Saltmarshes and mangroves (see Mangroves) are the dominant biological wetland types of the upper intertidal zone along the Australian coastline. Saltmarshes play a vital role in sequestering carbon, and function as habitat and nursery sites for diverse faunal communities, including birds, fish and insects (Laegdsgaard 2006). They are, however, among the most neglected type of wetland in Australia (Boon 2012), and are listed as a threatened ecological community under the New South Wales Threatened Species Conservation Act 1995 and the EPBC Act.
The position of saltmarshes high on the shoreline and their arguably displeasing aesthetic has led to extensive historical losses and a suite of present-day pressures (Saintilan & Rogers 2013). At local or regional scales, total loss of saltmarsh has exceeded 50 per cent in some areas. For example, it is estimated that Sydney Harbour has lost 85 per cent of its original saltmarsh habitat (Mayer-Pinto et al. 2015). Tropical saltmarshes are extensive, but are much less studied.
The threats to saltmarshes are diverse and variable in spatial scale. There is a widespread issue of mangrove encroachment (Saintilan et al. 2014), where the expansion of mangroves squeezes saltmarshes into landward barriers. Invasive species are a problem in south-eastern Australia (Hurst & Boon 2016), and include common cordgrass (Spartina anglica), groundsel bush (Baccharis halimifolia), pampas grass (Cortaderia selloana) and spiny rush (Juncus acutus). There is a historical legacy of pollution in sediments that, if disturbed, has the potential for broad impacts. Oil spills are an unpredictable threat; however, few have occurred in Australia and considerable contingency plans are in place.
A major threat to saltmarshes is clearing and drainage for mosquito control (Dale & Hulsman 1990). Saltmarshes are becoming increasingly fragmented, causing decreased biodiversity, resilience, sediment trapping and nutrient cycling; and altered food-web dynamics. Permitted and unpermitted tourist and recreational use, such as damaging 4WD activity or storage of tenders to access yachts, occurs in some locations. Other threats include tidal restriction, dredging, draining, eutrophication, acid sulfate soils, water pollution, saltwater inundation, grazing and erosion. In the future, climate change impacts are expected from sea level rise, rising carbon dioxide levels and temperature, increased frequency of extreme events, and acidification of floodwaters. Climate change processes may exacerbate to a ‘coastal squeeze’ on saltmarshes caught between land-based and sea-based pressures.
Historically, saltmarshes have been lost to land reclamation (including draining) and more recently to mangrove encroachment in New South Wales, Victoria, South Australia, southern Queensland and Western Australia. Approximately a decade ago, Victoria had approximately 192 square kilometres of saltmarsh and 32 square kilometres of estuarine wetland (Boon et al. 2011), but this is now greatly reduced. In South Australia, mapping in 2000 indicated that approximately 3.3 per cent of intertidal and 4.7 per cent of supratidal saltmarshes were degraded or displaying dieback; these proportions are likely to have remained largely unchanged, with losses occurring predominantly near urban centres.
Recent closure of salt fields north of Adelaide has provided an opportunity for saltmarsh restoration. However, 2 species of introduced seablite (Suaeda baccifera and S. aegyptiaca) are present and likely spreading in South Australia. Invasion of cordgrass (Spartina spp.) is also an increasing threat in Tasmania and Victoria, although management strategies are successfully implemented in Tasmania. In Queensland and Western Australia, salt production is a past and present cause of saltmarsh loss, but in the tropical north, development pressure is low and there are extensive saltmarsh and salt pan areas.
Saltmarshes are high-risk coastal habitats and current management is insufficient (Rogers et al. 2016). Currently, saltmarsh management is largely a local- and state-run operation, and variation between states in approaches can make it difficult to compare programs in terms of success and effectiveness. Development of recovery plans requires greater understanding of species of high functional importance, responses of saltmarshes to the plethora of threats, regulating factors such as nutrient inputs and herbivory, ecosystem function and services, and hydrology (relationship with tide, groundwater and fresh water; see Box COA8). Furthermore, recovery requires increased public recognition of the value of saltmarshes, and policy change to allow inland saltmarsh to respond to rising sea levels. Understanding of saltmarsh regeneration is still in its infancy, although early indications are of long restoration times and potential for mangrove encroachment into restored areas.
The short-term outlook for temperate saltmarshes has improved because of state and national conservation and planning in the past decade, along with growing public awareness. However, the longer-term outlook for temperate saltmarshes is poor, especially where site geology (e.g. incised valleys) or coastal development limits opportunities for inland retreat to accommodate rising sea levels and mangrove encroachment. The long-term outlook for tropical saltmarshes, where development is lower, is one of greater opportunity for inland retreat.