Australian Saltmarsh Ecology

Preface

As recently as 1990, Peter Fairweather described Australian saltmarsh as the least studied of all marine habitats, and ignorance of the ecological values of saltmarsh had been reflected in the relative lack of protection afforded to the habitat compared to other ecosystems. By way of contrast, mangroves have been recognised as an important fisheries habitat in Australia for nearly a century, and have a long history of protective legislation and regulations. Several decades of sustained research into mangroves through the 1970s and 1980s provided a comprehensive picture of their structure and composition and aspects of their ecology.

Over the same period, little attention was give to saltmarshes, in spite of their occupying as much as 16 000 square kilometres of the Australian coastline and supporting more than three times the number of vascular plant species found in mangrove forests. Throughout the 19th and 20th centuries saltmarshes were replaced by playing fields, residential and commercial land and agriculture. We now know that in the closing decades of the previous century, mangroves began replacing saltmarsh from the seaward edge, a trend likely to continue with elevated sea levels as a result of global climate change. The decline of coastal saltmarsh in the southern half of the continent has now come to the attention of policy makers, and in New South Wales coastal saltmarsh has been declared an Endangered Ecological Community under the NSW Threatened Species Conservation Act.

Fortunately, the growing awareness of the vulnerability of coastal saltmarsh has prompted more than a decade of research by a number of university and government scientists. While there is still much to be discovered about Australian saltmarshes, the time is ripe to dispel the myth that we know virtually nothing. This book provides the first synthesis of knowledge of Australian saltmarsh ecology. We hope it will stimulate greater interest in this fascinating habitat. The 10 chapters review geomorphology and biogeography, invertebrate ecology, the use of saltmarsh as a habitat by fish, birds and other mammals, and management issues including the control of mosquitos and the threat of invasive species. The picture which emerges is one of a vulnerable habitat which makes a unique and important contribution to the ecology of the coastal zone.

Paul Adam’s opening chapter places Australian saltmarsh in a global context. Saltmarshes occur widely on estuarine and sheltered open coasts, and are immediately recognisable through a combination of habitat, vegetation physiognomy and elements of floristics. Australian saltmarshes exhibit patterns of variation at local, regional and continental scales which are similar to those elsewhere, but nevertheless have unique features. The distinctiveness of Australian saltmarshes is strongest in the south. The flora of southern saltmarshes has similarity with that across Gondwana, but with a number of Australian endemic genera and species. Whether patterns in faunal distribution reflect those in the flora is not known at geographic scales, either in Australia or elsewhere.

Chapters 2 and 3 explore the biogeography and geomorphology of Australian saltmarshes. The possible impacts of climate change are introduced in these chapters. Saltmarsh diversity increases toward the colder latitudes, and a warming climate may well pose a threat to many species. In Chapter 3, Neil Saintilan, Kerrylee Rogers and Alice Howe present evidence that sea level rise in the southern and eastern regions of the continent may already be having a detrimental impact, promoting the colonisation of the saltmarsh environment by mangrove.

Several chapters then consider the faunal ecology of Australian saltmarsh. Pauline Ross, Todd Minchinton and Winston Ponder provide a comprehensive account of the mollusc fauna of Australian saltmarshes in Chapter 4. The authors describe the unique adaptations of gastropods to the challenges of the saltmarsh environment, and their close association with the saltmarsh flora, both for habitat and food.

In Chapter 5, Debashish Mazumder outlines the ecology of grapsid crabs, the dominant crustacean and in many ways the keystone of the saltmarsh ecosystem. Crabs in Australian saltmarshes have limited opportunity to spawn, but on the few occasions the tide inundates the upper intertidal saltmarsh a mass spawning ensues. This event provides a link between the trophic ecology of crabs and fish, and is explored by Rod Connolly in Chapter 6. Many species of fish enter the saltmarsh on the spring tide, including several species of direct commercial importance.

Chapter 7 considers the importance of saltmarsh as a habitat for a range of terrestrial species, including birds, bats and other mammals. The significance of saltmarsh for migratory shorebirds has only recently been appreciated in the published literature, partly because the saltmarsh is primarily used as a night-time roost, a time when few ecologists are active. The shallow pools of the saltmarsh afford good protection from many predators, as well as a secondary feeding habitat. Among the other night-time visitors to the saltmarsh are several species of insectivorous bat, including some threatened species.

