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State of the Marine Environment Report for Australia: The Marine Environment - Technical Annex: 1

Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland

Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995

ISBN 0 642 17399 0

Saltmarsh

Paul Adam

School of Biological Sciences
University of New South Wales
Sydney NSW 2052

Introduction

Definition

Coastal (intertidal) saltmarsh is defined as an intertidal plant community complex dominated by herbs and low shrubs. For the most part there is a clear structural distinction between saltmarsh and mangrove (which is defined as an intertidal community dominated by trees). However, at the southern limit of mangrove distribution, in Victoria, the structural distinction becomes blurred with stunted mangroves (Avicennia marina) being of lower stature than the tallest saltmarsh shrubs (Sclerostegia arbuscula). In these circumstances the separation of the two communities is floristic. Similarly, while at a few localities there is an intermingling of the lowest saltmarsh and the uppermost seagrasses(Figure 1), seagrass and saltmarsh are distinguished on the basis of floristics and physiognomy.

Composition and biogeographic variation

Saltmarsh vascular plant communities are characterised by being species poor and often clearly dominated by single species. Within a single marsh there is frequently a zonation from low to high elevations on the shore (e.g. Figure 2). For example, on the New South Wales coast the most frequent ordering of community dominants from low to high marsh (Sarcocornia quinqueflora- Sporobolus virginicus-Juncus kraussii) is subject to local modification along drainage lines and in depressions.

On the much larger scale of the continent itself, there is a striking trend of increased species richness and community complexity with increasing latitude (Saenger et al. 1977, Specht 1981, Adam 1990). Saltmarshes in northern Australia, although extensive, are very species poor, with frequently less then ten vascular plant species in their total flora. Compared to northern Australia, saltmarshes in Victoria and Tasmania which cover much smaller areas, frequently support at least three times as many species. Not only is the flora of individual sites much greater at higher latitudes, but the regional saltmarsh flora is also much larger.

In southern Australia a biogeographic distinction can be drawn between saltmarshes on arid or seasonally arid (Mediterranean climate) coasts and those on temperate coasts with relatively high rainfall (Bridgewater 1982, Adam 1990). On drier coasts saltmarsh vegetation is characterised by a diversity of succulent, shrubby chenopods with a tendency to more open vegetation towards the upper tidal limit, while on wetter shores vegetation is denser with more grassland and sedgeland communities. On the east coast there is a gradual transition from the more species poor, frequently S. virginicus- dominated, subtropical marshes to southern temperate marshes; but at Jervis Bay there is an important biogeographic nodal point where saltmarshes support a suite of species at, Adam & Hutchings 1987). A similar trend is seen on the west coast of Australia but here the estuaries are more widely spaced, making the transition less apparent.

More permanently brackish sites in upper estuaries or on the shores of some coastal lagoons support floristically different communities although there is a continuum of variation between fully saline and brackish sites. Where topography permits, saltmarsh grades upwards into shrubland or forest characteristically dominated by Casuarina species (C. glauca in the south-east, C. obesa in the west), Melaleuca species or Eucalyptus species (for example E. robusta in New South Wales).

Although there is a high degree of endemism at the species level in the saltmarsh flora of Australia at generic and family level there is a strong similarity between Australian saltmarshes and those elsewhere in the southern hemisphere. In addition, many affinities to saltmarshes in the northern hemisphere are displayed (Adam 1990). The patterns of variation in structure and composition of Australian saltmarshes also are very similar to those exhibited on other continents. When viewed in a world context therefore, Australian saltmarshes are not as distinctly 'Australian' as are the various terrestrial communities of the continent.

The vascular plants of saltmarshes are referred to as halophytes (salt-loving plants); but whereas a number of the species may show improved growth at moderate salinities there is no evidence that any are obligate halophytes in that it is possible for them to be grown experimentally under non-saline conditions. However, salt tolerance is achieved at the expense of growth (Adam 1990) so that in the field, under non-saline conditions, halophytes are out-competed by non-salt tolerant species.

