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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

Mangrove ecosystems in Australia: structure, function and status

Australian Institute of Marine Science
PMB No 3, Townsville MC, Qld 4810
Sydney NSW 2033

Mangrove and mangrove ecosystems: definitions

Mangroves are a diverse group of predominantly tropical trees, shrubs, palms and ground ferns growing above mean sea level in the marine intertidal zone. The term 'mangrove' is used to refer to the habitat although equally often the habitat is called a 'mangrove forest' or 'tidal forest'. It is also common for individual plants in this habitat to be referred to as mangroves.

As a group, mangrove plants share several highly specialised and well known adaptations - notably exposed breathing roots, support roots and buttresses, salt excreting leaves and viviparous water dispersed propagules (Saenger 1982). Individual species do not possess all of these characteristics. The 69 recognised species of mangrove plants in the world belong to some 20 families (Duke 1992), and the species do not have shared ancestry. The term 'mangrove' is therefore an ecological term and not a genetic one.

The set of habitats including mangrove forests, their associated waterways (eg creeks) and estuarine regions or embayments with extensive mangrove forests (and their fauna) are collectively referred to in this paper as components of the mangrove ecosystem.

Habitat and area coverage

Mangrove forests show their greatest development on tropical shorelines with extensive intertidal zones composed of fine-grained sediment; for instance, as found on low gradient or macrotidal coasts (Woodroffe 1992). The forests are at their most luxuriant in areas of high rainfall or abundant freshwater runoff. Mangroves are thus generally associated with low energy, muddy shorelines, particularly tropical deltas. However, they can grow on a wide variety of substrates including sand, volcanic larva or carbonate sediments (eg Chapman 1976).

The most recent survey of the Australian coast showed that there were approximately 11 500 km2 of mangrove forests in Australia (Galloway 1982). The largest forested areas occur in the humid tropics where there is abundant fine sediment and high rainfall and runoff from catchments. However, there are large local forests in the subtropics and as far south as Corner Inlet in Victoria at 38S (Table 1).

Floristics

The mangrove flora of Australasia (the area including New Guinea, New Caledonia, Australia and New Zealand) is one of the richest in the world, having approximately five times the species richness of all other regions excepting the neighbouring region of IndoMalesia (Duke 1992). Generally the greatest concentration of mangrove species in Australasia is found in southern New Guinea and north-eastern Australia, where 45 taxa of mangrove plants are shared.

Table 1. Area of mangrove forests in each of the Australian mainland States and their islands (after Galloway 1982).

State Area (km2)
Queensland
- tropics
- subtropics


4117
485

Northern Territory 4119
Western Australia
- tropics
- subtropics-temperate

2507
10
New South Wales 99
Victoria 12
South Australia 211

The most recent review of mangrove species in Australia (Duke 1992) recognised 39 taxa of mangrove plants belonging to 21 genera and 19 families (Table 2). The taxa include at least four rare hybrids of more common species. Only one species, the newly discovered Avicennia integra, appears to be endemic to Australia (Duke 1988). All other species (Table 2) are widely distributed on the island of New Guinea or in South-east Asia.

Figure 1: The pattern of mangrove plant species richness on the Australian coast. The number of species is generally much higher in the tropics, although the degree of aridity (shown here as the number of wet months per year) has a major control on species richness in the Pilbara region of Western Australia (based on Duke 1992)

Figure 1

The greatest species richness in mangrove communities of Australia is in the north-east humid tropics and there is a gradual decrease in the number of species with increase in latitude (Figure 1). Some estuarine systems on Cape York have up to 35 species of mangrove plants while to the south (Victoria and South Australia) only one species, Avicennia marina, occurs. Within the tropics of Australia, estuarine length, the size of the surrounding catchment, rainfall variation and the frequency of tropical cyclones have a significant effect on species richness on the east coast. These elements do not significantly affect the species richness of west coast mangrove forests however. Estuaries that are long and have large catchments tend to have more species than those with shorter and smaller catchments. Higher interannual rainfall variability and frequent cyclones tend to decrease species richness on the east coast tropics of Australia. The amount of freshwater runoff is inversely related to species richness in the western mangrove forests but not in eastern forests (Smith & Duke 1987).

