Mangrove and Saltmarsh Ecology

Ecological significance of mangroves

Mangroves can be found in all the major subclasses of coastal waterways in Australia including: tide-dominated deltas, tide-dominated estuaries, tidal creeks, wave-dominated deltas, wave-dominated estuaries and strandplains. Mangroves are adapted to the salt-water environment and to anoxic and sulphidic-rich sediments. For example, some are recognised by their breathing roots (i.e. pneumatophores) which obtain oxygen directly from the atmosphere when exposed at low tide. Other special features of mangroves include: buttresses and prop roots for support; salt excretion from leaf pores; and floating seedlings (viviparous propagules). Sheltered intertidal shorelines are also occupied by salt marshes, however salt marshes are generally dominant in more temperate and drier regions. Where seagrass beds are found adjacent to mangroves, many material links and shared plant and animal communities exist. Changes in the distribution of mangroves have been identified as an important indicator of broader environmental change. Source: OzCoasts

Ecosystem functions

Mangroves offer many benefits to both natural systems and humans:
  • Shoreline protection and sediment accretion; mangroves buffer the shoreline from the erosive impact of storms and waves, preventing erosion by stabilizing sediments with tangled root systems
  • Trap and bind sediments; reducing coastal turbidity and making cleaner waters
  • Major source of primary productivity in the form of plant materials; supporting many important trophic pathways by providing a source for food chains that support many terrestrial and marine organisms
  • Provide habitat for both marine and terrestrial organisms; a home for both plants and animals, including threatened and endangered species. For example, more than 230 species of birds have been recorded from Australian mangroves and, while only 8 or 9 species are restricted to mangroves in the Wet Tropics, the many other species visit and depend upon the mangroves for food, nesting or shelter. Source: Australian Institute of Marine Science
  • Provide nurseries for commercially important mollusc, prawn, crab, and fish stocks; replenishing estuarine and coastal fisheries. For example, it has been estimated that 75 percent of the commercially caught fish and prawns in Queensland spend at least some part of their life cycle living in mangroves (Sources: Australian Institute of Marine Science, Queensland Department of Environment and Resource Management, [1])
  • Mangroves are a sink for atmospheric carbon; helping to reduce global carbon dioxide levels and global warming: an analysis of the impact of mangrove plants on marine carbon inventories suggests that the mangroves account for more than 10% of the terrestrially derived dissolved organic carbon transported to the ocean, while they cover only 0.1% of the continents’ surface. The worldwide rapid decline of mangroves could have potential consequences on the atmospheric composition and the climate
  • Mangroves capture effluents from terrestrial runoff; providing a buffer for nutrients, metals and other toxicants entering coastal waters
  • Changes in mangroves may be a means to monitor change in coastal environments, as indicators of global warming, climate change, storm effects, sea level change, pollution, and sedimentation rates
Their removal can have economic consequences.

Unless otherwise indicated, source: OzCoasts

Ecosystem change

Shoreline development (e.g. coastal urbanisation and industrialisation and conversion to aquaculture) and changes in local hydrology are the biggest threats to mangrove habitat and may cause changes in mangrove extent. Regional and global processes that influence hydrology, such as climate variability, climate change and sea level may also cause significant changes to mangrove areas. Some specific threats include:
  • storms
  • baffling by mangrove root systems provides a physical trap for fine sediment with loads of heavy metals and other toxicants. Changes in pH, redox potential (dissolved oxygen) and salinity can render these toxicants more available to keystone mangrove animals, including crabs. Increases in heavy metals can also lead to inhibition of photosynthesis and respiration in mangroves, causing die back
  • mangroves are susceptible to pollution. Floating oil is a particular concern because it deposits on mangrove roots when the tide drops and contaminates the sediment. Mangroves die from the suffocating or toxic effects of the oil, or from the oiling of keystone burrowing animals (crabs and worms), which are essential to the health of the system
  • rotting drift algae from algal blooms, wrack from seagrass areas and excess input of sediment can smother mangrove pneumatophores causing senescence and death of trees
  • Phytothera introduced in landfill soils has been implicated in the die-back of mangroves
  • trampling can decrease the density of pneumatophores and the biomass of epiphytic algae causing a change in habitat structure
  • increased tidal amplitudes, caused by dredging and climate change, and changes in riverine nutrient and sediment loads, can cause the expansion of mangroves into salt marsh areas
  • increased freshwater contribution from stormwater drains and regional increases in rainfall can cause the expansion of mangroves into salt marsh areas
  • reduction in the duration of the wet season can lower water tables, and increase salinity stress
  • genetic modification of the mangrove genome due to pollution causes changes in species composition and fitness of mangroves
  • invasive weeds: for example, pond apple is a weed that superficially resembles mangroves and is of particular concern because of its invasiveness, potential for spread and its environmental and economic impacts. It now infests more than 2000 ha in the Wet Tropics, and threatens melaleuca wetlands in addition to mangrove communities.
Source: OzCoasts