There are numerous species of insects which may be attracting bats to the saltmarsh. One such species is the saltmarsh mosquito, Aedes vigilax. The ecology and management of the saltmarsh mosquito forms the subject of Chapter 8 by Pat Dale and Mark Breitfuss. The saltmarsh mosquito is a biting nuisance in many coastal communities and in some locations a vector of the Ross River virus. There are a number of other viruses which cause disease in humans which could be transmitted by mosquitoes, and with global warming the incidence of infection may increase. Perceptions of this risk will need to be addressed to ensure that public opinion continues to support wetland conservation. Several strategies for mosquito control are discussed and their ecological consequences described.

The final two chapters provide an overview of management issues and responses. Pia Laegdsgaard, Rob Williams, Jeff Kelleway and Chris Harty describe the effects of overgrazing, use of off-road vehicles, dumping of waste and reclamation. The legislative and policy responses of the various Australian jurisdictions are discussed and the importance of community awareness is stressed. Implementation of conservation measures for saltmarsh is dependent upon us knowing where it is, and the final chapter provides guidelines which should improve the mapping and monitoring of saltmarsh by natural resource managers.

While the book goes a long way towards redressing the common misconception that little is known about Australian saltmarsh, a common refrain in many chapters is that there is still much to discover. Several fruitful areas of research are proposed, notably an improved understanding of the ecophysiology of saltmarsh plants, the study of saltmarsh insects and their trophic dependencies, and a better appreciation of the ecology of saltmarshes in the tropical north and the arid west of the continent. Studies of ecosystem processes have been out of fashion for some time, although there are indications of a resurgence of interest. Saltmarshes, particularly in the USA, were amongst the earliest ecosystems subject to process studies, and these early results have entered textbooks as generalisations applicable to all saltmarshes. Given the differences in floristic composition, climate, tidal regimes and sediment fertility it is likely that quantitatively, Australian saltmarshes will differ from those in the USA, and it would be highly desirable if we had local studies – although these will require multidisciplinary teams and substantial budgets. It is the hope of the authors that this book will inspire the next generation of saltmarsh ecologists to answer some of these questions.

Finally, there are a number of people who deserve our thanks, not the least the numerous honours and graduate students whose diligent work has contributed so much to our present understanding of Australian saltmarshes, braving hot days, cold nights, mud and mozzies. Thanks are also due to the team at CSIRO Publishing for their enthusiasm and support, including John Manger, and Briana Elwood. Janet Walker did a superbly efficient job with editing, as did Frank Saintilan.

Neil Saintilan and Paul Adam

August 2008

CHAPTER 1

Australian saltmarshes in global context

Paul Adam

Introduction

Coastal saltmarshes are recognised globally as ecosystems of high ecological value which are increasingly under threat (Adam 2002; Valiela 2006). While there is increasing acknowledgement of their importance in Australia, and their ‘Cinderella’ status, demonstrated by Fairweather (1990), has improved over more recent times, they are still relatively unknown compared with the intensively studied marshes of Europe and North America.

Coastal saltmarshes can be defined as intertidal communities dominated by flowering plants, principally herbs and low shrubs. They are found on soft substrate shores of estuaries and embayments, and on some open low wave energy coasts. Coastal saltmarsh is also found on the shores of intermittently open saline coastal lagoons. When these lagoons are open to the sea they are tidal (although tidal amplitude may be attenuated in comparison to nearby open shores), but when closed, which is often the majority of times, water level fluctuations are climate driven and lack predictable periodicity.

Saltmarsh is distinguished from adjacent communities by both floristic composition and structure. Mangroves are dominated by trees (and amongst the world’s forests are unusual in the virtual absence of an understorey). The boundary between saltmarsh and mangrove is normally sharp, but on temperate coasts there are sites with mosaics of saltmarsh and mangrove where there are groves of Avicennia marina interspersed amongst saltmarsh and at the southern global limit of mangroves in Victoria mangroves are stunted and may be lower than the chenopod shrub Tecticornia arbuscula on adjacent saltmarsh. Seagrass beds are predominantly subtidal and are dominated by various monocots (although none are true grasses). The upper limit of saltmarsh is set by the level of the highest tide, but the nature of the transition to terrestrial vegetation will be determined by topography, and, in urban areas, human activity. Coastal lowlands have been very heavily modified in temperate and subtropical regions, so that natural transitions to terrestrial vegetation are becoming uncommon (Figure 1.1). Swamp forests on coastal flood plains often have an understorey of saltmarshes in the transition zone, which may be inundated with brackish water during storms.

