Impacts of concrete jetties on the composition of shore molluscan assemblages

Abstract

Rocky shores have undergone considerable modification worldwide through resource harvesting, disturbance and the introduction of artificial structures, among other impacts. Construction of artificial structures results in the destruction of natural habitats and the creation of novel colonisable surfaces. The ability of particular species to settle and survive on such surfaces will depend on the properties of the new substratum (Perkol-Finkel et al., 2006), the hydrodynamic regime (Lam et al., 2009) and the extent of colonisable area (Chapman, 2003), but construction of structures such as jetties often causes irreversible changes to these physical factors. For instance, jetties jutting out from the shoreline modify the hydrodynamic regime of the area in their vicinity. In particular, jetties have vertical or near-vertical sides, whilst the natural shores they replace are often gently sloping. Vertical faces tend to deflect waves, rather than enabling wave break and dissipating the wave energy, and hence molluscs on jetty walls are subjected to higher energy impacts than on sloping shores (Lam et al., 2009). Artificial structures may also retain or dissipate heat in a modified manner, such that they could be cooler than natural shorelines (Williams & Morritt, 1995). In addition, the composition of the substratum determines its surface chemistry, microtopography and durability, which in turn could affect settlement and recruitment of benthic organisms (Chapman, 2003; Spieler et al., 2001). Moreover, the surfaces of jetties and other artificial structures tend to be smoother than those on natural shores, which are naturally rugged (Chapman, 2006). Rugged surfaces present a multitude of microhabitats such as rock pools, crevices and overhangs, each with different microclimates, and molluscs that are constrained to such microhabitats may be unable to survive on smooth surfaces.