Changes to tidal flushing in the mangrove forests of Bicentennial Park
Professor Gee Chapman
Mangrove forests in urbanized estuaries like Port Jackson are very disturbed. Built structures, such as seawalls, narrow channels and bund walls can interrupt flow of water into these habitats. In addition to carrying nutrients, water carries small larvae of the animals that live on or in the sediments of the forest. These animals are very important components of biodiversity of mangrove forests. The flow of water thus "connects" populations of these animals among different patches of forest. Interrupting flow may degrade the quality of the habitat which may, in turn, decrease the diversity of animals that can live in it. In addition, it may decrease diversity directly by reducing the numbers of larvae that arrive from other places.
Bicentennial Park is a public park in Homebush Bay, containing large areas of mangroves and a very successful Education Centre. In 1996, as part of a series of projects done around the Olympic site, we examined the diversity of animals in patches of forest in Bicentennial Park. Other sites included Roseville Bridge and Boronia Park, which were considered less polluted or disturbed. We showed that, as long as patches of the forest in Bicentennial Park and other nearby sites were flushed with tidal water each day, the numbers and types of animals found were similar to those in less disturbed areas. Patches of forest that were cut off from tidal flushing by steep banks and bund walls had fewer and a different set of animals
This gave rise to a very exciting collaboration between the Centre and the Bicentennial Park Trust and managers. The managers of the park arranged to have new channels built to allow tidal waters to flood some areas that were previously cut off from tidal flushing. These are considered "Restored" because it is expected that the change in tidal flushing will improve the quality of these habitats and/or bring more larvae into the sites. Other unflushed areas have been left alone, thereby providing Control sites (i.e. left in a degraded condition). There are also Reference sites (i.e. areas that are already flushed by the tide each day). These represent conditions we would expect in the Restored sites if changes to tidal flushing have the desired effect. So, there are Restored, Control and Reference sites. Because it took quite a while between deciding to do the project and the channels being built, we also had the opportunity to collect quite a large amount of data about the diversity and abundances of animals in these different areas before the channels were built.
This design allows an unambiguous test of the effectiveness of changing tidal flushing on biodiversity of disturbed mangrove forests in Port Jackson. It incorporates data collected a number of times before and after the changes were made and the three sets of sites necessary to separate effects of restoration from natural changes. In addition to being scientifically important, this project is another successful collaboration between the Centre, which is responsible for the scientific aspects of the project and a managerial body responsible for important intertidal habitats around Sydney.
It is too soon to determine whether changing tidal flushing has had the predicted effect, but early signs are encouraging. The numbers and types of invertebrates in the sites being restored started to change within a few weeks and changes in the early stages were in the predicted directions.