Figure 1: Distribution of seagrass beds, mangroves and saltmarsh in the southern part of Botany Bay, New South Wales (redrawn from West et al. 1985).

Figure 1

Figure 2: Relationship of saltmarsh and mangroves to elevation (expressed in m above Australian Height Datum) at various sites along the Georges River, New South Wales (from Mitchell & Adam 1989a).

Figure 2

Distribution

Saltmarsh is found on the coasts of all States. Where mangroves also occur, saltmarsh occupies higher elevations (except in Victoria where the introduced grass Spartina anglica extends seaward of the mangroves). This characteristic zonation has been interpreted as reflecting a successional relationship (Pidgeon 1940) but there is little evidence to support this view (Mitchell & Adam 1989a). In northern Australia both mangrove and saltmarsh may be restricted to the lower, more frequently flooded intertidal zone while the upper intertidal takes the form of extensive hypersaline flats with only very sparse and localised vegetation.

Extensive saline areas are found in the arid and semi-arid zones of Australia - both naturally and as a result of landuse practices. Physiognomically, and floristically at generic level, the flora of these inland saline habitats is similar to that of saltmarsh on climatically dry coasts. Indeed, it can be difficult to determine the upper limit of saltmarsh on some arid coasts as the intertidal saltmarsh merges with the fully terrestrial vegetation.

On cliffs and headlands exposed to very high inputs of salt spray, plant communities floristically identical to those of intertidal saltmarsh are present (Adam, Wilson & Huntley 1988). However, the total area of such vegetation is small.

The best available estimate of the area of estuarine saltmarsh in Australia has been provided by Bucher and Saenger (1991) (Table 1). This estimate, based mainly on remote sensing data, does not differentiate between closed saltmarsh vegetation and more open high level tidal flats because these two habitats are part of a continuum of variation, and separation on the basis of remote sensing data is difficult. In addition, saltmarsh on open shores was not covered by the survey (P. Saenger pers. comm.). Inclusion of open coast areas in the analysis would probably increase the estimate for northern Australia where there are extensive saltmarshes on open coasts (such as around Broome).

There is a tendency in the literature to regard saltmarsh and mangrove as being geographically mutually exclusive, with saltmarsh as the temperate equivalent of the tropical mangrove. However, the data from Bucher and Saenger (1991) demonstrate that the greatest extent of saltmarsh occurs in tropical Australia and that the total area is considerably greater than that of mangrove shores. Limited data are available on the distribution and extent of particular saltmarsh vegetation types in southern Australia (Kirkpatrick & Glasby 1981, Bridgewater 1982, Adam, Wilson & Huntley 1988) but none as yet for northern Australia.

Value and usage

Productivity

Spartina alterniflora - dominated saltmarshes on the east coast of the United States of America are amongst the most productive natural communities on earth. There has been a tendency in the popular literature to generalise from this observation and assume that all saltmarshes are equally productive, but this assumption is not supportable. The majority of data on saltmarsh productivity are from north America, with fewer reports from elsewhere in the northern hemisphere, and these data indicate a considerable range of productivities. Given the physiological costs to plants (Adam 1990) of living in a saline environment however, all saltmarsh productivity could be regarded as being high compared with that from terrestrial grasslands at similar latitudes.

Tabler (1991) 1. Areas of estuarine saltmarsh (km2). Source: Bucher and Saenge

NSW VIC QLD WA SA TAS NT TOTAL
57.16 124.88 5321.91 2965.02 83.67 36.99 5004.84 13594.47

An important difference between Australian and overseas marshes needs to be considered before extrapolating from overseas data to Australia. Saltmarshes around much of the Australian coast occupy the upper intertidal, and even at their lowest limit on the shore are not subject to daily flooding by tides. The zone around much of Australia equivalent to that of the most productive S. alterniflora marshes in North America is occupied by mangroves. In physiological terms, the habitat of Australian marshes is more stressful than that of S. alterniflora, with greater fluctuation in salinity and higher maximum salinities. This greater stress is likely to lower the maximum potential productivity.