Species found in intertidal saltmarsh are also characteristic of seepage zones on seacliffs and rock platforms above the tidal limit, and on some of the most exposed cliffs and headlands, extensive swards (covering hectares) of saltmarsh species are found tens of metres above the sea but subject to high inputs of aerosolic salt.

Australia has very extensive areas of saline soil inland – some of these are of natural origin, but salinisation of agricultural and urban land is one of the major environmental problems to be addressed as a national priority. Some of the species of inland saline areas also occur in coastal saltmarshes, but the majority of the vegetation comprises species in the same genera as those on the coast, but endemic to the inland. At various geological times parts of the inland would have been shallow seas, and, at others, seed transporting birds could have carried propagules between the coast and inland, so that there will have been ample opportunity for gene and species exchange, as well as periods of isolation of local populations and opportunities for speciation.

Figure 1.1 Temperate saltmarsh. Newington, Parramatta River NSW. Marsh dominated by Sarcocornia quinqueflora, and fringing stand of Casuarina glauca.

Towards the head of estuaries, conditions may be brackish or fresh, but still subject to tidal influence. Fringing reed and tall sedge communities in the freshwater tidal zone have been very heavily impacted by urban and agricultural development, and by hydrological change as upstream water abstraction reduces freshwater input. There have been few studies of freshwater tidal marshes in Australia.

In tropical Australia the upper intertidal, flooded by the tides only infrequently, develops hypersalinity during the dry season. The vascular vegetation of these hypersaline flats is extremely sparse and contains only a small number of mostly succulent species (Batis argillicola, Cressa cretica, Sesuvium portulacastrum and Tecticornia australasica). Although the vascular plant cover is very low, there is a skin of microalgae and cynobacteria amongst the salt crust and extending some millimeters into the underlying sediment. The ecology of these flats has not been extensively studied, but they may make a considerable contribution to estuarine productivity; around the Gulf of Carpentaria considerable quantities of salt and nutrients are released from hypersaline flats during king tides (Ridd et al. 1988).

Similar extensive hypersaline flats occur on arid coastal zones elsewhere, and are known as ‘sabkha’ in the Middle East. There is no consensus as to whether these flats should be regarded as saltmarsh or as a separate ecosystem. This uncertainty renders it difficult to determine the extent of saltmarsh in Australia, as different estimates have been made on different bases. However, the area of flats is probably roughly the same as the area of fully vegetated saltmarsh.

While extensive hypersaline flats are a feature of tropical coasts, smaller bare patches are found within temperate marshes. On the central NSW coasts such patches were formerly frequent on the Parrramatta River, Cooks River, Botany Bay / Georges River (Hamilton 1919; Clarke and Hannon 1967). In the last few decades many of the patches have become vegetated, and the few that remain have been damaged by vehicle use (extensive new bare patches have been created by off-road vehicular use – Kelleway 2005). Whether the revegetation of bare areas is a response to natural environmental change, or whether it reflects human influence (such as greater discharge of stormwater into marshes) remains to be determined.

The saltmarsh environment

The saltmarsh environment is a challenging one for many plants, explaining the relatively small flora and its general similarity around the world. Although the flora is not a single taxonomic lineage, and the adaptations necessary to survive the saltmarsh environment have evolved independently on a number of occasions, relatively few families are represented within it.

The factor which distinguishes saltmarsh (and mangroves) from other vascular plant communities is tidal inundation. Tides are highly predictable, but the interactions between tides, weather, groundwater influences and vegetation result in complex patterns of environmental variation (Figure 1.2).

Figure 1.2 The interaction between environmental factors and vegetation in saltmarshes. Redrawn and modified from Clarke and Hannon (1969).

The tidal regime varies considerably around the coastline. In south-west Western Australia and parts of the Gulf of Carpentaria, tides are diurnal, with a single low and high tide a day on the cycle of 24 hours and 50 minutes. Elsewhere tides are semi-diurnal, with two highs and lows a day, or mixed, when the two highs per day differ considerably in height. Tidal ranges are also extremely variable – in southern Australia mostly low (micro- to mesotidal) but with higher ranges in bays and inlets, while in northern Australia tidal ranges are generally high with a maximum of more than 8 m in north-west Western Australia.

