salt marshes

Chapter 7. Salt marshes

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CHAPTER 7. SALT MARSHES

DOUG FOTHERINGHAM1 and PERI COLEMAN2

1Coastal Management Branch, Natural & Cultural Heritage, Department for Environment and Heritage, GPO Box 1047, Adelaide, SA 5001. Email: [email protected]

2Delta Environmental Consulting, 12 Beach Road, St Kilda, SA 5110

Figure 1. Salt marsh habitats at the head of Gulf St Vincent.

Introduction

Salt marshes comprise plant communities subject to periodic inundation by the sea. They reach their peak development in the southern parts of Australia, in contrast to mangroves, which have their largest extent and diversity in the more northerly parts of the country. Little specific research work has been undertaken on Gulf St Vincent’s salt marshes; however they fit into the arid type salt marsh described by Chapman (1960) and Adam (1990). Arid salt marshes of South Australia (SA) are defined by their domination throughout by dwarf shrubs of one family, the Chenopodiaceae. In Gulf St Vincent (GSV) this family contains the common genera Sarcocornia, Halosarcia, Sclerostegia, Maireana and Suaeda. This type of salt marsh is unlike the temperate salt marshes in wetter climatic zones of Australia, which are dominated by chenopods in the low marsh, and by rushes in the mid and upper marsh levels (Adam 2002), or the more tropical salt marshes that are dominated by grasses and salt pans.

GSV arid salt marsh contains a range of vegetation associations in a physical setting that is both hydrologically and chemically challenging. This chapter documents available information on these fascinating habitats.


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Figure 2. Profile surveyed across a salt marsh near Port Wakefield, with associated overlying plants.

Location and hydrogeological background

Distribution in Gulf St Vincent

In GSV, tidal salt marshes typically occur above the mean high tide (coinciding with the landward mangrove edge) and below the highest spring tides, in areas of calm water and good sediment supply, and typically in estuarine areas. Other types of salt marsh are also common in the Gulf – supra tidal salt marshes occur on salty land that is above the highest astronomical (lunar) tides but subject to seawater inundation by storm assisted tides, while salt lake fringing salt marshes occur around coastal sabkhas or saltpans (Fig. 1).

There are 12 700 ha of salt marsh in GSV, with ~ 6 000 ha along the eastern side, 2 000 ha at the head of the Gulf and 4 700 ha along the western shoreline (Fig. 3). The area of salt marsh in GSV forms a significant portion of the total area of salt marsh in SA, with over 15% of the State’s inter-tidal salt marsh and 22% of the State’s supra-tidal salt marshes.

Bucher & Saenger (1991) estimated that salt marshes, nationally, occupy ~13 594 km2. The majority of this area occurs in the less arid zones of Australia. While the area of SA’s salt marshes is small in comparison with the areas occupied by tropical salt marshes, they form a highly developed, chenopod-dominated habitat that differs markedly from the less complex, grass-dominated northern salt marshes.

For simplicity of mapping, the salt marsh classes have been classified as: inter-tidal salt marsh, supra-tidal salt marsh and stranded salt marsh.

Mangrove and salt marsh habitats within GSV have been classified and mapped by the Department for Environment and Heritage (DEH) as part of a State-wide mapping program. Over 60 salt marsh habitat classes have been identified based on landform, whether they are estuarine, tidal class, vegetation cover and integrity (Canty & Hille 2002). These have been combined into eight classes for State of Environment (SoE) reporting purposes, as follows:

Inter-tidal samphire – flooded by astronomical (lunar) tides and covered by samphire vegetation. Inter-tidal cynobacterial mat– flooded by astronomical tides and covered by cynobacterial mats (blue-

green algae). Inter-tidal sedges – flooded by astronomical tides but subject to freshwater influence resulting in a

sedgeland cover. Stranded tidal samphire – no longer subject to seawater flooding due to levees, roads or land sea level

changes and covered by samphire vegetation. Supra-tidal cynobacterial mats – elevated above astronomical tides but below the height flooded by

storm surges and covered by cynobacterial mats.


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Supra-tidal Melaleuca – not flooded by astronomical tides but still flooded by storm surges, and also has a freshwater influence resulting in stands of tea tree (Melaleuca halmaturorum).