Most estimates of saltmarsh biomass and productivity refer only to above ground material. The proportion of above ground productivity transferred to roots may be high, and the root:shoot weight ratio of plants may vary with salinity (Adam 1990). Productivity estimates also frequently exclude algal productivity. The soil surface in the wetter parts of saltmarshes may have a dense covering of algae (both microalgae such as diatoms and macroalgae such as Chaetomorpha and Enteromorpha) and these algal layers may be important contributors to local productivity. The significance of algal productivity to the ecosystem might be proportionately greater than its biomass contribution given that much higher-plant productivity becomes detritus whereas algae represent a high quality food source more directly available to grazers (Pomeroy et al. 1981).

Although claims have been made for high productivity in Australian saltmarshes there have been very few studies of either standing crop or productivity. The above ground productivity values reported for Juncus kraussii saltmarsh in the Blackwood River estuary (Western Australia) of 0.3-1.3 kg dry weight m2 per year by Congdon and McComb (1980) are towards the lower end of the range from temperate northern hemisphere saltmarshes. In the absence of further studies generalisations regarding productivity in Australian saltmarshes are unwarranted. With the exception of saltmarshes deliberately grazed by livestock, consumption of biomass by herbivores appears to account for a small percentage of productivity in the north American situation (Pomeroy & Weigert 1981). Whether this generalisation holds in Australia is not known.

In the absence of a large energy and nutrient flow through direct grazing, and with - at least in most sediments - little accumulation of autochthonously-produced organic material, much of the primary production of saltmarshes will be utilised in detrital pathways. These pathways, which include microbially-mediated decomposition, take place either within saltmarshes or in adjacent waters.

In the popular literature the assumption of high productivity is accompanied by acceptance of the 'outwelling hypothesis', by which coastal wetlands act as a source of detritus and nutrients exported to offshore waters. This argument has been used to justify the conservation of intertidal wetlands but, as Nixon (1980) emphasised, is not well supported by data. Internationally the role of intertidal wetlands in the ecology of estuarine and near shore waters is the subject of considerable research. The current understanding (Jansson et al. 1988) is that there is substantial internal recycling of energy within saltmarshes as well as some fluxes to adjacent ecosystems (which vary in magnitude and direction between sites). Nutrient budgets also vary between sites. However, even where budgets are balanced overall, the transformation of the chemical form of nutrients within saltmarshes may be an important mechanism for increasing bioavailability. Data on the linkages of saltmarshes to adjacent waters in Australia are lacking.

Habitat

Saltmarsh provides habitat for numerous organisms of both terrestrial and marine origin.

A high proportion of the commercially important fish species in Australia are estuarine dependent (requiring estuarine habitat at some stage of their life cycle). Currently for example, about 60% by weight and 70% by value of the commercial catch in New South Wales is estuarine dependent (Leadbitter & Doohan 1991). Many of these estuarine dependent species utilise intertidal wetlands for part of their lives, particularly as juvenile nursery habitat. The most important habitats are seagrasses and mangroves: Australian saltmarshes, unlike those overseas, generally have few permanent creeks and pans and so provide few fish habitats. Nevertheless, studies in both Queensland and New South Wales indicate that at least some saltmarshes may provide habitats utilised by fish (Gibbs 1986, Morton, Pollock & Beumer 1987, Morton, Beumer & Pollock 1988). Further research on the importance of saltmarsh to fish is required.

Coastal wetlands are popularly identified as important habitats for birds. The number of species breeding in saltmarshes is small but upper marsh vegetation provides nest sites for some species (for example the white-fronted chat Ephthianura albifrons). A large part of the population of one of the rarest species in Australia, the orange-bellied parrot (Neophema chrysogaster), overwinters on saltmarshes in Victoria where it feeds on the seeds of chenopods. Migratory waders feed largely on invertebrates in intertidal sand and mudflats but saltmarshes may provide secure high tide roosts. Conservation of waders is a matter for international concern and the Commonwealth is signatory to three agreements (the Ramsar Convention, the Japan-Australia Migratory Birds Agreement and the China-Australia Migratory Birds Agreement) which impose obligations to protect habitats utilised by migratory waders.