The tidal range determines the vertical extent of saltmarsh, but the horizontal extent will depend on the local topography and geomorphology and there are laterally extensive stands at sites with low tidal range and narrow fringes on coasts with high tidal ranges, although for a given surface gradient the higher the tidal range the wider the saltmarsh.

As a consequence of tidal inundation the soils in saltmarsh are saline; the lower on the shore the more frequent the inundations and the less variable the soil salinity. However, at higher levels of the shore salinities can vary considerably depending on the balance between rainfall and evaporation. Inundation will also result in anaerobic soil, although the duration of waterlogging will depend on sediment type and local drainage.

Tidal flooding has other effects on plants. Tidal currents, which increase with tidal range, may dislodge seedlings, so that recruitment may require sufficient long windows of opportunity between inundation to permit germination and development of sufficiently robust seedlings. Estuarine water may be turbid so that after tidal flooding vegetation may be coated with sediment, possibly reducing photosynthesis. Submergence may also change the effective day length and expose plants to a sudden temperature shock. The physiological consequences of these stresses have not been studied.

The interaction between the environmental conditions and species results in a general zonation of species (Adam 1990); the more frequently inundated lower marsh providing habitat for fewer species than the higher levels. Communities are also zoned, but at any given level on the shore there is often a mosaic of communities rather than a continuous band of a single community (Zedler et al. 1995). Local microtopographic change to drainage conditions is often reflected in the vegetation mosaic (despite the absence in many Australian saltmarshes of the well developed creek and pan systems which are a feature of saltmarshes elsewhere – Adam 1990, 2000).

The zonation of saltmarshes is often interpreted as the spatial expression at one point in time of succession. Conceptual models have been developed in which species colonise mud or sand flats and promote accretion and stabilisation of sediment. As the elevation of the marsh surface rises, frequency of tidal inundation declines and environmental conditions permit the establishment of other species which displace the primary colonists. Continued expansion of primary colonists seawards results in zonation. This basic model of sedimentation driven succession, with various additional complexities to account for variation in relative sea level, is sustained by empirical evidence, but interpretation of zonation as a reflection of succession in Australia is less certain.

Pidgeon (1940), influenced by the Clementsian approach which was then one of the major paradigms of ecology, proposed that the zonation of intertidal communities on the New South Wales coast could be interpreted as resulting from succession, and this view has become part of received wisdom. If true, it would be a very atypical successional sequence as it would imply that the primary colonists were trees (mangroves), subsequently replaced by dwarf shrubs and herbs. Pigeon’s model also postulated that the succession continues above the highest astronomical tide level through Casuarina glauca forest to eucalypt swamp forests. In the absence of a drop in relative sea level it is difficult to see that the proposed succession would be driven by allochthonous sedimentation or that autochthonous sedimentation (peat formation) would be sufficient to elevate the surface out of the tidal frame.

The relationship between saltmarsh and mangrove is complex (see Chapter 3) but it is difficult to accommodate a transition from mangrove to saltmarsh within the standard model of saltmarsh formation. There are few sites where active formation of new saltmarsh is occurring, with the exception of invasion by Spartina anglica. Long-term successional development in Australian Spartina marshes, if it occurs, has yet to be described.

Flora and vegetation of Australian saltmarsh

Accounts of saltmarsh flora and vegetation have been published for a number of parts of the Australian coast, including: inter alia by Hamilton (1919); Saenger et al. (1977); Adam (Adam 1981a, b; Adam et al. 1988; Adam and King 1990; Adam 1994); Bridgewater (Bridgewater, Rosser and de Corona 1981; Bridgewater 1982; Bridgewater and Cresswell 1993, 2003; Cresswell and Bridgewater 1998); Craig (1983); Kirkpatrick and Glasby (1981); Thannheiser (2001); Johns (2006); and Kelleway et al. (2007).

Saltmarshes occur globally, and most, although exhibiting local characteristics, have immediately recognisable similarities. Australian saltmarshes are no exception; in terms of physiognomy and composition (particularly at generic level) they are similar to saltmarshes elsewhere.

Within terrestrial biomes, as a general rule, species richness is highest in the tropics and declines at higher latitudes. This is also the case with mangroves, but saltmarshes show a strikingly different pattern (Adam 1990). Tropical saltmarshes in Australia are extremely depauperate, but species richness increases in temperate latitudes, with the highest number of species being recorded from Tasmanian marshes (Saenger et al. 1977; Bridgewater and Cresswell 2003).