Supra-tidal samphire – not flooded by astronomical tides but still flooded by storm surges and sparsely covered by samphire vegetation.

Supra-tidal sedges – not flooded by astronomical tides but still flooded by storm surges. Also with a freshwater influence resulting in a sedgeland vegetation cover.

Table 1 shows the total area mapped for each habitat found in GSV, its percentage and a percentage comparison of the SA total, including mangrove habitat.

Table 1. Area of salt marsh in Gulf St Vincent.

HABITAT DESCRIPTION Area (hectares) GSV % SA % Inter-tidal cyanobacterial mat 2.83 0.02 0.22

Inter-tidal mangrove 4 720.95 27.07 30.19

Inter-tidal samphire 3 576.50 20.51 15.70

Inter-tidal sedges 0.42 0.00 0.10

Stranded tidal samphire 3 765.50 21.59 41.89

Supra-tidal cyanobacterial mat 10.66 0.06 0.52

Supra-tidal Melaleuca 2.81 0.02 0.16

Supra-tidal samphire 5 357.78 30.72 22.41

Supra-tidal sedges 4.31 0.02 1.61

Total 17 441.77 100.00

At the time of European settlement, tidal and supra-tidal salt marshes extended up to 5 km inland behind the mangroves that fringe the Port River and Barker Inlet. They occurred around the Patawalonga Estuary, and along the upper reaches of the Port River, where the supra-tidal salt marsh graded into the freshwater swamps of what used to be the “Reedbeds” recorded in Fenner & Cleland (1935). In the intervening years extensive areas of salt marsh habitat have been developed for housing, industrial development and salt production. Loss of habitat has been particularly significant on the Adelaide plain (Burton 1982; Coleman 1998; Saintilan & Williams 2000).

Factors affecting salt marsh distribution

Salt marshes are the expression of a number of inter-related factors. The range of tidal inundation, amount of rainfall, riverine input and evaporation rate affect the salinity of the soils that support the marsh. Sediment supply, particle size, currents and wave action determine the amount of accretion that may occur at a site, while the slope determines whether the marsh will form as a narrow band or wide expanse. Even the organisms that occupy the marsh affect it. An example is the burrows of crabs that allow oxygenation of the deeper sediments. Some of these interacting facets are displayed in Figure 4.

Salt marsh soils in GSV occur above the shelly sediments of the St Kilda Formation. The surficial sediments vary and may comprise organic peats, shelly sands or sandy clays (Daily et al. 1976).

The entire range of salt marsh habitats in GSV, from the lowest lying Sarcocornia marsh through to the bluebush-dominated high marsh, may occupy elevations that are only about 60 cm apart, a reflection of the micro-tidal range of the Gulf. A topographic profile and vegetation cover across a salt marsh near Port Wakefield is shown in Figure 2, with its location marked on Figure 3. Although salt marshes may look flat from the air, or when viewed from a distance, the profile reveals the physical complexity that can exist. This complexity, in addition to a seaward gradient, produces significant spatial variation in drainage and tidal flooding.


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Figure 3. Map of salt marsh habitats around Gulf St Vincent.

A recent study in Barker Inlet of the change from mangroves to salt marsh (at an elevation of 0.95 m Australian Height Datum) revealed that the low marsh receives a fresh influx of tidal water on at least 146 occasions over an average year (Cook & Coleman 2003). The frequency of this inundation decreases in the


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higher marsh. The most elevated parts of the marsh only receive tidal flooding a few times in a year – usually in early winter, which is when the tides are at their largest in the Gulf. Fotheringham (1994) analysed data from 61 quadrats in Barker Inlet to examine the relationship between salt marsh species and seawater inundation. Outer Harbour tide gauge records from 1940-1992 were used to compile seawater inundation values for differing elevations in the marshes. Twenty one plant species were recorded on the most elevated sites where annual flooding frequency was <1% each year. There was a steady decline in species as sea-level inundation increased. Only two salt marsh species were recorded at sites where the incidence of seawater flooding exceeded 8% of the year. Mangrove communities replaced salt marshes, where tidal flooding is almost daily and yearly inundation exceeds 19%. Figure 5 shows the presence/absence of plant species in Barker Inlet in relation to the incidence of seawater flooding.

Figure 4. Environmental factors that influence salt marshes (after Lear & Turner 1977).

