Intertidal seawalls—new features of landscape in intertidal environments

Artificial structures, such as pier pilings, floating pontoons, breakwaters and seawalls are becoming common features of landscape in shallow coastal waters of urbanised areas, in some areas, replacing considerable portions of natural habitats. In Sydney Harbour (NSW, Australia), approximately 50% of the shore is composed of retaining seawalls or other built habitats. Intertidal seawalls and other similar structures provide new surfaces for colonisation by benthic organisms and therefore provide new intertidal habitats for estuarine animals and plants. They have the potential to supplement natural habitat by supporting natural assemblages, in terms of species composition and relative abundances. Alternatively, artificial structures may support only a subset of natural assemblages, provide habitat for additional species not found on nearby rocky shores, or provide habitat in which some species are consistently missing and additional species found (a combination of the previous two models). Understanding the ecological role that seawalls and other intertidal structures have in urbanised estuaries is important in planning of developments along foreshores. This study compared intertidal assemblages, at each of three different heights among seawalls and nearby horizontal and vertical rocky shores to specifically test the model that seawalls support the same assemblages of animals and plants as are found on natural rocky reefs. Hypotheses from this model were tested at three study locations by measuring abundances of animals and plants in quadrats in replicate sites on each structure. Results indicated that, at mid- and highshore levels, there were frequent differences between assemblages on natural shores and seawalls. In contrast, at lower levels, differences between assemblages on seawalls and vertical rocky reefs were small compared to differences between sites on each structure. Nevertheless, it appeared that the seawalls and rocky shores examined supported a similar suite of species and most differences were due to relative abundances and frequencies of occurrence of the species present. It is, however, premature to conclude that seawalls support similar assemblages to rocky shores until more intensive sampling has been done, specifically to search for rare species. In addition, until we have more knowledge on ecological processes in these assemblages, seawalls should not be considered as surrogates for natural shores.