Within individual marshes, species richness is generally lowest at low, more frequently tidally inundated elevations and increases higher up the shore, although if freshwater input permits the establishment of tall competitive dominants, such as Phragmites or Typha, in the upper marsh, species richness is again low.

The broad geographic scale patterns of variation in species and community distribution within Australian saltmarshes are similar to those elsewhere (Adam 1990). The distinction between tropical and temperature saltmarshes is seen not just in changes in species richness, but also in the distribution of individual species. Some of the tropical species are widespread outside Australia on hot dry shores (Batis, Sesuvium, Cressa) but Tecticornia australasica (Figure 1.3) is an Australian endemic element in the flora.

On more temperate shores the flora has a large, widely distributed element (at both generic and species level) as well as a strong Gondwanan element. Adam (1990) has argued that there is an overall similarity of flora and vegetation in saltmarshes in South Africa, south-western and south-eastern Australia, New Zealand and temperate South America. Links include the upper marsh rush Juncus kraussii, Sarcocornia spp, Triglochin striata, Cotula coronopifolia and Samolus spp. One species common to Australia and New Zealand is Selliera radicans (Figure 1.4), from a family (Goodeniaceae) absent from northern hemisphere saltmarsh floras. (On the central NSW coast there is evidence of a recent decline in S. radicans, only partly explained by habitat loss (Adam et al. 1988; Kelleway et al. 2007).

Figure 1.3 Distribution of Tecticornia australasica (from literature records, personal observation and records in the Australian Virtual Herbarium).

Figure 1.4 Distribution of Selliera radicans (from literature records, personal observation and records in the Australian Virtual Herbarium).

On coasts with a strongly seasonal Mediterranean climate, saltmarsh vegetation is characterised by shrubby chenopods (formerly in the genera Halosarcia and Sclerostegia, but following a recent taxonomic revision by Shepherd and Wilson (2007), Tecticornia spp.). The dwarf subshrub Frankenia is also characteristic of Mediterranean zone saltmarshes.

Some of the less common species from brackish upper marsh communities exhibit remarkable transhemisphere disjunctions, which, if they have been correctly identified, may reflect the legacy of past long distance dispersal events, possibly by migratory waders. Examples include Limosella australis, which, amongst other occurrences in Australia, is found in upper saltmarshes on the south coast in NSW, but is an endangered wetland plant in Wales, and Isolepis cernua, widespread, although not abundant, in upper marsh flushes in eastern Australia but which is much rarer in northern Europe. A recent discovery in Australian saltmarshes is the dwarf Eleocharis parvula, one of the most inconspicuous saltmarsh species. Is this a recent introduction, or a cryptic species which had been previously ignored? In the northern hemisphere E. parvula has a circumboreal distribution, but with many disjunctions. Clearly E. parvula has not been deliberately introduced into Australia, and it is difficult to envisage a mechanism for accidental introduction by human agency. The habitat of E. parvula in Australia is similar to that in which it occurs in the northern hemisphere and it is not impossible that it was introduced to Australia by birds, possible a long time ago. We may never be able to determine the origin of E. parvula in Australia, alt hough molecular comparison with northern hemisphere populations may in the future provide insights.

Many saltmarsh species have very wide distributions, both at the local scale, within individual marshes, and geographically. This wide amplitude is made possible by the species being made up of many genotypes (Adam 1990). There have been few studies of the genecology of Australian saltmarsh species, but one of the most widespread saltmarsh grasses, Sporobolus virginicus, has been shown to be genetically very variable (Smith-White 1981, 1988). The presence of genetic variation may facilitate the response of species to climate change, but also has implications for the use of planting material in rehabilitation or recreation projects. The wide distribution of species may suggest that any material could be used (including commercial cultivars) in these projects, without care being taken to match the genotype to the new environment.

Non-vascular flora

The vascular plants are of the visibly dominant component of saltmarshes. Other plants may, however, play important roles in the ecosystem.

While there have been many studies of algae in Australian mangroves, saltmarsh algae have been rarely studied. However, they are likely to be as important as algae in saltmarshes elsewhere – contributing to primary productivity, stabilising sediment surfaces, being the food source for filter and surface feeders and, in the case of cyanobacteria, which form part of the algae skin on the sediment surface, functioning as nitrogen fixers.