With the exception of salt marsh habitat adjacent to the few river outlets of the Onkaparinga, Little Para, Gawler, Light and Wakefield rivers, most of GSV salt marshes have very little freshwater input. Therefore, the combined effects of frequency of tidal inundation, rainfall and evaporation are responsible for determining the soil salinities in the marsh, as well as the soil moisture. Soil salinity and moisture content are higher in low marsh soils than in high marsh soils (Table 2), with a lower pH (Coleman unpublished data). This is an outcome of seawater saturation and bacteria that reduce sulfates in seawater.

Table 2. Average soil physical and chemical observations

Low marsh Mid marsh High marsh Moisture % 60 37 20 Chloride (mg/kg dried soil) 85 293 23 572 10 364 pH 7.59 7.86 8.26

Biological features

The physical factors discussed above are reflected in the distinct vegetation zones in the inter-tidal salt marshes of GSV (Fig. 6), and the low to high inter-tidal salt marsh habitats and several hind-marsh habitats are described below.

Inter-tidal salt marsh habitats

Low (or submergent) marsh occurs immediately landward of mangroves (if present) or adjacent to inter-tidal sea grass meadows, at elevations where the tide floods the land daily. In GSV, the low level marsh is dominated by low-growing samphires, mostly the herbaceous Sarcocornia quinqueflora. The plants grow in close, almost lawn-like swards called “kangaroo lawns” in other States. Where low marsh is subject to


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disturbance, the sea blite, Suaeda australis is found. It quickly colonises areas where the Sarcocornia sward has been damaged by storms depositing driftwood and seaweed, or by the activities of bait-diggers. Fotheringham (1994) found Suaeda australis was the predominant species and occupied ~13.8 ha on each side of the Bolivar sewage discharge channel.

Figure 5. Presence/absence of plant species in Barker Inlet in relation to seawater inundation.

Green algae, such as Enteromorpha spp., universally known to children as “green guts”, and the hair-like Chaetomorpha billardieri, commonly grow in the pools that form within the low marsh. There are also beds of submerged seagrasses e.g. widgeon grasses (Ruppia and Lepilaena spp.) and eelgrass (Zostera spp). These flowering plants provide food for flocks of black swans that summer in the sheltered coastal habitats of the Gulf. Swans, in particular, feed on the starch-filled turions of the widgeon grasses.

Cyanobacterial mats often underlay the low marsh vegetation, as well as forming a distinct mat on bare surfaces.

World-wide, low salt marsh forms an important food resource for estuarine fisheries (Weisberg & Lotrich 1982). The daily tidal flushing allows fish to harvest the abundant invertebrates found in this zone. The


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surface mud of the marsh hosts a benthic algal slime comprising colonies of diatoms, the microscopic plants that are major contributors to the productivity of the marsh (N. Saintilan pers. comm). The ubiquitous mud crab, Helograpsus haswellianus, feeds on this benthic film of organisms. During the breeding season the mud crabs release millions of tiny zoeae (the larval stage of the crabs) into the outgoing spring tides, causing a feeding frenzy in the schools of small fish waiting in the salt marsh creeks.

Herbivorous molluscs occur in the moist conditions of the low marsh (Womersley & Thomas 1976). Species include samphire ear shells (Laemodonta ciliata), conniwinks (Bembicium spp.), topshells (Monodonta constricta) and detrital feeders such as sand snails (Salinator fragilis and S. solida). Periwinkles (Nerita spp.), while more commonly found on mangrove trees and rock embankments, may also be found in salt marshes. The snails graze the algal film that coats both the mud flats and the stems and jointed branchlets of the samphires.

Fishing and foraging birds, such as the herons, many migratory shorebirds and ibis are the top-level predators in this part of the marsh (McComb et al. 1995).

At a slightly higher elevation a zone of emergent salt marsh occurs. Emergent (mid and high) marshes look decidedly different from low marsh, with the uniform low sward being replaced by more shrubby species of chenopodiaceous plants. As conditions become drier at higher elevations, other families of plants, such as the grasses, become evident. These higher marsh habitats do not receive daily tidal inundation—some areas may only receive a tide annually—although most are flushed by the spring tides each month.

Figure 6. Salt marsh vegetation zonation.

