Mangrove Proliferation and Saltmarsh Loss in the Hunter Estuary
Total Page：16
File Type：pdf, Size：1020Kb
Mangroves in the Hunter Estuary The Whistler 1 (2007): 38-45 Mangrove proliferation and saltmarsh loss in the Hunter Estuary Chris Herbert 17 The Quarterdeck, Carey Bay, NSW, 2883 The Hunter Estuary is in a state of ecological crisis. The diverse mosaic of vegetation communities that previously existed in the estuary is rapidly degrading into a mangrove monoculture with a consequent loss of biodiversity. It is concluded that deepening the harbour and harbour channels by dredging, has led to a considerable increase in the tidal range in the estuary. This is considered the main mechanism responsible for the rapid landwards incursion of mangroves into, and displacing, the saltmarsh community. In order to restore the balance between mangrove and saltmarsh communities, it is proposed that existing floodgates be managed adaptively to manipulate tidal inundation. In addition, in areas where critical shorebird habitat is under threat of mangrove encroachment, flow-control structures should be constructed to manage tidal flow into the remaining uncontrolled tidal creeks, downstream of Hexham Bridge. INTRODUCTION harbour that is now the largest coal-exporting port in the Southern Hemisphere. Myriads of estuarine Mangrove proliferation and the concomitant loss islands, separated by winding tidal creeks, have of the saltmarsh community is a phenomenon been amalgamated into one super-island, recognized throughout southeast Australia. This Kooragang Island. As a result more than half of trend is more pronounced in developed estuaries the original estuarine shoreline has been lost. (Saintilan & Rogers 2002). Therefore, it should be Most of the flanking intertidal floodbasins have no surprise that the Hunter Estuary, the most been closed off by floodgates that have never been developed estuary on the New South Wales coast, opened since installation. The prawn and fish is experiencing serious ecological changes industry has suffered and migratory shorebirds (Figure 1). The process is now so rapid that the visiting the estuary have declined in numbers estuary is in danger of becoming a monoculture of from more than 10,000 during the 1970s to about mangroves within a few years, with all the 3,500 as their habitat has degraded. The Hunter resulting problems related to loss of biodiversity. Estuary is no longer a pristine wilderness that can be left to “natural processes”. Natural processes Since the 1950s mangroves have increased in area have been overwhelmed by “progress”. The from about 1300ha to about 1700ha, despite the estuary is man-modified and must now be loss of 240ha to industrial and urban development. managed by man to achieve positive outcomes for At the same time, the original saltmarsh area of the biodiversity that remains. 2133ha, was reduced by 67% (1428ha) by 1994 (Williams et al. 2000). Since then, this trend has A number of authors have suggested various continued and, in some areas, perhaps at a more reasons for the changes, but have generally rapid rate (personal observations). Saltmarsh is concluded that that there is no single explanation now listed as an endangered community. This for all estuaries in southeastern Australia. Without trend together with the previous loss of saltmarsh a definite explanation there has been no attempt to and mangroves from the extensive Hexham suggest alternatives for controlling the changes, if Swamp and Tomago Wetlands, by the closure of indeed that is possible or even desirable. The floodgates during the 1970s, has put tremendous purpose of this discussion is to briefly review pressure on the ecological integrity of the Hunter suggested reasons for mangrove proliferation and Estuary that increasingly resembles a canal saltmarsh loss and to discuss the most likely system. Many people see changes in the estuary as mechanism for these changes as it relates to the a natural progression. However, the Hunter Hunter Estuary. Remediation measures are also Estuary, originally a shallow estuarine delta, is suggested. being transformed into a deep-water industrial 38 Mangroves in the Hunter Estuary The Whistler 1 (2007): 38-45 DISCUSSION 100 years. Contrary to this expectation mangroves are vigorously expanding their range at what Mangrove and saltmarsh communities occur in a appears to be an increasing rate (personal restricted, narrow, vertical zone determined by observations). In addition, a large area of local tidal variations above and below mean sea- saltmarsh on Area E, Ash Island (western part of level. In the Hunter Estuary this vertical zone Kooragang Island, Figure 1), suffered no measures less than two metres at the mouth and mangrove incursions during this period. However, decreases rapidly upstream. Mangrove and during the late 1990s mangroves did rapidly saltmarsh communities flourish in sedimentary encroach and displace saltmarsh within only a few environments undergoing active deposition. These years, but only after creek culverts were removed environments are essentially flat with very low allowing increased tidal flushing. This effectively