Little is known about the utilisation of saltmarsh by other faunal groups and this is a topic requiring further study.

Direct exploitation

There is limited direct exploitation of saltmarshes in Australia. In northern Australia many saltmarshes are accessible to livestock but the effects of grazing and trampling have not been studied. In southern Australia a number of saltmarshes are heavily grazed, but although this results in obvious changes to vegetation and soil structure the long term impacts are unknown.

Losses and threats

Reclamation

In settled areas saltmarshes have been reclaimed for port, industrial and housing development, road construction, parks and sports fields. In more recent decades saltmarshes at sites well removed from existing urban centres have been threatened by developments for recreation and tourism (marinas, resorts and canal estates). Construction of solar salt production ponds in Western Australia has also resulted in some loss of saltmarsh. Some reclamation for agriculture (mainly pasture) has occurred but this has probably involved much smaller areas than have been lost from freshwater wetlands on coastal floodplains. Although the pattern and extent of reclamation have been documented for particular locations there appear to be no inventories at State and national levels. Compared with the total extent of the habitat (Table 1) losses are likely to have been small, but they are concentrated in the south-east of the continent where the initial total area was small and where biodiversity is highest. The losses are therefore likely to be significant both nationally in terms of effects on biodiversity and regionally in terms of loss of habitat functions. Nevertheless, it is difficult to predict the specific impacts of losses in view of the paucity of quantitative information on ecosystem functions.

Degradation

A much larger area of saltmarsh than has been lost from reclamation has been damaged by various forms of habitat degradation.

Adjacent to settlements many saltmarshes are subject to illegal rubbish dumping and to disturbance through the construction and maintenance of easements for pipelines and powerlines. Vehicular use (4-wheel drive, trail bikes and 'BMX' bikes) alters the microtopography and drainage, leading to changes in vegetation. Even if access is prevented (an impossibility in most instances) recovery may take many years. Trampling - such as that associated with educational excursions - may cause long term damage to vegetation, in particular where succulent species predominate. Stormwater drains frequently discharge into saltmarshes. Apart from introducing gross pollutants, nutrients and weed propagules, freshwater discharge can cause local erosion and, through altering the salinity regime, promote the spread of fresh or brackish water species such as Phragmites australis and Typha species at the expense of more salt tolerant species (Pen 1983, Zedler, Paling & McComb 1990).

Saltmarshes are depositional sinks and pollutants from both terrestrial and marine sources may accumulate in them. Little is known about the nature and impact of such pollutants on saltmarshes in Australia. Sewage discharge and runoff from agricultural catchments may promote algal productivity in estuaries, and the accumulation of decaying masses of algae on saltmarsh may cause damage to the underlying vegetation (Hodgkin et al. 1985). Oil spills close to sea ports have affected some marshes (Anink et al. 1985) but these impacts have been inadequately studied. Many saltmarshes are potentially vulnerable in the event of a major oil spill in Australian waters.

Weed invasion

The harsh physico-chemical environment of saltmarshes could be assumed to provide protection against invasion by exotic species. Nevertheless a number of significant invasive weeds threaten the natural biodiversity and community structure of saltmarshes.

In the low marsh the only significant weed is the cord grass Spartina anglica, deliberately introduced at a number of sites in the early 20th century (Boston 1981). It has grown vigorously in Victoria and Tasmania, although populations in New South Wales and South Australia have remained small. In Victoria, cord grass has established to seaward of Avicennia marina, so changing the zonation pattern.

Cortaderia selloana (pampas grass) is a vigorous invader of disturbed bushland. It has considerable salt tolerance and has invaded a number of saltmarshes in southern Australia.