The shrubby samphire (Sclerostegia arbuscula), thick headed samphire (Sarcocornia blackiana), salt bluebush (Maireana oppositifolia), trailing hemichroa (Hemichroa pentandra), creeping brookweed (Samolus repens), silky wilsonia (Wilsonia humilis), and the grey samphire (Halosarcia halocnemoides) are very common in the mid marsh of GSV, along with the pretty pink-flowered sea-heath (Frankenia pauciflora). This latter plant was one of the species noted by the botanist James Backhouse, in his diary of his visit to Adelaide in 1839 (Backhouse 1841).

The variety of plants increases substantially in the high marsh. Grasses and herbs occur where only the storm-pushed tide surges of winter can inundate them. The corky (or brown-head) samphire (Halosarcia indica), marsh saltbush (Atriplex paludosa), salt marsh grass (Puccinellia stricta), introduced curly ryegrass (Parapholis incurva), sand spurriers (Spergularia spp.), annual groundsel (Senecio glossanthus), coast celery


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(Apium annuum), pigfaces (Carpobrotus rossii and Disphyma crassifolium), streaked arrowgrass (Triglochin striatum), and silky wilsonia (Wilsonia humilis) are all found in GSV high marsh habitats. The distribution of species depends on whether regular freshwater enters the hind-marsh or not. For example, Triglochin striatum occurs in salt marshes that are regularly flushed with freshwater while Atriplex and Maireana are found where the salt marsh changes to dunes or other dryland habitats.

Being drier, the higher level marsh supports many more terrestrial animal species. Several species of galling insects parasitize the samphires (Kolesik 1997), and on hatching these provide a food source for the colourful Christmas spiders (Austracantha minax), the leaf-rolling spiders (Phonognatha spp.), bats such as the white-striped mastiff bat (Talarida australis), and birds including the marsh, or whiskered, terns (Chlidonias hybrida) and white-fronted chats (Epthianura albifrons). Less frequent tidal inundation prevents fish from completely penetrating this zone, and so pools that occur in the emergent sa1t marsh can provide a breeding ground for saltwater mosquitoes (Ochlerotatus vigilax), a species that may carry Ross River virus (Russell 1995).

High marsh environments support a range of insectivorous and seed eating birds. The endangered orange-bellied parrot (Neophema chrysogaster) feeds on seeds from several salt marsh plant species (Laegdsgaard 2006), and has historically used GSV salt marshes as over-wintering grounds.

Larger animals frequent the high marsh, and reptiles are particularly common. Both the eastern and western blue-tongue lizards (Tiliqua scincoides scincoides and T. occipitalis) are found in GSV high marsh, along with the shingle back, or sleepy lizard (T. rugosa), and many species of small skinks that are preyed upon by the top-level consumers, the kites and other raptors (Laegsdgaard 2006). The larger lizards eat a wide range of plant matter, supplemented by small, slow moving animals such as snails and beetles. Snakes found commonly in the high marsh include the white lipped whip snake (Drysdalia coronoides) and the eastern brown snake (Pseudonaja textilis). Mammals are not common; however the water rat (Hydromys chrysogaster) is sometimes seen in the high marsh, especially where freshwater enters from the hind-marsh.

Hind-marsh habitats

The hind-marsh is often called the marsh border and, in GSV, may consist of several habitat types. In arid, flat areas in the northern parts of the Gulf, sabkhas (a Persian word for these features, which occur along the Persian Gulf) may form behind the salt marsh fringe. These are large, flat, low-lying clay pans separated from the Gulf by small sand berms or beaches. The sabkhas maintain a hydraulic connection with the sea and seawater is drawn in, under the edge of the pans in response to the high rate of evaporation from the sabkha surface (Flood & Walbran 1986). As a result, sabkhas can be recognised by their salt crusts, which are present on the land-most margins of the pan, rather than centrally as occurs in drying salt lakes. This is a very harsh environment. Flooding occurs intermittently from the sea or when rivers such as the Light River break out across the clay pans. Evaporation greatly exceeds rainfall, and soil surface temperatures are frequently >40oC.

Sabkhas support relatively few species, but those that do occur tend to be specialists, resulting in high endemicity. Blue-green cyanobacterial mats form dessication polygons on the pan surface, salt-tolerant samphires (Halosarcia halocnemoides and H. pruinosa) edge the pans, and H. flabelliformis (the rare fan samphire) occurs on clay-rich pan surfaces, where water is replenished from flooding rivers or groundwater.