Mangroves and sea level rise

An emerging issue is the relationship between mangrove extent and sea-level changes associated with global warming. Intertidal vegetation, such as mangrove, may respond to sea-level rise by migrating upslope, or increasing their elevation through processes of vertical accretion or sedimentation so that they remain within the same tidal range. Without such a response, mangroves suffer from a contraction in extent at the shoreline due to erosion, or submergence and death. Since the response of mangroves to sea-level rise are numerous and vary depending on the rate and degree of sea-level rise, identifying links between changes in mangrove area and sea-level rise are difficult. By coupling analyses of change in mangrove extent with ground-based analyses of vertical accretion and surface elevation change, linkages between changes in mangrove extent and sea-level rise can be identified. Source: OzCoasts

Ecological significance of salt marshes

Coastal saltmarshes are communities of plants and animals that grow along the upper-intertidal zone (above the mean spring-tide height) of coastal waterways, mainly in temperate regions. Saltmarsh vegetation is usually less than half a metre high. Saltmarshes are habitats for salt-tolerant plants (halophytes, including grasses, herbs, reeds, sedges and shrubs), a wide range of infaunal and epifaunal invertebrates, and low-tide and high-tide visitors such as fish and water birds. They occur in areas that are either too cold or too dry for mangroves. The diversity of saltmarsh plant species increases with increasing latitude. This contrasts with mangrove diversity, which is highest in the lower latitudes of the tropics. In Australia, when saltmarshes and mangroves coexist, saltmarshes are typically found at higher elevations, where they are inundated less frequently than mangroves. When seagrass beds are found adjacent to saltmarshes and mangroves there may be many material links and shared plant and animal communities.

Where they co-occur, mangrove and saltmarsh ecosystems compete with each other, and changes in climate or hydrology can result in one expanding at the expense of the other.

Saltmarsh sediments generally consist of poorly-sorted anoxic sandy silts and clays. Carbonate concentrations are generally low, and concentrations of organic material are generally high. As with saltflats, the sediments may have salinity levels that are much higher than that of seawater. These sediments are also usually anoxic and have large accumulations of iron sulfides. Disturbing these acid sulfate soils can cause sulfuric acid to drain into coastal waterways. Saltmarshes are often associated with saltflats or exposed bare areas.

Source: OzCoasts

Saltmarshes are claimed to be one of the most biologically productive habitats on the planet, rivalling tropical rainforests. The daily tidal surges bring in nutrients which tend to settle around the roots of the plants, nourishing algal blooms that underpin the biodiversity [Wikipedia].

Ecosystem functions

Salt marshes fulfil a variety of vital roles in processes operating in coastal systems

Biological importance
  • Primary productivity supports estuarine food webs, including juvenile fish and crustaceans
  • Provides a habitat for both marine and terrestrial organisms, some of which are endangered
Physical importance
  • Protection of coastlines from the erosive effects of storms and extreme tides
  • Traps and binds sediment aiding in the process of land accretion
  • Hydrologic impacts on water quality and maintenance of groundwater
Economic importance
  • Salt marshes are farmed as grazing leases for cattle production
  • Landforms of commercial value for development, although this is often at the expense of the salt marsh
  • Important food source for commercially important fish
  • Value of ecosystem services relating to water treatment functions
  • Salt mining for commercial uses
Source: Ozcoasts

Ecosystem change

The majority of issues affecting mangroves also impact on salt marshes.