The rush, Juncus acutus, has invaded a range of wetlands in south-eastern Australia and in saltmarshes has displaced the native J. kraussii at a number of sites.

Groundsel bush, Baccharis halimifolia, is native to upper saltmarsh communities in the eastern United States of America. In Australia it is a major weed in coastal areas of southern Queensland and northern New South Wales and appears to be spreading southwards. Groundsel bush forms dense stands in disturbed saltmarshes and adjacent communities such as Casuarina glauca woodland.

Finally, there is a large number of small annual or short-lived, alien species found in upper saltmarshes in southern Australia (particularly on sandy soils) that do not appear to displace native species.

Insect control

Large populations of mosquitoes and sandflies associated with saltmarshes constitute a nuisance to humans. Through spread of diseases for which they are vectors, insects are a threat to human health in some circumstances. With an increasing population (both permanent and transient) living close to saltmarshes there are likely to be increased pressures from residents on local councils for the control of insects. Such control - through spraying of pesticides and/or hydrological modification - is already practised at many localities but the wider consequences of many of these control programs are unknown.

Sea level rise

As a consequence of global warming associated with the 'Greenhouse effect' sea level may rise. Intertidal wetlands have adjusted to previous sea level fluctuations but the consequences of a rise in the near future may be different from those of the past.

If the sea level were to rise, a regression of the seaward boundary of intertidal wetlands would occur. Where topography and other circumstances permit this regression would be accompanied by an extension landward (Bird 1988, Vanderzee 1988). For much of northern Australia there is no impediment to landward migration, so seaward losses would likely be matched by inland gains. However, in much of south-eastern and south-western Australia alienation of the hinterland for a variety of usages would mean severe limits on opportunities for landward movement and sea level rise would be accompanied by net loss of habitat. In the case of saltmarsh this loss would be exacerbated by the landward expansion of mangroves into former saltmarsh.

Increased temperatures accompanying the 'Greenhouse effect' may result in changes to the geographic distribution of individual species, while alteration to rainfall and storm regimes may also affect the composition of vegetation.

Invasion of saltmarsh by mangrove without a significant sea level rise has occurred at a number of sites in New South Wales over the last century (Mitchell & Adam 1989b). The factors responsible for this spread are unknown.

Conservation and management

A number of significant saltmarshes are included within national parks or nature reserves and, of those, several have been placed on the list of internationally significant wetlands under the Ramsar Convention (one such, the Towra Point Nature Reserve in New South Wales, owes its protected status to purchase by the Commonwealth to meet obligations under the Japan-Australia Migratory Birds Agreement).

However, the majority of saltmarsh is outside formal reserves. Protection from development can be conferred through planning, and increased public concern for wetland protection (particularly for coastal wetlands) has meant that planning authorities have, over the past decade, taken an increasingly sympathetic view towards saltmarsh protection. Although most planning decisions affecting saltmarsh are taken at the local council level, guidelines and more formal policies may be suggested at the State level. In New South Wales for example, the majority of saltmarshes outside the Sydney region are included in State Environmental Planning Policy 14 (Coastal Wetlands) which makes many proposals affecting coastal wetlands 'designated development'. This classification requires the production of an Environmental Impact Statement and concurrence from the Director of the Department of Planning to any consent to development. Although the policy does not prohibit development in saltmarshes it has been a major factor in slowing the rate of loss of saltmarsh habitat in New South Wales since its introduction in 1985.

The majority of the extensive tropical saltmarshes are not likely to be threatened by reclamation or development; however, there are few controls on access for grazing. While grazing does not destroy sites information is lacking on the impacts of grazing and associated activities such as burning to promote new growth.

In order to slow habitat degradation there is a need for implementation of catchment management regimes which address the input into estuaries of stormwater, nutrients and pollutants. It is also necessary to recognize the adverse impacts of uncontrolled access to saltmarshes and, even for sites which are not conservation reserves, to prevent the use of 'off road' vehicles.