Floral communities

Regional scale

Variations in factors such as air temperature, rainfall, river runoff, sediment type (and deposition rate), tidal amplitude and geomorphology along the Australian coast produce a variety of regional coastal settings for mangroves to grow. Interactions between such factors and the physiological tolerances of individual mangrove tree species control the growth forms and community structure of mangrove forests in different regions of the country (Clough 1992, Smith 1992). Thus in the high rainfall, humid tropics of north-eastern Queensland there are tall (up to 40 m), highly productive, closed canopy forests dominated by species of the Family Rhizophoraceae - in particular, species belonging to the genera Rhizophora and Bruguiera. This region also harbours the largest potential species pool of mangrove vegetation in Australia (see above); hence, a large variety of floral associations are observed in north Queensland estuaries (eg Bunt & Williams 1981). In this as in other regions of Australia, the observed floral communities on a local scale (ie within estuary) depends on position along both the estuarine salinity gradient and vertical height on the intertidal gradient (see below for details).

As conditions become more arid within the tropics, water and salinity stress on mangroves increase in the intertidal zone and open canopy woodlands or short (1-5 m), low productivity open shrublands may develop. Such shifts in forest types are well illustrated by decreases in average tree height, species richness (and hence forest composition) and greater development of mangrove shrublands from the more humid tropics of the Kimberley coast to the extreme arid zone tropics of the Pilbara coast in Western Australia (Semeniuk, Kenneally & Wilson 1978).

Open woodlands of the single species Avicennia marina, dominate mangrove habitats at latitudes greater than 30S in Australia. Stem densities, basal areas and growth forms of Avicennia vary with region. For instance, on the east coast north of Sydney, the growing season is long enough for trees to reach 10 m in height. By contrast, Avicennia trees at Corner Inlet in Victoria (38S) are stunted (less than 5 m in height) and have low stem densities.

Table 2. Australian mangrove plant species (based on Wells 1982 and Duke 1992). This list contains only obligate mangrove taxa. Putative hybrids are denoted by a 'X' before the specific epithet. * = present in that State.

    States and territories of Australia
Family and species WA NT QLD NSW VIC SA
Acanthaceae          
Acanthus
ebracteatus
    *      
Acanthus
ilicifolius
* * *      
Arecaceae            
Nypa fruticans   * *      
Avicenniaceae            
Avicennia marina * * * * * *
Avicennia integra   *        
Bignoniaceae            
Dolichandrone spathacea     *      
Bombaceae            
Camptostemum schultzii * * *      
Caesalpiniaceae            
Cynometra iripa     *      
Combretaceae            
Lumnitzera racemosa * * *      
Lumnitzera X rosea     *      
Lumnitzera littorea   * *      
Ebenaceae            
Diospyros ferrea     *      
Euphorbiaceae            
Excoecariaagallocha * * * *    
Lythraceae * * *      
Pemphis acidula            
Meliaceae            
Xylocarpus granatum * * *      
Xylocarpus mekongensis * * *      
Myrsinaceae            
Aegiceras corniculatum * * * *    
Myrtaceae            
Osbornia octodonta * * *      
Plumbaginaceae            
Aegialitisannulata * * *      
Pteridaceae            
Acrostichumspeciosum   * *      
Rhizophoraceae            
Bruguiera gymnorrhiza   * * *    
Bruguiera sexangula   * *      
Bruguiera exaristata * * *      
Bruguiera parviflora * * *      
Bruguiera cylindrica     *      
Ceriops australis * * *      
Ceriops tagal   * *      
Ceriops decandra   * *      
Rhizophora apiculata   * *      
Rhizophora X lamarckii   * *      
Rhizophora stylosa * * * *    
Rhizophora mucronata     *      
Rubiaceae            
Scyphiphora hydrophyllacea * * *      
Sonneratiaceae            
Sonneratia alba * * *      
Sonneratia X gulngai     *      
Sonneratia caseolaris     *      
Sonneratia X 'merauke'   *        
Sonneratia lanceolata   * *      
Sterculaceae            
Heritiera littoralis     *      

