Distribution of estuarine vegetation

Two studies have looked at the distribution of estuarine vegetation.
Saltmarsh, mangrove and seagrass are known to provide important vegetated habitats for many species of fish and invertebrates, some of which are of commercial and/or recreational importance. The distribution of these plants in various New South Wales estuaries are reported to have changed over time. One place where change has been reported is the Parramatta River and its downstream extension, Sydney Harbour.

There would appear to have been a large net loss of seagrass from 129 ha to 52 ha, with large losses at some locations (Clontarf) and small gains at others (e.g., Iron Cove). In contrast, the area of mangrove has increased from 148 ha to 185 ha. Area of saltmarsh appears to have been steady at less than 10 ha. The results show that seagrass has been lost to a large degree and is presumably susceptible to further loss. This situation needs to be immediately addressed due to the importance of seagrass as a fish habitat. The increase in amount of mangrove is also of concern as it confirms studies done elsewhere in the Parramatta River that show an expansion of mangrove that parallels human settlement and use of the river.

West, G., Williams, R.J. and Laird, R., 2004. Distribution of estuarine vegetation in the Parramatta River and Sydney Harbour, 2000. Final Report to NSW Maritime and the Australian Maritime Safety Authority. NSW Department of Primary Industries - Fisheries Final Report Series No. 70. 37pp. ISSN 1449-9967.

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 very 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.

Photos of Port Hacking at intervals from 1930 to 1999 were examined 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. 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.

Williams, R.J and Meehan, A.J., 2004. Focusing management needs of the sub-catchment level via assessments of change in the cover of estuarine vegetation, Port Hacking, NSW, Australia. Wetlands Ecology and Management. 12: 499-518.