In steep areas, or where embankments contain or cross the marsh, high marsh quickly grades into the local terrestrial habitat. Under appropriate management conditions, seawalls and embankments provide gradational regimes that support a very rich flora. Adam (1990) reports that species occurring in the transition zones between salt marsh and grazing/agricultural land are heavily impacted, and embankments frequently serve as refuges for these rarer species, such as silky wilsonia (Wilsonia humilis), found quite commonly along the embankments and seawalls edging the salt marshes. Similarly Atriplex cinerea, the coastal saltbush, occurs. It is frequently confused with other blue-grey saltbushes such as old man saltbush (A.nummularia), but has beautiful tapering leaves and quite showy pink male flower spikes. Male and female coastal bitterbushes (Adriana klotszchii) are present on some of the seawall embankments north of Adelaide and on Yorke Peninsula, and these provide larval hosting for the locally uncommon bitterbush blue butterfly (Theclinesthes albocincta). This butterfly has very specific requirements - not only does the butterfly lay its


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eggs only on the flowers of male bushes of the Adriana, it also requires the attendance of small black ants that apparently protect the larvae (Fisher 1978).

Where freshwater enters the salt marsh, ‘backswamps’ or freshwater tidal swamps may develop. In some areas levees have been built in an attempt to make freshwater lakes, such as the one at Buckland Park, built in the late 1890s. Currently this lake only receives tidal inundation during the highest winter tides. Being located in the depositional environment of the Gawler River delta it is inevitable that eventually this artificial lake will fill completely with sediment, and the freshwater habitat may convert back to salt marsh.

In other areas, such as the Little Para River, the transition between freshwater swamps and the salt marsh occurs as a long smooth salinity gradient. These wetlands are dominated by rushes and sedges that can tolerate an occasional tidal inundation—Bolboshoenus caldwellii and Phragmites australis are common, with Typha orientalis in the freshest areas. In some areas the salt marsh grass, Pucinellia stricta, can be found.

The pools of water in fresh hind-marshes are edged with the cosmopolitan yellow water buttons, Cotula coronopifolia, introduced aster, Aster subulatus, delicate mauve monkey flowers, Mimulus repens, tangles of lignum, Muehlenbeckia cunninghamii, arrow grasses, Triglochin striatum, and thickets of the tea-tree, Melaleuca halmaturorum. Away from tidal inundation, but where the groundwater is still fairly saline, swamps of the thatching grass, Gahnia filum, the host plant for the rare yellowish sedge skipper butterfly, Hesperilla flavescens flavia, can be found (Sands & New 2002). Back-swamp pools host a wide variety of plankton and a rich diversity of aquatic insects.

In areas where salt marshes occur near deposits of larger sediments, chenier ridges (shell-based ridges), beach berms and dunes may penetrate into the hind part of the salt marsh. The name chenier comes from the French word meaning oak trees, and similar shell ridges in Louisiana support oak trees. All these dunes are very well drained, and the dune vegetation lives on the transient moisture of precipitation, ordinary dew and internal condensation (Packham & Willis 1997).

Common plants on these well-drained, elevated habitats include the sea box, Alyxia buxifolia, native apricot, Pittosporum angustifolium, boobialla, Myoporum insulare, wattles, Acacia spp., moonah, Melaleuca lanceolata, and coastal daisy bushes, Olearia spp. In spring the showy native grass, Stipa elegantissima, provides an attractive display with its fluffy windmill flowers as does the short stem flax lily, Dianella brevicaulis, with its candelabras of blue flowers.

Dune and ridge areas support many vertebrates (goannas, skinks, dragons, snakes and small rodents) and are frequently infested with feral plants and animals such as boxthorn, Lycium ferocissimum, several species of introduced snails, and rabbits (Coleman & Eden 2005).

Foodwebs in salt marshes

The species occurring in GSV salt marshes have already been described. They form the most identifiable components of the salt marsh food web. Trophic interactions in the salt marsh occur as three separate “sub-webs” (McComb et al. 1995). These sub-webs (Fig. 7) are based on the benthic surface of the salt marsh habitat, the aquatic habitat (pools, channels, pans and creeks) and the aerial habitat (including the vertical elements of the vegetation).

