Improving the quality of intertidal habitat on seawalls in Sydney Harbour
Professor Gee Chapman
Seawalls are important features of urbanised harbours. They are built for many reasons, e.g. to prevent the collapse of soft banks, or to provide access to the shore for boats. Seawalls also add new "hard" substrata on which marine animals and plants can live. There can be a great profusion of seaweeds and attached (oysters, mussels, cunjevoi) and mobile (snails, starfish, sea urchins) animals on seawalls, especially below low tide. Although the intertidal area on seawalls is not very large, it contains a diversity of marine life.
Seawalls differ from natural rocky shores in important ways. First, around Sydney, they tend to be steep, often vertical. Second, seawalls have fewer cracks, crevices and overhangs compared to rocky shores and some habitats (e.g. rock pools) are completely missing. Local councils spend a lot of money filling in cracks and holes in seawalls - for aesthetic reasons as well as safety. This has the potential to remove complex habitats that might be very important for the survival of many intertidal organisms, particularly those that depend on holes and crevices.
This project is working in collaboration with local Councils and others who are building or repairing seawalls, to test the model that the addition of small complex structures to new areas of seawall will allow a more diverse or natural variety of plant and animal life to develop.
As one example, many seawalls in North Sydney have been repaired to be structurally sound and also to be used experimentally to test the effects of different forms of building walls on the marine life. In some parts of the wall, holes between the blocks have been filled or the grouting made flush with the sandstone blocks. In other parts of the wall, holes are left unfilled or the grouting indented, leaving "crevices" between the blocks. In another project elsewhere in the harbour, small holes and grooves are being made in the sandstone blocks themselves, again in an attempt to increase local marine diversity by increasing the complexity of their habitat.
Seawalls are features of urbanised harbours - albeit artificial features. They are here to stay and getting more numerous and widespread. It is extremely important that any effects they have on the local ecology be well understood. For us to be able to maintain and, where possible, enhance local marine biodiversity, we need research into the consequences of the ways we build urban infrastructure, such as seawalls. This is another set of projects where the Centre is collaborating with engineers and local Councils in attempts to improve the quality of marine habitats around the city.
Analysis of the results of the crevice experiment have shown that the crevices increased the diversity of algae and sessile animals, indicating that it is a cheap, but effective method of increasing intertidal diversity of some components of the assemblage. During the experiment, a heavy recruitment of mussels caused reduced cover by other animals, but this was rapidly reversed by removing the mussels on one occasion