Monitoring of Saltmarsh / Mangrove Habitats
Westernport Bay covers 270 square kilometres, is relatively shallow and contains extensive areas of saltmarsh/mangrove and seagrass communities. Information on these communities, derived from monitoring programs, helps guide decisions made to protect Westernport, including the international important Ramsar wetland site.
Mangrove- Salt Marsh CommunityThe mangrove-salt marsh vegetation of Westernport Bay is of regional, national and international significance. Mangroves in Victoria are at their southern limit, with the most well developed communities occurring in Westernport Bay. Salt marshes are frequently associated with mangroves and may abut them, with salt marsh growing inshore of the mangroves. This zonation is common in Westernport Bay. The mangrove zone occurs inshore of seagrass, sand and mud flats. These distinct communities occur generally parallel to the shore, except where drainage channels or creeks alter the surface. The mangrove-salt marsh vegetation is generally 100 to 300 meters wide. At Watson Inlet it is up to 1,000 metres wide.
The mangrove-salt marsh vegetation plays an important role in stabilising the coastal system, in nutrient cycling and in providing wildlife habitat. Many changes have occurred in the cover and distribution of the salt marsh and mangrove area since the early 1800s. The majority of these changes are either directly or indirectly related to human activities. Mangroves were cleared to create boat access, to produce barilla ash for soap production, and to reclaim land for industrial and port development. In turn, this exposed nearby mangroves and salt marsh to the damaging effects of tidal scour, erosion and increased sedimentation. Sandy deposits in areas once occupied by mangroves impeded their regeneration. Mangrove-salt marsh vegetation has also been affected by cattle grazing, by the effect of vehicles and by decreased salinity associated with freshwater entering the bay from drained swamps.
The banded zones of salt marsh and mangrove in Westernport Bay represent a long-term successional sequence, with mangroves advancing seawards to be replaced by salt marsh. It is now recognised that this does not always occur. In some areas of New South Wales, salt marsh is threatened by the landward movement of mangroves. This is a reversal of longer-term vegetation change. Landward movement of mangroves into salt marsh is recorded in some areas of Westernport Bay.
A 2000 report reviewed the literature on mangrove salt marsh vegetation in the Bay. The review helped design a monitoring program to provide information on the state of vegetation and the factors affecting it. This information will help guide action to protect this vegetation.
Monitoring of Saltmarsh / Mangrove HabitatsA May 2000 workshop at Hastings identified four sites where the state of the saltmarsh-mangrove vegetation could be monitored and the factors affecting it understood.
Monitoring at each site will be based on remote sensing and field measurements. Remote sensing measurements are based on a comparison of aerial photos taken as early as the 1930s and as late as 1999, and provide a measure of long-term change.
Two sets of direct measurement will be made on a yearly basis at each site. The first involves measuring the elevation of the sediment surface. The second involves measuring the height, diameter, number of basal stems and crown foliage diameter of individual mangroves; and for salt marsh, the species present and the proportion of the area they cover.
ReportsRoss, R. (2000). Mangroves and Salt Marshes in Westernport Bay, Victoria. Arthur Rylah Institute for Environmental Research.
Seagrass CommunityA 2001 report mapped the distribution of the different seagrass species in Westernport Bay and also compared the distribution at 1998-2000 with that observed over the previous 30 years.
Mapping of the 1998-2000 distribution was based on remote sensing and direct observations. High quality aerial photography, flown at a scale of 1:20,000, was used to identify seagrass areas that were then confirmed by field observations. The mapped areas of seagrass were digitised and the seagrass species and density attributes for each area, based on the field data, were entered in a Geographic Information System.
Four species of seagrass were recorded: Zostera muelleri, Heterozostera tasmanica, Halophila australis and Amphibolis antarctica. The two species of Zosteraceae, Heterozostera tasmanica and Zostera muelleri, could not be differentiated by the remote sensing techniques used and therefore were grouped into a single category of Zostera/Heterozostera.
Some 155 square kilometres of seagrass and macroalgae were recorded. Of this, 130 square kilometres was recorded as being either seagrass or a mixture of seagrass and algae. The dominant category of vegetation by area was the mixed category ‘Dense Zostera/Heterozostera with algae’ and it covered some 45 square kilometres. 'Amphibolis with Macroalgae’ was the second dominant category and covered some 20 square kilometres. ‘Undefined Algae’ covered some 25 square kilometres of the vegetation mapped.
Previous seagrass mapping studies in Westernport Bay identified a marked decrease in seagrass and macroalgae cover between 1973/74 and 1983/84. Over this 10 year period about 70%, in area, of seagrass and macroalgae was lost. Observations based on the 1994 and 1998-2000 distributions showed that the seagrass and macroalgae cover had increased from approximately 60 square kilometres in 1983/84 to 95 square kilometres in 1994 to 155 square kilometres in 1998-2000. However, the comparison needs to be treated with some caution as different mapping techniques and field verification methods were used over this 30-year time span.
The general pattern of change was consistent with a more detailed qualitative assessment of aerial photography from four sites for the years 1956 to 1999. It showed a pattern of seagrass decline in Westernport commencing in the late 1970s and continuing through the 1980s, followed by a recovery of seagrass areas in the late 1990s. For each site the greatest area of seagrass cover was observed up to and including 1974 and the lowest area of seagrass cover was observed between 1985-90.