The benthic foodweb is located on the mud surface. Much of the productivity of the marsh is driven by detritivores. Fungi and bacteria on the marsh surface decompose most of the plant litter from salt marsh vegetation (Laegsdgaard 2006). Detritivoral bacterial and fungal colonies produce organic slime that is colonised by surface-coating producers such as diatoms. This slime is eaten by protozoa, nematodes, harpacticoid copepods, annelid worms, rotifers, and small crustaceans. One of these crustaceans is the little mud crab that releases its larvae (zoeae) only during spring tides, when water covers the whole salt marsh. At these times the water leaving the marsh may carry up to 7 000 larvae m-3 (Saintilan 2004). These small zooplankton are an incredibly valuable food source for the spat of fish and other marine creatures.

The waters of the small creeks and standing pools support a distinct, but connected, food web―the aquatic foodweb. In salt marshes, this web is dominated by copepods, ostracods, small fish, and fly and


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mosquito larvae. Phytoplankton in the water column, along with zoea and detritus from the benthic zone and riverine input provide the food sources for this web. The top-level predators are the wading hunting birds such as the egrets, herons, spoonbills, sandpipers, stilts and stints.

The aerial food web is found within and above the canopies of the salt marsh vegetation. Grass and plant hoppers, gall wasps and midges, beetles, spiders, and snails provide rich pickings for bats, lizards, chats (these small birds are salt marsh specialists), magpie larks, kites, and harriers (Lane 1987; Major 1991; Priest 2002).

Figure 7. Salt marsh foodwebs.

Threats

Alienation by filling, stranding or reclamation

As metropolitan areas expand, housing removes habitat areas for all marsh species, but critically for the rarer ones. Housing also brings increased traffic to areas that were once only rarely visited. Salt marshes have been systematically “reclaimed” (were they ever ours?) around every major metropolis in Australia. Reclamation consists of building seawalls and then filling the stranded areas. In GSV, land reclamation has occurred in the salt marshes of the Onkaparinga, Le Fevre Peninsula, and Barker Inlet. A network of seawalls was built around Barker Inlet over a period from the 1890s to the 1950s, resulting in nearly 80% of the salt marshes in the affected area being stranded. Unforeseen consequences, such as the development of coastal acid sulfate soils and changes in the hydrology in the areas surrounding the reclaimed land have caused additional habitat loss.

Reclamation for salt production areas north of Adelaide and at the top of the Gulf at Price caused extensive loss of salt marsh. There is still a potential risk of further loss to salt production developments. Analysis of mapping information shows that 20% of the mangrove habitat, 36% of inter tidal samphire and 40% of the supra-tidal habitats within GSV are currently located within Mine Tenements.

There are very few feasible ways of reversing the effects of stranding, filling, and acid development, so conserving the small remaining patches of habitat within the metropolitan area is an important management issue (Saintilan & Williams 2000). Besides the obvious need to prevent filling, it is necessary to ensure that habitat stranding behind embankments is addressed when roadways and railways are built that traverse coastal embayments and estuarine salt marshes.


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Introduced species

Harsh environmental conditions appear to dampen weed invasion in salt marshes that are regularly inundated by the sea. However there are some introduced salt marsh specialists that occur in GSV. Ricegrass (Spartina X townsendii) was planted at Port Gawler in 1931 (Fotheringham et al. 1995), but fortunately has not spread. From the experience of invasion in Victoria and Tasmania it has the potential to become a serious threat to low- and mid-marsh habitats (Hedge & Kriwoken 2000; Boston 1981). During 2005 all known patches of rice grass were physically removed by Primary Industries SA who are currently ensuring there is no regrowth.

An introduced seablite, Suaeda baccifera, originating from the Baltic region, has also been detected in low- to mid-marsh habitats in the Port River and Barker Inlet. The potential impact of this weed has not been assessed. The sea lavenders (Limonium spp.) are very prevalent weeds in the salt marshes and are naturalised across southern Australia. The salt marsh grass genus Puccinellia includes several introduced species. A wider variety of weeds and herbs is found in the upper part of the hind-marsh rarely flooded by seawater.

