Focusing management needs of the sub-catchment level via assessments of change in the cover of estuarine vegetation, Port Hacking, NSW, Australia
Seagrass, mangrove and saltmarsh are important habitats and the need to protect them is well recognised. Seagrass is a prime estuarine habitat for commercial and recreational species, but relative to sand, mud and reef, the portion of the floor of estuaries it covers is infinitesimally small. Monitoring change in its distribution is of fundamental importance, as is differentiating between the natural and/or human-causes of change, and applying appropriate management responses.
We used a geographic information system (GIS) to analyse photos of Port Hacking at intervals from 1930 to 1999 to assess long term changes in cover of seagrass, mangrove and saltmarsh. During the middle of the century the cover of seagrass was stable at around 180 ha before declining to a minimum of 66 ha in 1977 and then increasing to 82 ha in 1999. The area of mangrove increased steadily, from 15 ha to 33 ha, while the area of saltmarsh progressively declined from 14 ha to 9 ha.
Port Hacking was divided into nine zones to examine change in its subcatchments. Seagrass has been continuously present in all nine zones, and saltmarsh in each of three zones. Mangrove was present in only five zones in the early record but now occurs in seven zones. The most dramatic changes took place at Cabbage Tree Basin and the Hacking River, where large reductions in area of seagrass, and substantial increase in mangrove, were observed.
Bait gatherers and/or storms may have damaged seagrass at the mouth of Gunnamatta Bay. In 1965 NSW Fisheries closed the whole of the bay to digging, but given the slow growth of seagrass, recovery may have been impeded by the large storms of May 1974 and June 1975. Other cultural factors, such as shellgrit mining, dredging and reclamation, have also had an influence on the distribution of macrophytes in Port Hacking. Urban development of the catchment increases the products of erosion and well as stormwater discharge. Sediments create new substrate for mangrove while stormwater increases turbidity and reduces the amount of light falling on seagrass. Some loss of saltmarsh has been caused by upslope expansion of mangrove.
Provided conservation guidelines of NSW Fisheries are adhered to, the recovery of seagrass should continue. We should attempt to re-establish saltmarsh at sites from which it disappeared. It is likely the cover of mangrove will continue to increase, potentially posing the issue of whether to remove it in certain circumstances. Monitoring of vegetation cover in estuaries is an ongoing management need.