Mangroves comprise several species of trees and shrubs that grow along sheltered intertidal shores, mainly in tropical & subtropical coastal waterways. However, in Eastern Australia, mangroves do extend along temperate coastlines where their distribution overlaps with saltmarsh communities. In these instances, mangroves usually establish in low elevation sites where inundation is more frequent . Australia has 39 mangrove species, which is more than half the global number [1, 2].
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 . However, they are most common in tide-dominated waterways (e.g. deltas, estuaries and tidal creeks) and in wave-dominated deltas .
Mangroves are adapted to the salt-water environment and to anoxic and sulfidic-rich sediments (see acid sulfate soils). For example, some are recognised by their breathing roots (i.e. pneumatophores) which obtain oxygen directly from the atmosphere when exposed at low tide (see photos 1-3). 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 saltmarshes 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 (Photo 4) . Changes in the distribution of mangroves have been identified as an important indicator of broader environmental change for State of the Environment reporting .
Photo 1. Breathing roots (pneumatophores) of Avicennia marina (photo by Jon Knight, UQ) obtain oxygen directly from the air.
Photo 2. The knee roots of Bruguiera gymnorrhiza (photo CSIRO Land and Water, Environmental Remote Sensing group) provide for oxygen uptake by the mangrove roots in anoxic mangrove muds.
Photo 3. The prop roots of Rhizophora stylosa (photo Jon Knight, UQ) enable these mangroves to grow low in the intertide zone where the roots become inundated during high tides.
Photo 4. Mangroves are an integral component of the intertidal environment of Moreton Bay Qld, where they grow in conjunction with expansive mud flats and seagrass beds (foreground) (photo Jon Knight UQ).
Mangroves offer many benefits to both natural systems and humans, and their removal has several economic consequences.
Photo 5. Avicennia marina is the most extensive mangrove in Moreton Bay Qld. Pools like these ones are key habitats for mosquito breeding (photo by Jon Knight, UQ).
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:
Photo 6. Die back in mangroves can result in extensive defoliation and death of mangroves, as in these Avicennia marina mangroves in Moreton Bay Qld. The most likely cause of this die back was a hail storm in 1999 (photo by Jon Knight, UQ).
Photo 7. Accumulation of town rubbish in mangrove near Tea Gardens, NSW (photo by C. Wenger)
Photo 8. Invasion of mangroves near Tea Gardens, NSW, by: (a) Lantana; and (b) Bitou bush (photos by C. Wenger).
Due to the identification of regional trends of mangrove expansion, there has been considerable interest in the relationship between mangrove extent and sea-level changes associated with global warming [For example 7, 18]. 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 .
Mapping changes in the distributions of mangroves is relatively straightforward, and can be undertaken by community groups such as Waterwatch. Aerial photography and satellite imagery can be used, although ground-truthing by local agencies is advised (photo 9). Identifying the causes of mangrove expansion or contraction may be difficult to identify and consideration should be given to natural forces including storm damage (see Photo 6), and changes in rainfall, tidal regime and sea level.
While mapping mangrove distribution provides considerable information about changes in extent, it rarely provides information about the condition of mangroves. Exceptions do occur when mangrove death occurs over large spatial scales, which can be observed from aerial photographs or satellite imagery . For this reason it is advisable to accompany analyses of changes in extent with ground-based measures . Waterwatch include several indicators in their monitoring programs that may serve as early warning indicators of stress to mangroves. These indicators include the quantification of litter production, rates of seedling regeneration, the extent of canopy cover as well as aspects of mangrove structure. the Department of Environment, Water, Heritage and the Arts provides guidelines for State of the Environment reporting.
Photo 9. Research in Avicennia marina mangroves may involve being thigh-deep in black anoxic mud (photo Jon Knight, UQ). The possibility of sinking deep into mangrove mud is ever present.
Mangroves and coastal saltmarsh of Victoria: Distribution, condition, threats and management comprises the first State-wide assessment of the wetlands that fringe the coast of Victoria. The 514 page report examines the diversity of wetland types and plant communities along the Victorian coast and provides analysis of the ecological condition and major threats to coastal wetlands in Victoria. It also includes the first fine-scale mapping of all current mangrove and saltmarsh wetlands in Victoria.
OzCoasts now delivers mangrove conceptual models (community dynamics, Man-made causes of defoliation, dieback and death, and natural causes of defoliation, dieback and death) for the Queensland Department of Employment, Economic Development and Innovation.
Interactive Habitat Extent and Distribution Mapping Interface. The National Intertidal/Sub-tidal Benthic Habitat Classification scheme (NISB) habitat classes include: mangroves, saltmarsh, seagrass, macroalgae, coral reef, rock-dominated, sediment-dominated and filter feeders (such as sponges). These habitats occur between the approximate position of the highest astronomical tide mark and the location of the outer limit of the photic benthic zone (usually at the 50 to 70 metre depth contour). High spatial resolution polygons with thematic attributes based on NISB are available in the NRM Reporting module, together with national, state and regional summary maps for each habitat.
The OzCoasts database contains mapped areas for a large number of Australian waterways (Figure 1). No differentiation has been made between mangroves and Melaluca stands in OzEstuaries, however this data-set is large and internally consistent. Accurate mapping has been undertaken by a number of state and local governments and academic institutions. For example, The Coastal Habitat Resources Information System (CHRIS) has an interactive map facility that enables users to make detailed vegetation maps for the Queensland coast (see coastal wetland layers) in which mangrove standards are differentiated at the level of genera (Figure 2).
Australia's Mangroves (by Norm Duke) is an authoritative guidebook on the mangroves of Australian coastal waters. The book has over 500 colour photographs together with clearly illustrated keys, and an innovative water-proof field key in the form of a wheel. There are also distribution maps, fruiting/flowering charts and detailed botanical descriptions.
Figure 1. Mangrove areas (green) in the Adelaide River (NT) (from OzEstuaries).
Figure 2. Screen dump showing detailed vegetation map for tidal creeks near the Fitzroy Estuary and Keppel Bay based on CHRIS.
More information on habitat removal/disturbance.
Mark Breitfuss, Australian School of Environmental Studies, Griffith University
Norm Duke, University of Queensland
Jon Knight, University of Queensland
Kerrylee Rogers, NSW Department of Environment and Conservation