Operations Manager, Griffith Centre for Coastal Management, Griffith University.
On the periphery of one of Australia’s fastest growing coastal cities, is a habitat diverse and unique. Extensive mangrove and saltmarsh communities would have once inhabited the northern waterways of the Gold Coast. There now remain many remnant communities that provide key environmental functions and support a myriad of life.
Mangroves are important for their high ecological value. In addition to providing a habitat for a range species, they are a nursery for juvenile fish and crustaceans. Up to 70% of the commercially important species spend a portion of their lifecycle in and around mangroves. They are a sediment sink; providing a natural filter for nutrients and pollutants from across our landscape and act as a protective measure for our coastline as they absorb and dissipated wave energy within their complex root structures. Evidence shows us that mangrove forests protected coastlines during the Indian Ocean Tsunami of 2004(i).
The economic value of mangroves was acknowledged in a global study in 2005, in which Australia’s wetlands (saltmarshes and mangroves) were valued at $1.7 million per square kilometre(ii). A study in 1990 placed a value of $8,380 per hectare on the mangroves of Moreton Bay based on the market value of the commercial fish species caught during the study(iii).
Mangrove forests were once seen as wastelands and extensively cleared and drained for development. This led to a major loss of habitat worldwide and increased the risks of storm related inundation and shoreline erosion. Mangroves are currently protected in Queensland and a permit is required for any activities involving mangroves.
These unique trees grow on the fringe between the marine and terrestrial environment. Key fea tures of this habitat include variable levels of salinity, fluctuating tides, low wave energy and fine sediments consisting predominately of fine-grained mud with sand and high silt content. Over time, a waterlogged environment emerges with little oxygen penetration into the sediments creating anaerobic conditions and their characteristic odorous smell. These conditions support a range of bacteria that utilise sulphate in place of oxygen to assist in breaking down organic
matter. The process includes the reduction of sulphate (SO2-4) to hydrogen sulphide (H2S).
Stands of mangroves support an abundance of life that starts with photosynthesis within the leaves of the trees. The leaves, once shed from the mangrove tree, provide a key food source of carbon and nutrients that support a complex and extensive food web. This is a highly productive ecosystem upon which a wide range of organisms feed and shelter (including fungi, bacteria, molluscs, fish, crustaceans, insects polychaete worms, mammals, reptiles and birds) and each tidal change carries nutrients into other coastal habitats.
The digging activity of organisms such as crabs leads to the alteration in the biological, physical and chemical features of the sediments; bioturbation. This process, within the top few centimetres, is considered a “life-support” for the mangrove habitat; providing conduits for the transfer of water and dissolved nutrients, aeration of the sediments and the redistribution of organic matter. Additionally, filter feeders such as sponges, barnacles and shellfish attach to the submerged mangrove roots and filter nutrients and silt from the water leading to improved water quality.
Mangroves exhibit physiological mechanisms of salt exclusion, salt excretion and salt accumulation. Their distribution and growth is generally limited by exposure to saline waters, sediment grain size, height above average sea level, exposure to waves, and air/water temperature. They are halophytes (salt tolerant) with specialised roots with thick cuticles to restrict salt absorption, have specialised glands that transport salt to old leaves that act as storage and are subsequently shed and can excrete salt through transpiration (salt crystals are visibly present on the leaves of Aegiceras corniculatum).
A visible feature of mangroves is their adapted roots that provide both an extensive support system for the trees and specialised features for oxygen exchange. These specialised roots, called pneumatophores, allow the roots to obtain oxygen for respiration in the anaerobic surroundings. Prop roots (lateral extensions from the trunk into the sediment), knee roots (roots that arise from the sediment like a bent knee) and peg roots (finger like roots emerging from prevailing environmental conditions. Additionally, large air spaces called aerenchyma transport oxygen through to the submerged roots.
Mangroves also exhibit a specialised adaptation to reproduction called vivipary and cryptovivpary. In these cases, the mangrove bears propagules (where the seed germinates while attached to the ‘mother’ mangrove) rather than seeds, and nutrients are transferred to the propagule from the mother tree, providing a ‘head start’ and increasing their salinity tolerance. Once detached, the buoyant propagules can be dispersed long distances before settling in an appropriate environment.
Common species to the Gold Coast’s waterways include Avicennia marina: the grey mangrove, Aegiceras corniculatum: the river mangrove, Rhizophera stylosa: the red mangrove and less commonly the large-leafed orange mangrove, Bruguiera gymnorrhiza.
Mangroves are a valuable environmental asset. They are specialised to their habitat, protect our coastline from wave inundation and erosion and support a myriad of organisms that, in many cases, support the economic needs of our society through fisheries. The loss of mangrove forests worldwide is extensive and alarming. The protection of mangroves and their associated habitats is important across many coastal landscapes for the many values they provide.
(i) EJF (2006) Mangroves: Nature’s defence against Tsunamis— A report on the impact of mangrove loss and shrimp farm development on coastal defences. Environmental Justice Foundation, London, UK.
(ii) Blackwell, B. (2007) The ecoservice values for some of Australia’s natural coastal assets: How much are our coasts worth and what’s missing from the mosaic? Presented at the 2007 ANZSEE conference: Re-inventing Sustainability: A Climate for Change. 3-6th July 2007, Noosaville, Queensland.
(iii) Morton, R. M. (1990) Community structure, density and standing crop of fishes in a subtropical Australian mangrove area. Marine Biology, 105, 385-294.