Introduction to Mangroves
Mangroves are plants that are found in the intertidal zone between the sea and land. They are mainly found in the tropics, subtropics and temperate coastal areas in estuaries, river banks and marine shorelines. Australia has the third largest area (11,500 km²) of mangroves in the world and about half of the world's mangrove species.
Characteristics of Mangroves
Mangroves have adapted to tolerate a range of salinity, temperature and moisture levels. They grow in soft mud and have a well-developed system to transport water, minerals, salts and sugars. The characteristics of mangroves determine the zone where their genera can be found parallel to the shoreline (Figure 1).
Mangroves have a variety of root systems including aerial and structural roots (stilt and cable) which provide stability and exposed breathing roots (Pneumatophores) covered with special breathing cells (Lenticels) to adapt to the muddy low-oxygen soil. (Table 1)
Figure 1: The zones in which the common types of mangroves in Australia grow (Source: N Duke and D Kleine, University of Qld 2007)
Mangroves have adapted to variable salinity in their environment by:
• Excluding salt through a filtration system in their roots e.g. Orange Mangrove
• Controlling the salt level by concentrating it in leaves and bark e.g. Milky and Red Mangroves
• Excreting excess salt through leaves e.g. Grey and River Mangroves
Mangroves reproduce in different ways depending on their environment by:
• Producing seeds that float to aid dispersion and to avoid overcrowding e.g. Orange and Milky Mangroves
• Producing live seedlings which when mature, drop into the water and remain inactive until they reach the soil e.g. Red, River and Grey Mangroves
Location of Mangroves
Mangroves grow in all mainland States and Territories that are on the coast. (There are no mangroves in Tasmania or the ACT).
The Importance of Mangroves
Mangroves significantly contribute to our environment in a variety of ways.
1) Ecology: Mangroves enrich the soil with their litter, provide nutrient rich habitats and breeding grounds for both marine and terrestrial animals (Figure 2).
Figure 2: The contribution of mangroves to the local environment.
2) Commercial value:
• Food: Around 80% of global commercial fishing relies on mangrove forests. They are an invaluable source of food for thousands of communities around the world. Mangroves are also important for oyster growers and as a source of pollen for bee keepers.
• Wood: Mangrove wood is extremely valuable as it is resistant to rot and insects. It can be used for building due to its strength and light weight.
• Tourism: Some countries offer snorkelling tours to view the variety of sea life in between the interwoven roots of the mangroves.
3) Coastal protection: The dense roots of mangroves create a barrier to protect the land from soil erosion and people from severe weather events such as cyclones and Tsunamis.
4) Filtration system: Mangroves act as large filtering systems that filter sediment thus protecting coral reefs and sea beds from heavy metals and other pollutants.
5) Carbon Emission Storage: Mangroves isolate carbon two to four times more than and can store three to five times more carbon emission per area than tropical rainforests thus reducing the effect of climate change.
6) Indigenous Value: Indigenous communities regard mangroves as an important resource for hunting and gathering and also for enhancing cultural awareness and traditional values and skills. They rely on mangroves for food and medicinal purposes, shelter and for constructing canoes and weapons.
Measures to ensure the survival of mangroves
A number of measures can be taken to ensure the survival of mangrove forests:
1) Decreasing mangrove exposure to waves
Strong waves and exposure to high water levels can destroy mangroves
Constructing rock, timber and sediment fencing to the mean-high water level about 3–5 meters in front of the river bank will help dissipate and absorb the wave energy and creating an area of still water before the mangrove growth zone. Successful examples of this treatment can be found in the Manning and Hasting river estuaries (Figure 3).
2) Improving coastal water quality
A number of factors affect coastal water quality:
• Mangroves commonly survive best at low water salinity. Both high and low salt concentration i.e. change from brackish to fresh water can damage mangroves.
• Large amounts of sediment in rivers from land deforestation, over grazing and cropping can can smother the mangrove forest’s filtering ability
• High levels of heavy metals such as copper, lead, mercury as well as accidental oil/fuel spills and effluent from sewage systems and drains can damage mangrove forests
Controlling sediment by preventing overuse of land adjacent to mangrove forests, avoiding the construction of dams and irrigation systems close to mangrove forests to optimise salinity levels and avoiding pollution from sewerage and industrial waste washing into mangrove forests.
3) Increasing mangrove forest area
Planting mangroves by propagating seeds in pots and transferring them into tidal zone nurseries which can be protected with fencing (Figure 4).
4) Protecting existing mangroves
• Fencing the intertidal zone thus preventing livestock access to mangroves
• Designing jetties, boat ramps, boardwalks and roads to encourage the public to access the shoreline without damaging the mangroves.
• Prohibiting tall structures and buildings close to the shoreline
Figure 3 and 4 sources
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