Local scale: estuarine and intertidal gradients

Two key factors control forest community structure at specific mangrove localities within the (mainly) tropical regions of the Australian coast. These factors are the forest's position along the salinity gradient of an estuary, and its height in the intertidal zone. These factors in turn control the patterns of soil porewater salinities and soil wetness - key physiological constraints on mangrove species growth and competitive potentials (Ball 1988, Smith 1992). An idealised zonation pattern for mangrove plant communities in the humid tropics of north-eastern Australia is illustrated in Figure 2a. In the high salinity region of the estuary the forest composition changes from a Sonneratia alba or Avicennia marina fringe in the lowest section of the intertidal inhabited by mangroves (ie above mean sea level), through a mixed forest of Rhizophora (R. apiculata, R. stylosa and R. x lamarkii) and Bruguiera (usually B. gymnorrhiza and B. parviflora) to higher intertidal forests of Ceriops species and Avicennia marina. In low salinity regions of the idealised estuary, forest composition changes from a fringe of species such as Sonneratia caseolaris or Nypa fruticans through a zone dominated by Xylocarpus granatum to the high intertidal zone, whose characteristic species is Heritiera littoralis (Figure 2a).

Such an idealised zonation pattern is rarely, if ever, observed. The major reason for this is that the medium scale geomorphic features such as infilled creek beds, sand ridges and spits and areas of sedimentation and erosion occurring throughout the intertidal region provide a mosaic of microhabitat types available for colonisation and growth of mangroves (Thom 1982). This situation is illustrated by a surface map of mangrove forest community types in a section of the Daintree River estuary (Figure 2b). Local floral communities may, therefore, be highly complex. For instance, Bunt and Williams (1981) reported 29 species associations ('communities') in northern Australia based on a species pool of only 35 mangrove plants.

In regions where there is little rainfall, local patterns of mangrove floral communities are much simpler. On the Pilbara coast of Western Australia only seven mangrove species are present. Because there is little or no upstream-downstream salinity gradient, increases in the sediment porewater salinities with increasing intertidal height provide the strongest physiological gradient controlling patterns in local mangrove communities (Semeniuk 1983). In most localities on the Pilbara coast a seaward fringe of Avicennia marina (in areas of accreting sediment) gives way to a zone of Rhizophora stylosa followed by zones dominated by Ceriops australis and Avicennia marina in the mid to high intertidal region. Extensive bare salt pans (where soil porewater salinities are more than 90 parts per thousand) are the dominant feature of the higher intertidal areas of the region. In some locations, groundwater seepages from confined aquifers lower soil porewater salinities sufficiently to allow the growth of Avicennia marina on hinterland margins.

Figure 2:
a)
Idealised intertidal and upstream (low salinity) - downstream (high salinity) distribution of mangrove forest community types for a high rainfall north Queensland estuary; and
b) a real situation for a small section of the Daintree River estuary (data taken from Le Cussan (1991).

Figure 2

1 = Rhizophora communities: total of 6 species; 2 = Bruguiiera gymnorrhiza/Osbornia complex: total of 11 species; 3 = Bruguiera gymnorrhiza complex, with Xylocarpus mekongenisis, B. parviflora, Rhizophora species and Ceriops tagal; 4 = Ceriops Tagal complex with Xylocarpus granatum, Bruguira gymnorrhiza, Ceriops decandra and other species; 5 = Ceriops open scrub; 6 = Heritiera/Exocoecaria/Bruguiera/Cynmetra terrestrial margin mangroves; 7 = saltpan.


