Year

2001

Degree Name

Doctor of Philosophy

Department

Faculty of Science

Abstract

Posidonia australis Hook f. is the dominant seagrass in a number of estuaries in south eastern Australia, where it provides habitat and food for a variety of marine fauna, and acts as an important nursery area for juvenile fish, including species of commercial importance. Elsewhere in Australia, there have been numerous reports of significant declines in the abundance of P. australis, largely as a result of anthropogenic impacts. These losses have caused great concern about the conservation status of P. australis, as it is thought to be slow to recover from damage and difficult to transplant. This study investigated a number of issues related to P. australis in southern New South Wales (NSW), an area where P. australis has not been previously studied in detail. Research was carried out in six estuaries in the region: Port Hacking, Jervis Bay, St Georges Basin, Wagonga Inlet, Bermagui River and Merimbula Lake.

The status of P. australis in southern NSW estuaries is poor. Five of the six estuaries in the region have lost significant areas of P. australis over the last fifty years, largely due to anthropogenic impacts, such as shell grit mining, engineering works and deterioration in water quality caused by foreshore development. These losses ranged from about 15% in Bermagui River to 30% in St Georges Basin. P. australis increased in area in one estuary, Wagonga Inlet. This increase was probably the result of engineering works at the entrance, which increased tidal range and tidal prism and enabled P. australis to expand into deeper waters.

Long term natural recovery of damaged P. australis meadows in southern NSW was slow but measurable. In Jervis Bay, seismic survey 'holes' created in the late 1960's have been progressively recolonised by P. australis over the last thirty years. P. australis rhizomes have spread from the surrounding beds at an average rate of 21 ± 2 cm.y-1. It was estimated that the 'holes' should be completely revegetated at various times over next century, the earliest being 2034 and the latest 2071. In Port Hacking, areas once dredged for shell grit mining have been recolonised by P. australis, and a number of small P. australis patches now exist in the mined area. Unfortunately, it was not possible to determine whether the patches were the result of seedling development or regrowth from fragments that were too small to be seen on the historical aerial photographs.

Natural recovery of damaged P. australis was also observed in St Georges Basin, NSW . On the edge of surviving P. australis beds a large number of plagiotropic (horizontally spreading) rhizomes were observed to be colonising surrounding substrate at an average rate of 21.4 ± 1.0 cm.yr-1. At four sites, P. australis seedlings were found to have recolonised the substrate between these larger beds. For the first time in this region, small P. australis plants were also found. However, seedling abundance was low and only two seedlings had matured sufficiently to produce a plagiotropic rhizome. It was estimated that, on average, P. australis seedlings may take up to four years before a plagiotropic rhizome is produced. Natural recovery by sexual reproduction in P. australis communities is therefore likely to take several decades.

Monitoring P. australis communities in southern NSW estuaries was able to detect changes in P. australis health due to natural variability, as well as possible anthropogenic impacts. Monitoring P. australis shoot density was particularly effective, as small natural variation, and the absence of a strong seasonal trend, meant that sustained declines in health appeared an obvious downward trend in the time series graphs. Generally, natural variation in P. australis shoot density fell within 20% of the mean for each site, while more extreme changes in health, possibly due to anthropogenic impacts, fell above 30 % . Therefore, a change of > 30% represents a change in P. australis health that is of concern with respect to management. This trigger level should be used to monitor the health of P. australis in future monitoring studies.

Experimental transplanting of P. australis in Port Hacking to restore damaged meadows was successful and increased the amount of P. australis habitat in the Port. Three of the five sites survived for the duration of the study, and shoot numbers increased at two of the sites, by 60% at the control site and more than 300% at the Burraneer Bay site. The transplant units (P. australis sprigs) also exhibited significant rhizome growth, rhizome branching and shoot growth. Burraneer Bay in Port Hacking provided a good example of an area which has sustained significant loss of P. australis but now appears ready for further restoration.

Overall, this study confirmed that P. australis is vulnerable to a wide variety of anthropogenic impacts, but that the response to these impacts is complex. P. australis is also extremely slow to recover naturally and is not transplanted easily. However, P. australis in southern NSW estuaries has not declined to the extent that it has in more developed estuaries, such as Botany Bay, and has a greater capacity to recover than previously thought. This study also demonstrated that suitable sites for restoration do exist in southern NSW estuaries, but each site needs to be carefully identified in light of the long term changes that have occurred to the seagrass beds. Prior to a feasibility study, restoration needs to be instigated at a number of sites, in order to compensate for the loss of P. australis that has occurred over the last fifty years. Meanwhile, the remaining areas of P. australis in southern NSW estuaries need to be conserved and protected from waterway and catchment development. A number of management and research recommendations have been made to assist in this process.

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