Penrhyn Estuary Habitat Enhancement Plan: Results of Seagrass Monitoring

As part of the Port Botany Expansion Project, land reclamation, boat ramp construction and dredging works altered seagrass habitat in Penrhyn Estuary such that approximately 317 sq. m of seagrass were lost. Prior to... [ view full abstract ]

As part of the Port Botany Expansion Project, land reclamation, boat ramp construction and dredging works altered seagrass habitat in Penrhyn Estuary such that approximately 317 sq. m of seagrass were lost. Prior to construction, seagrass in these areas had undergone a significant natural decline from the 47,100 sq. m (of mainly Zostera capricorni) measured in 2006. The remaining seagrass was monitored during and after construction in parts of Foreshore Beach not directly affected by the works and in a Rehabilitated Area of Penrhyn Estuary that included the flushing channel and lower reaches of the estuary. The aim of the monitoring was to determine if changes to seagrass area were consistent with those predicted.
The findings of three post–construction monitoring surveys indicate there is a narrow, large bed of Halophila spp. containing small patches of Zostera capricorni and Posidonia australis that extends off Foreshore Beach in 2-3 m water depth. Small isolated patches of remnant P. australis and Z. capricorni persist at Foreshore Beach and post-construction conditions are suitable for the survival of these species. Hence, larger beds of these species may be able to re-establish in the future given normal processes of succession. Although numerous patches of the colonising seagrass Halophila spp. and Z. capricorni have been reported at times in the Rehabilitated Area of Penrhyn Estuary, typically these did not persist. Turbidity may be limiting light penetration to the deeper parts of the flushing channel and offshore movement of mobile sediments may be smothering seagrasses in the shallower areas of the Rehabilitation Area before they can fully establish.
Beds of P. australis located within the dredging footprint were also removed and transplanted to Quibray Bay prior to the works. Transplanting showed early signs of success but in the most recent survey (November 2013), decreases in shoot densities of P. australis in the main planting areas suggest these areas, which were initially unvegetated, may not have been the most appropriate sites for transplantation. Investigations into the various methods of transplantation have shown that transplanting whole or trimmed plants was the best way to transplant P. australis.