Physiological adaptations to physicochemical gradients in the mangrove environment are not the only factor implicated in controls on mangrove forest structure at the local scale. Recent work in Australia has shown that seed predation by crabs and insects is also important in controlling the within forest distribution patterns of mangrove tree species (reviewed by Smith 1992). For instance, almost 100% of the propagules (precocious seedlings) of Avicennia marina dispersed into the mid intertidal region of north Queensland mangrove forests are consumed by crabs of the subfamily Sesarminae. Placing cages around A. marina propagules planted in the mid intertidal causes a significant increase in the survival of propagules relative to controls subject to consumption by these crabs. Such predation helps to explain the bimodal distribution of A. marina, where the species occurs in low and high intertidal mangrove habitats but not the mid intertidal habitat (Smith 1992).

Ecosystem structure and function

Interactions between physics, chemistry and biology

Hydrodynamic factors play a major role in the structure and function of mangrove ecosystems. Biogeochemical and trophodynamic processes are often driven by physical dynamics and forest structure and growth are intimately linked to tidal movement of waters.

In a recent review of mangrove hydrodynamics, Wolanski, Mazda and Ridd (1992) provided a succinct summary of the links between physical, chemical and biological processes in mangrove ecosystems (Figure 3). Water circulation is quite different in the two major constituents of mangrove ecosystems - the creeks and the forested portion of the system. Strong tidal flows occur in the long, often branching creek systems as a result of the tidal prism caused by the surrounding forested swamp. This causes a strong dispersion at the downstream end of creeks, which in turn helps flush material out to sea. In the upstream end of creeks there is weak dispersion and hence trapping of materials, often for up to several weeks. This can result in very low oxygen concentrations in the water column of such creeks.

Figure 3:Links between physical dynamic processes and chemical biological consequences in mangrove ecosystems. Note features special to creeks and forested area of mangrove swamps (from Wolanski, Mazda & Ridd 1992)

Figure 3

Frictional forces and the presence of forested areas lead to an asymmetry of tidal currents, with ebb currents much greater than those in flood. This asymmetry maintains deep, self scouring, tidal channels. The high vegetation density in forests leads to high friction, retards flow, and results in trapping of water within forests. The result is that anoxic conditions can occur in waters near the sediment surface and the efflux of nutrients from sediments may increase.

The complex topography of the forested regions can also lead to secondary three dimensional currents and smallscale topographically controlled fronts which aggregate floating mangrove detritus in long lines and enhance particulate export from forests. Biological structures such as crab burrows and decaying roots provide pathways for water (and salt and nutrient) filtration through mangrove sediments. This filtration prevents excessive accumulation of salt arising from evapotranspiration.

Finally, mangroves develop best on shores with low gradients. Mangrove swamps and nearshore waters often share a body of water called a 'coastal boundary layer', which mixes only slowly with offshore waters (Wolanski & Ridd 1990). Slow mixing means that there is often longshore transport of mangrove derived materials which are released to offshore waters only at distinct headlands (Wolanski & Ridd 1990).

Fauna

During the last decade there has been extensive work on the fauna associated with Australian mangrove forests and the role of heterotrophs in mangrove system dynamics (recent reviews by Alongi 1990, Robertson 1991, Alongi & Sasekumar 1992, Robertson & Blaber 1992).

Bacterial standing stocks in mangrove sediments and on decaying leaf litter range from 0.2 to 35.9 x 1010 cells per g, while the productivity of these bacterial populations are amongst the highest levels recorded for benthic bacteria and range from 0.6-5.1 g of carbon per m2 per day (Alongi & Sasekumar 1992). The densities of benthic ciliates and flagellates are low in mangrove forests (range of mean densities; 6-260 cells per cm2 of sediment) relative to other intertidal sedimentary habitats in Australia (Alongi & Sasekumar 1992). One possible reason for this is the low food quality of mangrove detritus.