Climate change

The 2007 estimation from the Intergovernmental Panel on Climate Change is that sea level will be ~ 0.5 m higher by 2100. This will have a significant impact on the salt marsh habitats in the Gulf due to the flat gradients, which have been measured at between 1:200 and 1:1 500 in the gulf (unpublished data). When the sea-level rises, mangrove and salt marsh communities will shift inland.

Roads and levee banks are now located behind many of the salt marshes. These form barriers that will prevent retreat by tidal plant communities as sea level rises, or as land subsidence occurs. If the high marsh communities cannot retreat because of the barriers, whether natural or artificial, the marshes will be replaced by mangroves.

Mangrove incursion

Before European settlement, salt marsh species could retreat landward in the event of sea or land level changes, permitting mangrove expansion inland. Now, with levee banks, roads and houses backing the coastal area, the marshes have nowhere to retreat, and are being replaced by monocultures of mangrove communities (Harty 2004), that will in their turn eventually face a similar problem.

Saintilan & Williams (2000) have shown that mangroves are migrating across salt marsh throughout Australia, for varying reasons, the most common of which are local land subsidence, greater freshwater input, and higher sediment loads in stormwater. However, a single causative factor for mangrove incursion has not been identified (Wilton 2002), and it is likely that precipitating factors vary from location to location.

Salt marshes are very important feeding and roosting grounds for resident and migratory shorebirds, and Straw (2003) has shown that, as a response to predation, shorebirds prefer to roost > 30 m from mangroves. As mangroves invade salt marshes, and areas for retreat are unavailable, it is important that sufficiently large open areas of salt marsh remain to provide habitat for these birds.

In Barker Inlet, where the seawalls of the 1890s were breached between St Kilda and Dry Creek during the 1930s, Burton (1982) showed that mangroves have migrated landward at speeds of up to 18 m per year, transgressing the salt marsh. The mangroves now abut the newer seawalls of the saltfields that were built on the high marsh in the 1930s. The rate of this incursion varies depending on interactions of driving forces including terrigenous supply, site hydrodynamics, land subsidence, and sea level rise. Mangrove encroachment, combined with containment of salt marsh by levees, is causing losses of salt marshes in Barker Inlet at a rate of ~5 ha a year (Fotheringham 1994). On Torrens I., mangrove encroachment has been measured at 2 m a year.

Coastal Acid Sulfate Soils

When coastal acid sulfate soils (ASS) (Chapter 9) occur in salt marshes that have been cut off from tidal inundation, organic carbon and carbonate oxidation result in subsidence, and soils become acidic.


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Breakdown of soil structure may also occur in stranded saline clay soils as a result of Ca:Na exchange on the surface of clay particles, which results in the formation of sodic soils once freshwater rinses the chlorides, along with calcium, from the soil (Rengasamy & Walters 1994).

Hydrological changes

Tidal restrictions cause a range of ecological impacts, including decreased flow rates, changes in the area and frequency of tidal inundation, and increased sedimentation. A decrease in the quantity of water entering a marsh may cause an increased variation in soil and water temperatures, lower concentrations of water oxygen, increased salinity, lower carbonate availability, and changes to sedimentation and drainage patterns. These effects in turn impact on the health of the flora and fauna dependent on the salt marsh.

While tidal restrictions obviously impact adversely on salt marshes, the impacts of increased tidal inundation may not always be beneficial for the ecosystem. Ramsar (2002) guidelines discuss the potential effects of over-inundation in salt marshes that have been re-opened to the tide after being stranded for some time. A very fine balance needs to be achieved between too much tidal inundation and too little. McComb et al. (1995) in their study of the Peel Harvey Inlet, WA, found alterations in inundation levels as small as 10 cm could cause the obliteration of mid-level marsh and its replacement with low-level marsh.

Impoundment of water, either tidal or freshwater, has immediate impacts on salt marsh vegetation. Many marsh species that tolerate regular short periods of inundation will rot quickly if drainage is interrupted for an extended period (Adams & Bate 1994).

Areas of salt marsh that do not get sufficient tidal inundation become leached free of salt by the rain. Where soils are sandy, the freshened soils will rapidly be colonised by weed species. Saline clays that are leached may become sodic (Rengasamy & Walters 1994). These soils tend not to support vegetation and are readily eroded.

