments either from river or the shelf results in that the tidal channels of this estuary controlling the dispersal of sediment into the system. A 3-year study of morphodynamic equilibrium has been carried out in the tidal channels of Bahía Blanca Estuary with the aim to investigate the evolution of the system. Detailed changes in the spatial bathymetry linked to the observed flow and sediment processes give a clearer indication of the tidal circulation in the channels. This paper discusses morphologic variability and processes of the erosion, deposition and transport of sediment, which determines the morphological evolution of the system. Current measurements showed that channels appeared to be ebb-dominated with peak speeds of up to 1.2 m/s. Examination of current durations suggests that flood-time dominance is about 7 hr, while ebb period is 5 hr. The cross section bathymetry is usually bimodal with two channels separated by a shallow area. The deeper channel experiences ebb-directed net, while the shallower channel could be influenced by the net flood current. The net sediment transport into of these systems is shown to be governed more by the degree of tidal velocity asymmetry (ebb flow). In change the degree of time asymmetry (flood flow) governs the trends of erosion by the longer duration of tidal flood. Channel migration causes a shift from degradation in the north sector to aggradation in the south side, during adjustment. As a consequence, the estimated rate of migration is on the order of 25 m yr-1. Comparing the migration pattern between tidal and fluvial channel, we can to say that in rivers, where the water flows only one way, all morphologic features tend to migrate downstream. In tidal channels, by contrast, the erosive features appear to migrate in opposed directions to the dominant sediment transport direction. It is the erosive processes at tidal channels, which are mainly associated with flood currents dominance. [ABSTRACT FROM AUTHOR]