Gaps in knowledge

There has been very little research on Australian saltmarshes and most of what there has been concentrates on describing the vegetation. While there is now a general overview of southern Australian saltmarsh vegetation (Kirkpatrick & Glasby 1981, Bridgewater 1982, Adam, Wilson & Huntley 1988) there are still many sites for which little has been recorded. In northern Australia there has been little detailed inventory of saltmarsh resources.

Data on the fauna of saltmarshes are particularly scarce - and the terrestrial invertebrate component (insects, spiders etc) is virtually unrecorded.

Ecosystem studies of Australian saltmarshes have been few. Topics such as productivity, energy and nutrient flows and linkages to other ecosystems, although the subject of speculation, remain largely unstudied.

There is an urgent need for research on:

Following recognition of the extent of habitat degradation in south-eastern Australia there is increasing interest in rehabilitation of damaged saltmarshes. It will be important to monitor any rehabilitation programs and to carry out experimental work in order to develop appropriate methodologies.

Australia is fortunate in having extensive saltmarshes (Table 1), although many in the south of the continent have been severely degraded. In order to properly assess the status of saltmarshes and to develop measures for their conservation and management a much greater understanding of the functioning of saltmarsh ecosystems is required.

References

Adam, P. 1990. Saltmarsh ecology . Cambridge: Cambridge University Press. 461 pp.

Adam,P. & Hutchings, P. 1987. The saltmarshes and mangroves of Jervis Bay. Wetlands (Aust.), 6: 58-64.

Adam,P., Wilson, N.C. & Huntley, B. 1988. The phytosociology of coastal saltmarshes in New South Wales, Australia. Wetlands (Aust.), 7: 35-84.

Anink, P.J., Roberts, D.E., Hunt, D.R. & Jacobs, N.F. 1985. Oil spill in Botany Bay: short-term effects and long-term implications. Wetlands (Aust.), 5: 32-41.

Bird,E.C.F. 1988. Physiographic indications of a sea-level rise. pp. 60-73, in G.I. Pearman (ed.), Greenhouse. Planning for climate change. Melbourne: CSIRO.

Boston,K.G. 1981. The introduction of Spartina townsendii (s.l.) to Australia. Occas. Pap. Melbourne State College Dept Geogr., 6. 57 pp.

Bridgewater, P.B. 1982. Phytosociology of coastal salt-marshes in the mediterranean climatic region of Australia. Phytocoenologia, 10: 257-296.

Bucher, D. & Saenger, P. 1991. An inventory of Australian estuaries and enclosed marine waters: an overview of results. Aust. Geogr. Stud., 29: 370-381.

Congdon, R.A. & McComb, A.J. 1980. Productivity and nutrient content of Juncus kraussii in an estuarine marsh in south-western Australia. Aust. J. Ecol., 5: 221-234.

Gibbs, P.J. 1986. The fauna and fishery of Wallis Lake. In Wallis Lake - present and future. Occas. Pap., Aust. Mar. Sci. Assoc. (NSW), 86/2: 1-7.

Hodgkin, E.P., Birch, P.B., Black, R.E. & Hillman, K. 1985. The Peel-Harvey estuarine system. Proposals for management. Rept West. Aust. Dept Cons. Envir., 14. 53 pp.

Jansson, B.O, McIntyre, A.D., Nixon, S.W., Pamatmat, M.M., Zeitschell, B. & Zijlstra, J.J. 1988. Coastal-offshore interactions - an evaluation of presented evidence. pp. 357-363, in B.-O. Jansson (ed.), Coastal-offshore ecosystem interactions. Berlin: Springer.

Kirkpatrick J.B. & Glasby, J. 1981. Salt marshes in Tasmania - distribution, community composition and conservation. Occas. Pap., Univ. Tasmania Dept Geogr., 8: 1-62.

Leadbitter, D. & Doohan, M. 1991. Wise use of wetlands - sustaining our fish harvest. pp. 133-148, in R. Donohue & W. Phillips (eds) Educating and managing for better wetlands conservation. Canberra: Australian National Parks and Wildlife Service.

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