Exposure of seawalls to waves within an urban estuary: effects on intertidal assemblages

Despite being largely protected from sea swell by headlands, shores within Sydney Harbour, New South Wales, Australia are subjected to considerable wave action from wind waves and the numerous recreational and commercial boats and ships using the waterways. The aim of this study was to test whether assemblages on artificial shores, that is, seawalls, either exposed to or sheltered from waves would show similar patterns to those published for natural shores exposed to or sheltered from sea swell. Specifically, the hypotheses were tested that sheltered seawalls would have greater cover of algae, whereas exposed seawalls would have greater cover of sessile invertebrates and greater abundance of mobile invertebrates. It was found that encrusting algae had greater cover on sheltered seawalls, whereas cover of turfing algae was greater on exposed walls. Sessile filter-feeders generally had greater cover on exposed seawalls, although a dominant space occupier, the oyster Saccostrea glomerata, showed either no difference, or greater cover on sheltered seawalls among different locations. The pattern for the dominant grazer on seawalls, the pulmonate limpet Siphonaria denticulata showed the predicted pattern low on the shore, but the opposite pattern at mid-tidal levels. Despite some inconsistencies the results were similar to those predicted from studies on natural shores, showing that assemblages on urban structures respond to wave action in a way that is predictable from studies on natural shores. Experiments were carried out where assemblages were transplanted to sites with differing exposure to waves to determine whether differences in recruitment or post-recruitment mortality were responsible for patterns of difference. We found that some sessile invertebrates did not survive when transplanted to sheltered seawalls, suggesting that difference in survival of organisms was responsible for observed patterns. This was not, however, found when the experiment was repeated at other locations. It is likely that processes affecting the assemblages are temporally and spatially very variable, or that more than one process interacts to cause observed patterns. Identifying such complexity requires rigorously structured sampling designs and appropriate manipulative experiments.