Inventory of estuarine vegetation in Botany Bay, with special reference to change in the distribution of seagrass
Seagrasses play an important role in estuarine ecosystems by stabilising sediments and providing food and shelter to a wide variety of organisms. It is therefore important to monitor changes in the distribution of seagrass and identify the natural processes and human impacts involved so that appropriate management strategies can be adopted.
Aerial photographs taken of Botany Bay in 1995 at a scale of 1:16 000 were analysed in a Geographic Information System (GIS) to produce maps of estuarine vegetation for the whole of the bay. Presumptive boundaries of monospecific and mixed seagrass beds as well as mangrove and saltmarsh were delineated in the laboratory and confirmed in the field. The area of each vegetation type as them calculated and compared to the results of studies previously undertaken within the bay.
The overall area of seagrass in Botany Bay is larger than that calculated by West et al (1985) for 1981 and by Larkum and West (1990) for 1984-1985. It should be noted that different methodologies and classification schemes were used these two studies. As well as an increase in area, small scale changes in distribution were observed. For example, between the parallel runway and Port Botany the area of seagrass appears to be decreased substantially, whereas more seagrass is present long Lady Robinsons Beach. In contrast, along the foreshore of Silver Beach, Towra Point and part of Quibray Bay, the area of P. australis seagrass appears to have changed little since the investigations made in 1992 (The Ecology Laboratory 1995).
Depending on management needs, additional field investigations are necessary to describe the species composition of the mixed beds, bed densities and to determine the nature of successional sequences and their ecological significance.
The management of Botany Bay continues to be a topical issue for the community, and local, state and commonwealth governments (Botany Bay Mayoral Taskforce 1998). Recently, Wetlands (Australia) dedicated an entire issue to the Botany Bay Symposium held in September 1995. Strong recommendations about management of the environment (Hatfield 1997) and planning within the catchment (Chanell 1997) were made. One of the key issues addressed was the need to preserve the ecosystems of high conservation value in an area of intensive industrial use and dense residential development (Adam 1997). Our project addressed that need through three objectives:
- To create a digital map of the present estuarine vegetation of Botany Bay
- To distribute this information in the context of management needs for current development proposals
- To update the history of seagrass distribution in Botany Bay by
- Extending the work of Larkum and West (1990) with a calculation of the total amount of seagrass present in 1995, the most current year for which suitable aerial photographs were found
- Following the precedent of Larkum and West (1990) and examining change in area of seagrass on each of the northern and southern shores of the bay,
- Investigating the change of seagrass along critical parts of the northern and southern shorelines