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

Abstract

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.

Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

“…They are composed of taxonomically‐diverse algae and cyanobacteria, including coralline algae, which are important non‐coral contributors to net calcification in the Great Barrier Reef (Adey 1998). In fact, some algal assemblages have been reported as being mainly composed of coralline algae (Blockley and Chapman 2008; Connell et al 2014), but calcifying algae did not dominate the dead coral surfaces in our study. Mixed algal assemblages provide habitat for an abundant and diverse community of small invertebrates (Klumpp et al 1988; Smith et al 2001; Kramer et al 2012).…”

Colonisation success of introduced oysters is driven by wave-related exposure

“…Oysters seem to be an exception to the general pattern of filter feeders. An increased occurrence of M. gigas and other oyster species under sheltered conditions has been reported, for example, from coastal marine (Ruesink 2007) and estuarine areas (Robinson et al 2005) as well as from harbour sites (Blockley and Chapman 2008). The success of oyster colonisation at wave-protected sites may be explained by (1) the relative retention of water masses in the harbours and the associated reduced drift of the larvae and (2) the reduced whiplash effect on newly settled larvae, i.e.…”

Oceanography and Marine Biology

“…Altered hydrodynamics also influence depositional processes, ultimately increasing the proportion of finer particles in superficial sediment layers (Martin et al 2005). By contrast, in highly populated harbours, intertidal infrastructure can be exposed to continuous washing by boat-generated waves (Blockley & Chapman 2008). Regular maintenance and renovation works further alter the regimes of disturbance at which artificial structures are exposed (Airoldi & Bulleri 2011).…”