The meiobenthos of Australian mangrove forests is dominated by nematodes and harpacticoid copepods, and their densities are generally low (less than 500 individuals per 10 cm2) by comparison with other soft sediment habitats. Densities in the humid tropics of Australia are greatest during the wet season. In the dry tropics, the reverse is true as sediment temperatures can exceed 40C in the summer. The soluble tannins derived from decaying mangrove tissues have a significant negative impact on the meiofauna of Australian mangrove forests and may be a major reason why their densities are so low. However, the very low nitrogen content of mangrove litter (the food of some nematodes) is also responsible for low densities (Alongi & Sasekumar 1992). Although it has been hypothesised that meiobenthic organisms play a major role in food chains and nutrient cycles in coastal sedimentary environments, recent work in Australia suggests that the meiofauna of tropical mangrove forests do not affect significantly the remineralisation of mangrove detritus because of their low densities although they may occasionally be important as a dietary component of nekton (Robertson 1988, Tietjen & Alongi 1990). Sediment bacteria are responsible for the bulk of detrital turnover in mangrove forests (Alongi 1990).

Studies of the complete macrofauna of mangrove forest sites in Australia are somewhat rare. Hutchings and Saenger (1987) provided an overview of mangrove macrofauna up to the mid-1980s. Detailed species lists of elements of the infauna and epifauna of mangrove forests are available in Hutchings and Recher (1982), Davie (1982), Wells (1983) and Hanley (1985). In this review it is possible only to provide general comments regarding mangrove macrofauna.

Relative to other intertidal sedimentary environments mangrove forested areas generally appear to have fewer species and lower densities of macrofauna. Bare intertidal mudflats associated with mangroves have a comparatively rich and diverse fauna. The epifauna in the creeks associated with mangroves has a low density when compared to adjacent subtidal habitats such as seagrass meadows (Daniel & Robertson 1990).

Alongi and Sasekumar (1992) suggested that there are several reasons for low infaunal and epifaunal densities and species richness in tropical mangrove forests. These include control by physical factors (monsoons, high temperatures and desiccation), poor quality of mangrove detritus as food (eg a very high carbon to nitrogen ratio in mangrove leaves), chemical defences by mangrove trees and predation by epifauna and nekton.

Decapod crustaceans, including the anomuran families Callianasidae, Thalassinidae and Coenobitidae and the brachyuran families Grapsidae, Hymenosomatidae, Ocypodidae, Portunidae and Xanthidae, are usually the numerically dominant group within the mangrove macrobenthos. However, gastropods of the genus Cerithium, Littorina, Littoraria, Nerita, Cassidula and Ellobium are often locally dominant on the surface of mangrove sediments and on live and dead mangrove vegetation.

Macnae (1968) has given a general account of the behaviour of many macrofaunal species inhabiting Australian mangrove forests. However, recent work in the Australian tropics has shown that elements of the macrofauna play major roles in food chains and nutrient transformations in these intertidal forests. Grapsid crabs of the subfamily Sesarminae consume or bury up to 80% of the annual litter fall in tropical Australian mangrove forests (Robertson 1991). In so doing they have significant influences on the retention of litter nutrients (nitrogen and phosphorus) in forests that are flushed less regularly by tides, and hence on the total tidal export of materials from mangrove forests. Through their extensive burrowing activities the crabs also control aeration of mangrove sediments in some forests, thus ensuring the growth and survival of trees (Smith et al. 1991).

There is a diverse fauna associated with decaying wood in mangrove forests (Hutchings & Saenger 1987). In a 1989 unpublished study of wood-dwelling fauna in north Queensland, S.M. Cragg and A.I. Robertson found more than 120 species of animals inhabiting decaying wood. Surprisingly, insects were the most species rich taxa among the fauna with 17 species, while crabs and polychaetes were represented by 16 species. Teredinid bivalve molluscs (ship worms), represented by 12 species, were an abundant group occurring in more than half of the logs sampled.

Ship worms have a major role in the decomposition of trunk and branch wood in mangrove forests

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