Species Richness

Australia's mangroves have the fourth highest species richness of any country, after the Philippines, Indonesia and Papua New Guinea [2]

Species Ecology

Meiofauna
  • intertidal zonation and seasonality of tropical meiobenthic communities on Cape York peninsula [3]

Zonation of Species in Mangrove Ecosystems

Mangrove distribution at local scales is determined predominantly by tides and salinity, but influenced also by competition between species and selective predation by fauna [2]. Research has shown that differential predation by crabs on mangrove propagules across the intertidal region may be an important factor influencing mangrove distribution patterns [4].

Zones of dominant mangrove species tend to run parallel to the shoreline or to the banks of tidal creek systems. The seaward side of the community is likely to be dominated by a fringe of Avicennia marina, as it is best adapted to early colonisation and a wide range of soil conditions. Avicennia marina is Australia’s most common mangrove species because of its ability to tolerate low temperatures and a variety of other intertidal conditions. A pioneer, it is likely to be the first species to grow on newly-emerged mud banks.

Sonneratia alba often grows in this zone as well, but it is a more tropical mangrove. Rhyzophora stylosa is usually found behind this zone.

The next zone might be inundated only by periodic spring tides at the times of new and full moons. The soil will be firmer, but more saline due to the evaporation of water leaving behind salt which will not be diluted until the next spring tide. The more specialised Ceriops species can be found in this zone, although commonly saltmarshes or saline herblands with succulent plants thrive here. Avicennia marina can appear again, while less saline soils might be covered with a thick forest of Bruguiera species.

A number of factors may determine what happens to the landward side of this zone. In conditions of high rainfall — as occurs in north Queensland, particularly in the Daintree — regular flooding may lead to freshwater swamp areas dominated by the less salt-tolerant littoral margin (shore) species (such as Hibiscus tiliaceus and Barringtonia acutangula). Behind this may be a zone of paperbark swamps or wetlands and Dillenia alata, as littoral (shore) vegetation merges into rainforest.

In areas of high seasonal rainfall, such as the Gladstone to Townsville region in Queensland, the reverse may be the case, with evaporation and little fresh water input leading to an increase in salinity. This could be a saltmarsh or salt flat zone where only Ceriops tagal, Aegialitis annulata and Avicennia marina grow in patches bordering coastal saline herblands.

There is a similar change of species along rivers, the zones corresponding roughly to decreasing salinity levels and ranges of other factors. Avicennia marina tends to be found throughout river systems, including the upper limit of tidal influence where fresh water is abundant.

The greatest concentration of mangrove species is usually at the mouth of tidal creeks and rivers, where salt and fresh water mix in ideal proportions and floodwaters deposit plenty of material to build up the banks. [[Rhizophora_stylosa][Sonneratia alba]] is frequently found here. While there are certain patterns to mangrove zone development, local conditions will always dictate which mangroves are found and where.

Unless otherwise referenced, adapted from: Queensland Department of Environment and Resource Management

References

  1. Manson, F.J., Loneragan, N.R., Harch, B.D., Skilleter, G.A. and Williams, L. (2005). A broad-scale analysis of links between coastal fisheries production and mangrove extent: a case-study for northeastern Australia. Fisheries Research 74: 69-85. Available online: http://www.sciencedirect.com/science?_ob=MiamiImageURL&_imagekey=B6T6N-4G3SC67-1-H&_cdi=5035&_user=2322062&_pii=S0165783605001098&_check=y&_orig=search&_coverDate=08,2F31,2F2005&view=c&wchp=dGLbVlz-zSkzk&md5=5c48e7cea08a04167db3c90121c25333&ie=/sdarticle.pdf (more)
  2. Duke, N.C. (2006). Australia's Mangroves. The authoritative guide to Australia's mangrove plants. University of Queensland, Brisbane. (more)
  3. Alongi, D.M. (1987b). Intertidal zonation and seasonality of meiobenthos in tropical mangrove estuaries. Mar. Biol. 95: 447-458. Available online: http://www.springerlink.com/content/g21u357155405621/fulltext.pdf (more)
  4. Smith III, T.J. (1987). Seed predation in relation to tree dominance and distribution in mangrove forests. Ecology 68(2): 266-73. Available online: http://www.jstor.org/stable/pdfplus/1939257.pdf (more)

-- JohnBusby - 12 Apr 2010