Beach morphodynamic impact on a macrobenthic community along a subtidal depth gradient

The subtidal macrobenthic community of a temperate beach in southern Portugal was studied along a depth gradient (1.3 to 32 m deep), which was long enough to comprise both highly hydrodynamic and calmer areas, to assess the influence of the spatial and temporal differences in hydrodynamic impact at the seafloor on the organization of a macrobenthic community. These differences were assessed by studying the variations in wave climate, depth of closure and extension of the mixed profile. A spatial zonation was detected from highly hydrodynamic shallow depths, 7.2 m deep shorewards, to the calmer deeper areas, from 8.4 m seawards. Accordingly, in the shallowest depths, species richness and densities were lower, with the inhabiting species, mainly crustaceans and surf clams, adapted to the environmental severity. Both species richness and densities increased along the depth gradient, and the community became dominated by polychaetes, nematodes and nemerteans. Seawards, within the relict sediments (deeper than 25 m), species richness and abundance decreased again. The community patterns also changed in response to the occurrence of strongly hydrodynamic episodes (such as storms), which had a higher impact than the seasonality. It was concluded that harsh physical environmental conditions reduce species richness, both spatially and temporally. With respect to density, other parameters related to life cycle characteristics, recruitment success and biotic interactions may also have some impact.