Mangroves are sometimes seen as muddy swamps infested with mosquitoes and crocodiles. Removing mangroves was once seen as a sign of progress. So, what is the point of preserving them?
Mangroves systems are critically important habitats for a wide range of species, providing nursery, feeding and refuge areas and underpinning coastal food webs that support many commercial and non-commercial species.
Mangroves are an asset
Marine plants are a vital community asset supporting important commercial, recreational and indigenous fisheries. Mangroves are an integral part of this community resource.
Mangroves protect the coast by absorbing the energy of storm-driven waves and wind. The only two yachts undamaged by Cyclone Tracy in Darwin in 1974 were sheltered in a mangrove creek. In 2006, mangroves protected vessels and the coastline during Cyclone Larry in far north Queensland. The damage bill would have been much higher if it wasn't for the existence of intact mangrove forests. As well as providing a buffer for the land, mangroves also interact with the sea. Sediment trapped by roots prevent silting of adjacent marine habitats where cloudy water might kill corals or smother seagrass meadows. In addition, mangrove plants and sediments have been shown to absorb pollution, including heavy metals. Mangroves are also very effective at storing carbon.
Worldwide, vast tracts of mangroves have been destroyed to make way for unsustainable development; Queensland still has relatively large areas of Australia’s tallest and best developed mangroves.
The Report on the Effects of the January 2011 Flood on the Mangrove Communities Along the Brisbane River summarises the effect of the January 2011 flood on the mangroves between Breakfast Creek and the Bremer River in South East Queensland.
Protection and management
All marine plants are protected under Queensland law through provisions of the Fisheries Act 1994 which is managed by the Department of Agriculture, Fisheries and Forestry. The destruction, damage or disturbance of marine plants without prior approval from Fisheries Queensland is prohibited.
Number of species
Worldwide there are about 65 recognised species of mangrove plants. Up to 39 mangrove species and hybrids are known to occur in Queensland, although figures can change as the definition of a mangrove is not clear. Some plants, such as cottonwood, are not universally regarded as mangrove.
Some wetland species (Avicennia integra, Avicennia marina var. australasica, Excoecaria agallocha var. agallocha, Excoecaria agallocha var. ovalis, Acanthus ebracteatus, Acanthus ebracteatus subsp. ebarbatus) are possibly found only in Australia while others occur widely throughout the Indo-West Pacific region.
The north-east coast of Australia is home to the greatest diversity of mangroves and associated plants. This region was close to the centre of origin and dispersal of mangroves. The climate is similar to that under which they first evolved, and the sheltered shallow waters of numerous estuaries are ideal for growth.
The distribution of mangroves has been mapped through the Queensland wetland mapping.
A teaspoon of mud from a north Queensland mangrove forest contains more than 10 billion bacteria. These densities are among the highest to be found in marine mud anywhere in the world and are an indication of the immensely high productivity of this coastal forest habitat.
A square meter of mangrove plants produces about 1kg of litter per year (mainly leaves, twigs, bark, fruit and flowers). Some of this is eaten by crabs, but most must be broken down by bacteria and fungi before the nutrients become available to other animals. Dividing sometimes every few minutes, bacteria feast on the litter, increasing its food value by reducing unusable carbohydrates and increasing the amount of protein—up to 4 times on a leaf that has been in seawater for a few months. Fish and prawns eat the partly decomposed leaf particles. They, in turn, produce waste which, along with the smallest mangrove debris, is taken up by molluscs and small crustaceans. Even dissolved substances are used by plankton or, if they land on the mud surface, are browsed by animals such as crabs and mud whelks.
This process is not confined to the mangroves. While some litter is recycled on the spot, this system is one of the few to export much of the organic matter it produces. Studies of the mangroves at the northern end of Hinchinbrook Island have shown that they export more than 12,500t of litter a year into Great Barrier Reef waters. This material is deposited over 260km2 of seabed. Here bacteria densities are almost as high as those in the mangrove mud and they do much the same job, breaking down the litter to be consumed by bottom-dwelling fauna, e.g. prawns and fish.
The seafood industry is the fifth largest primary industry in Queensland, with an annual commercial catch worth several hundred million dollars. Many commercially caught fish and prawns depend directly on mangroves at some time in their lives or feed on food chains leading back there. Since those species making up the remainder of the catch probably also owe much to nutrients exported from the mangroves, these coastal forests can be seen as one of our major assets.