Human impacts

The damage done by the recreational use of off-road vehicles in salt marshes is conspicuous, particularly in the salt marshes close to urban areas, including those around Adelaide and Port Pirie. Salt marshes are particularly susceptible to damage from off-road vehicles, as the vegetation grows slowly and the soil structures are delicate. Off-road vehicle use results in a graffiti of large tracts of compacted, disturbed soil, that may no longer be capable of supporting vegetation.

Bait digging for molluscs and worms occurs in marshes close to urban areas and can impact heavily on salt marshes. The impacts can include: the over-exploitation of the bait species themselves, damage to inter-tidal seagrasses, direct loss of marsh vegetation, damage and disturbance to other interstitial macroinvertebrates not being collected, reduction in food sources for migratory and resident shorebirds, disturbance in the feeding patterns for migratory and resident shorebirds, and increased rates of erosion from the disturbed areas.

Salt marshes in GSV have frequently been used as drainage or detention areas for the discharge of storm waters and sewage. The resulting eutrophic conditions may cause blooms of annual algae such as Enteromorpha spp. and Ulva spp. Deposited wrack from these blooms can rot in piles in the marsh causing “rotten spots” or unvegetated areas.

Some GSV salt marshes are close to ports and urban centres, and these marshes are frequently used as dumping grounds for old vehicles and household waste. The hydrocarbons escaping from abandoned vehicles may have a severe impact on salt marsh flora and fauna (IPIECA 1994). Another source of oil pollution is from accidental discharges from shipping and boating.

Grazing

Where salt marsh is grazed heavily by hard-hoofed animals, damage similar to that from off-road vehicles may occur (Bridgewater & Cresswell 1999). Compacted tracks and pugged pools fragment the marsh. Some species are grazed preferentially (Laegsdgaard 2006) and this can eventually change the character of the


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marsh. Gaps may be invaded by weed species, while the deepened pools may not drain between the tides, providing habitat for midges and mosquitoes.

Conclusions

In all, the salt marshes fringing GSV represent a significant percentage of SA’s total. The interplay of seawater inundation, drainage, freshwater input, sedimentation, shelter, soil, and the organisms inhabiting the marsh, has resulted in a variety of habitat types. Each habitat is occupied by specific vegetation associations comprising highly adapted and specialized plants. These generally form open to dense low shrublands. These salt marsh plant communities are important to both marine and terrestrial fauna not only as feeding sites but also as nursery, roosting, and breeding habitat, and so are closely linked to marine and terrestrial food webs. Hence, there are strong flora and fauna conservation reasons why the GSV salt marshes should be protected from further degradation and development. There is, however, another equally important reason, and this concerns the interconnection between salt marshes and the adjacent mangrove, seagrass and deeper estuarine habitats. There is a degree of interdependence between all these habitats. Affect one and the others are affected. This interdependence is well illustrated in the Gulf at Bolivar where seagrass loss due to water quality changes has altered sedimentation patterns in the adjacent mangroves contributing to mangrove dieback. Wave protection by the mangroves has been reduced and the adjacent salt marshes and levee banks are now being damaged by winter storms.

The GSV salt marsh habitats have great importance as buffer areas between the marine and terrestrial environments. They intercept large rainstorm and inland flood events reducing the discharge of sediments and pollutants to the Gulf waters. The survival of adjacent seagrass meadows, which are highly sensitive to water quality changes, may depend on the retention of salt marsh buffers. The salt marshes also buffer the adjacent terrestrial environments from the flooding effects of coastal storm surges. Their importance as buffers against seawater inundation will only grow as sea-level continues to rise.

Areas that have been reclaimed from salt marshes but not yet developed, such as the large area of stranded salt marsh at Gillman, should be assessed for restoration potential. Whether restoration is feasible will depend on the degree of physical changes that have occurred. If the seawater inundation pattern can be restored, if there has been little filling and if seed is available, then it is likely that the salt marsh habitat can be restored.

There are large gaps in our knowledge of salt marshes. They are rewarding and fruitful areas for research. Unfortunately, the community, including community leaders, do not understand well their importance; hence, it is important that the knowledge gained be passed on to the wider community so that in time salt marshes will be valued by the community as highly as their taller compatriots, the mangroves.

Acknowledgments

The authors express sincere appreciation to Matthew Royal for his assistance in the preparation of this manuscript, and thank the Department for Environment and Heritage and Coast Protection Board for their support and use of mapping and photographic data.

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