gradients between areas of extremely low relief. decreased the hypersalinity of the saltmarsh soil, Therefore, even slight changes to tidal amplitude as additional rainfall was predicted to do. But over and/or relative sea-level have a large effect on the the very short period of only a few years, the horizontal distribution of these environments. reduced salinity was much more likely to have been the direct result of opening the area up to full The following discussion reviews various local tidal amplitude (especially as this ongoing mechanisms proposed to explain the proliferation process was taking place during a prolonged of mangrove and the simultaneous loss of drought). saltmarsh as they relate to the Hunter Estuary and compares them with the views expressed here that It is considered that increased rainfall has had no tidal range increase is the main causal mechanism significant effect on mangrove proliferation and for the ecological problems now manifest in the saltmarsh loss in the Hunter Estuary. estuary. Agricultural Practices Several possible mechanisms for mangrove incursion into former saltmarsh habitat have been It has been suggested that mangroves may have discussed in the past: recolonised areas previously cleared of mangroves in the past in Moreton Bay (Morton 1994). This • Precipitation; may well be the case for parts of Ash Island that • Agricultural practices; were cleared for dairy farming in the late 1800s. It • Sedimentation and nutrients; has also been observed in those areas that grazing cattle prevented mangrove propagules from • Subsidence; establishing into mature plants. Continuous cattle • Global sea-level change and grazing would then keep these areas free of • Altered tidal regime. mangroves. On the other hand, withdrawal of cattle from areas on Ash Island, particularly Area These points are used for the following discussion E and Milhams Pond, has seen the sudden concerning the Hunter Estuary. proliferation of mangroves over wide areas of saltmarsh that previously never supported Precipitation mangroves. Thus, it is considered that grazing by cattle has merely served to delay the spread of In southeast Australia average annual mangroves until recently, in contrast to areas precipitation has increased since 1945 (Pittock where mangrove proliferation has been occurring 1988, in Saintilan & Williams 1999). It was for the past 30 years or more in areas that have suggested that hypersaline conditions within never been grazed (e.g. Kooragang Nature saltmarsh soil could be diluted sufficiently to Reserve). allow mangrove colonization. In an area of In the past mangrove branches have been utilized mangrove expansion in the Hunter Estuary, to construct racks for oyster farming and may also Buckney (1987) noted a loss of vigour following have been used as fuel for burning shells for lime an El Nino drought period in 1982, leading him to manufacture. The extent of these activities has not believe that increased rainfall may have been investigated here, but they were probably contributed to initial mangrove expansion. If this more commonly carried out in the North Arm of is so then there should be a noticeable decline in the Hunter River from Fern Bay to Sandy Island the health of mangroves that have developed since where oyster leases are still present and where then in relation to the present prolonged drought dredging of oyster banks for shells was practiced. that is now regarded as the most severe in the last 39 Rare wrens at Warakeila The Whistler 1 (2007): 46-48 Hexham Bridge North Arm Fullerton Cove Hunter River Tomago Milhams Wetlands Pond Cobbans Creek Sandy Ash Island Island Kooragang Nature Reserve Area E Kooragang Hexham Island Fern Swamp Bay South Arm Hunter River Stockton Bridge N Pacific Highway Pacific Ocean 1 km Railway line Newcastle Road Harbour Figure 1. In the lower part of the Hunter Estuary a maze of former estuarine islands has been amalgamated into one super-island called Kooragang Island. Dredging has converted the shallow estuarine delta into a deep-water port by deepening the seaward end of the South Arm, Newcastle Harbour and the entrance channel. 1975 1983 1991 1 9 Mean Sea-level Five-year means Figure 2. Sea-level trends at Fort Denison, 1915-1998 (modified from Saintilan & Wilton 2000). The five- year means sea-level curve, if smoothed even more, appears to be a sinusoidal curve with a periodicity of about 80 years. 40 Mangroves in the Hunter Estuary The Whistler 1 (2007): 38-45 Other areas where mangrove destruction probably estuarine surface increases in elevation mangroves took place in the lower part of the estuary are now should retreat. covered by industrial development or housing. Rogers et al. (2006) found that on Kooragang Island the rate of sediment accretion was about It is evident that agricultural practices, where twice the rate of subsidence caused by sediment cattle grazing is continuing, have prevented compaction in areas of mangroves, and mixed mangrove expansion. The sudden expansion of mangroves and saltmarsh, resulting in a net mangroves in areas of saltmarsh where cattle have increase in surface elevation.