Response of intertidal benthic macrofauna to migrating megaripples and hydrodynamics

ABSTRACT: Migrating flow-transverse mesoscale intertidal bedforms (megaripples or dunes) may pose disturbance but may also provide heterogeneity in microhabitats to the inhabiting fauna. We investigated how the macrofauna community responds to these migrating intertidal bedforms, based on surveys in the Westerschelde estuary. Considering the entire estuary, low- and high-energy intertidal areas differed in macrofauna, and high-energy flat areas had a macrofauna community intermediate to those in low-energy flat areas and high-energy areas with megaripples. In megaripple areas on a polyhaline and a mesohaline tidal flat, the macrofauna community depended on hydrodynamics, morphodynamics, grain size, elevation and steepness of the megaripples. The relative importance of the environmental variables for structuring the macrofauna community differed for each site. Within the megaripples, conditions on crests, at flanks and in troughs were distinctly different: crests had more chl a and coarser sediment than troughs, while flanks had intermediate levels; troughs were higher in carbon and mud content than flanks and crests. The microhabitats supported a different macrofauna community but with a very large overlap in species. Troughs typically had higher species richness, while crests had higher densities of, in particular, (mobile) surface deposit feeders. Part of the macrofauna could benefit from the habitat heterogeneity within the megaripples, but the effect was relatively small. The distribution of macrofauna in the intertidal zone was particularly regulated by overall current velocities and bedform morphodynamics.