Bacteria and fungi play a major, although in the Australian context largely unquantified, role in decomposition and chemical transformation and provide food for filter feeders. Although it has long been known that a number of European vascular halophytes are vascular-arbuscular mycorrhizal (VAM, summarised in Adam 1990), it is only recently that more detailed studies have been undertaken (Davy et al. 2000). There has been no systematic investigation of VAM in Australian halophytes, although Samolus repens does support VAM (pers. obs).

Bryophytes and lichens are not generally considered to be components of saltmarshes, although in some northern hemisphere marshes bryophytes form a characteristic element in the vegetation (Adam 1990). In Australia, bryophytes and lichens are largely absent from saltmarshes, although there are occasional occurrences at the highest driftline or as epiphytes on shrubby chenopods.

Additional factors

The environmental factors incorporated in Figure 1.2 are universal in space and time – the interplay of the factors with the pool of Australian halophytic species results in a range of distinctive communities, in the same way that the same factors applied to a different range of species elsewhere would produce a different suite of communities. Human activity results in additional factors coming into play.

Climate change

Increased concentrations of greenhouse gases, including carbon dioxide and methane, are predicted to lead to global warming and other changes in climate conditions. One consequence of global warming will be a rise in sea level, initially as a result of thermal expansion but in the longer term further contributed to by melting of icecaps. The effects of global sea level rise will not necessarily be translated uniformly into changes in relative sea level, as tectonic movement or increased sedimentation could counter the rise in water level at the local scale. However, the Australian coast is tectonically relatively quiescent, and sediment supply, although increased through catchment erosion since European settlement, is still not great. The rise in sea level is thus likely to result in increased inundation of saltmarsh and the retreat of the tidal limit inland. In southern Australia, where many saltmarshes now abut infrastructure and urban or agricultural development, this will result in coastal squeeze, as the seaward edge of saltmarsh is lost to mangrove invasion or inundation beyond flooding tolerance, and the expansion inland is prevented by lack of habitat. However, in northern Australia, where much of the coast is still undeveloped, there are few impediments to the establishment of saltmarsh over what are now terrestrial communities.

Although warming is likely to be a universal phenomenon, other aspects of the climate – rainfall amount, intensity and temporal distribution, general storminess and frequency and intensity of major storms – are likely to vary at local and regional scales although currently available mathematical models do not permit detailed modelling of probable changes. However, changes in any of these factors are likely to be reflected in changes in saltmarshes – changes in rainfall regimes will alter the patterns of variation in soil salinity, change in storminess, but particularly in the intensity of major storm events, could result in erosion of saltmarsh vegetation.

Warming may result in an expansion southwards of the range of northern species, and through competition this might produce a contraction in southern species. Increased temperatures might favour mangroves at the expense of saltmarsh, and might also favour some weeds over native species.

While there remains uncertainty over the details of climate change and the biological consequences, even the most extreme climate change deniers would recognise that the atmospheric carbon dioxide concentration has increased and will continue to do so. This in itself will have profound effects on saltmarshes. Simplistically it might be thought that an increase in carbon dioxide will result in greater photosynthesis and, hence, greater ecosystem productivity. However, an increase in carbon dioxide will lead to greater growth of plants with the C3 photosynthetic pathway, altering the current competitive balance between C3 and C4 species. There will also be an increase in water use efficiency so plants will transpire less and the soil moisture regime will be changed. An increased carbon dioxide concentration is also likely to result in a decreased leaf protein content, which will have flow-on effects through the ecosystem. On current information it is not possible to predict whether lower leaf nitrogen will result in greater herbivory as herbivores need to consume more to achieve the same nitrogen input or lesser herbivory as the decline in resource quality deters herbivores. Experimental studies in the glasshouse, and in the field in American saltmarshes, confirm that carbon dioxide concentrations to the levels predicted over the next century will result in shifts in the relative abundance of C3 and C4 species (in favour of C3), changes in water use efficiency, root-shoot ratio and in nitrogen content (Drake et al. 1989), and it is probable that such changes will be experienced on saltmarshes globally.

Pollution

Climate change could be regarded as a global consequence of pollution, but there is a range of more local pollution events which could have considerable impacts on saltmarshes. Given that estuaries have been sites for