Mangroves [47]

Field Notes For Paddlers

By Richard Birdsey

One of my greatest pleasures is gliding past shores dense with mangrove trees or quietly sitting under the canopy and listening to a symphony of dripping water, popping crabs and the splashes of feeding fish. Having studied mangroves at university I thought I’d tell you a bit about the ecology of mangroves and their crucial role in maintaining the balance of coastal marine ecosystems. I’d also like to suggest how you, as a user and beneficiary of coastal areas, can help to protect these vitally important, but often misunderstood, forests.

What Are Mangrove Forests?

Mangrove forests are wetlands (sometimes unkindly called swamps) which are classified as ‘forested intertidal wetlands’. They are a complex mix of terrestrial (land), freshwater and marine ecosystems. While mangroves are the dominant canopy trees, many other species of terrestrial trees, shrubs and grasses are also found in Australian mangrove forests, e.g. saltbush, melaleucas, hibiscus, reeds and numerous species of grass, moss and lichens. Under the trees live a hugely complex marine community ranging from minute bacteria, algae, insects, invertebrates, fish and birds. All of these plants and animals are fed, sheltered, housed and protected by mangrove trees.

Taxonomic studies suggest there are about 25 families of mangroves containing approximately 75 species of trees globally. Five species of mangroves are found in New South Wales:

  • the Grey Mangrove (called the White Mangrove in South Australia and Victoria) – Avicennia marina,
  • River Mangrove – Aegiceras corniculatum,
  • Milky or Blind-your-eye Mangrove – Exoecaria agallocha,
  • Red or Stilted Mangrove – Rhizophora stylosa and
  • the Large Leafed Mangrove – Bruguiera gymnorrhiza.

Mangroves inhabit a wide range of coastal and estuarine environments. Grey and River mangroves are found along the entire length of the NSW coast while the other three species are restricted to parts of northern NSW. They prefer low-energy environments not constantly exposed to powerful waves. Mangroves are highly zonal, meaning different species will inhabit specific habitats like fringing islands, shorelines, sheltered bays and estuaries. For instance, you will find different species of mangroves living at the entrance to Port Hacking compared to those up in the estuarine reaches near Audley.

Estuarine mangroves are highly adapted to live in one of the toughest environments imaginable, immersed and emersed daily in salt water. Mangrove forests are pretty tough places to conduct research. I have fond memories of stinking thigh-high sucking mud, squadrons of sandflies and wasps, wild pigs and, where I worked in Queensland, stifling humidity and crocodiles. Ah, the good old days…

Where Did They Come From?

Biogeography, as the name suggests, is the study of the geographic distribution of plant or animal species. By plotting the number and types of species found in different parts of the world we can start to understand the origins of plants and animals and how they spread around the globe. Mangroves are thought to have originated in the area now known as Indo-Malaya and, borne on powerful ocean currents, spread westward. Forests sprang up in India and East Africa. Fossils suggest this pattern of colonisation started during the Cretaceous Period, between 66 and 23 million years before present. Seeds also travelled eastward and lush forests developed in the Americas and Caribbean. Some time later seeds moved south, colonising Australia, New Zealand and the South Pacific.

The whole story of their global spread is vastly more complex than this, particularly when continents were appearing, disappearing and moving around at the same time seedlings were drifting around in ever-changing sea currents.

Why Are Mangrove Forests so Important?

Brimming with life, mangrove forests are biological powerhouses. Vast amounts of energy in the form of biomass (leaves, algae, animal flesh, etc), sediment, chemicals and nutrients are stored or transferred to terrestrial, marine and freshwater food webs. The fuel powering these food webs is a constant stream of mangrove leaves, bark, fruit, flowers and seedlings raining down into the surrounding mud and water. Studies of tropical mangroves suggest each tree creates about one kilogram of tree litter per square metre per year. This equates to about 600 tonnes of material coming from a square kilometre of forest each year – a huge amount of food!

Once a leaf falls to the mud it is quickly colonised by bacteria, fungi, filamentous algae and other microorganisms. One teaspoon of tropical mangrove mud can contain 10 billion bacteria. Mangrove leaves have a thick, waxy layer used to reduce the loss of precious salt-free water in the plant’s tissues. Crabs and other benthic (bottom living) invertebrates cannot eat the leaf so it must be partially decomposed by the bacteria to make it more palatable. The microorganisms break down unusable carbohydrates in the leaves increasing protein levels of the leaf up to four times. However I wouldn’t recommend decomposing leaves for a quick salad as you paddle past – stick to bananas!

The decomposing leaves and their delicious protein-rich bacteria and fungi are then eaten by crabs, molluscs (snails, clams, pipis, etc), prawns, fish, etc. The herbivores are in turn eaten by fish, birds and other carnivores. The wastes left by predators (food scraps, crap, etc) are broken down by the microorganisms and herbivores and around the energy goes again.

A huge amount of decomposing plant material is carried out into the open sea where it supports thriving plankton communities. Prawns and other invertebrates feed on the plankton and pelagic (plants and animals that live in the water column – opposite of benthic) and benthic fish feed on them. The mangrove nutrients are vitally important for coastal fish populations and commercial species in particular. Studies of Queensland commercial fisheries suggest that up to 80% of the $300 million worth of commercially caught fish and prawns there depend on mangrove forests either as a source of food, spawning grounds, nurseries for juveniles, etc.

Other than a source of food and nutrients they also play an important role in providing habitats for many species of birds and animals, shaping our estuaries and rivers, flood mitigation, bank stabilisation and filtering out water pollutants – particularly nitrates and phosphorus. For humans they also are a limitless source of raw materials (e.g. timber), medicines, shelter, tools, food recreation, education and research. However, the key to this treasure trove is conservation, careful management and sustainable use. But, as discussed in later sections, we are destroying mangrove forests at a horrifying rate.

How do Mangroves Survive in Salt Water?

As mentioned above, mangroves live in one of the toughest environments possible. Living in an intertidal zone presents mangroves with problems terrestrial plants don’t have. How can the plant get fresh water when surrounded by salt? How do the roots get oxygen if they are buried in anoxic (oxygen-less) sediments? How do mangrove seeds take root if the tide can easily pick them up and carry them away? These wonderful trees have come up with a number of unique adaptations to allow them to survive and thrive.

“Pass The Salt” – Literally.

Mangroves have developed three systems to protect themselves from salt: exclusion, excretion and concentration. Excluders have special tissues in their roots and lower stems (areas inundated by the tide) which act as a barrier to salt, prevent it from entering the plant’s tissues. All species are excluders to some extent. ; Some species such as the Stilted Mangrove can exclude up to 90% of the salt! Traditional coastal cultures know that fresh water can be obtained from mangroves. Excreters, such as the Grey Mangrove, use special glands on their leaves to concentrate the salt and allow it to evaporate off the surface of the leaves. These glands are thousands of times more effective in concentrating and removing salt than our sweat glands. You may see mangrove leaves covered in a film or crust of salt as a result of this system. The last method, concentration, is where salt is moved to old leaves, flowers, bark or fruits which then drop off, taking the concentrated salt with them.

“Rooted” – Again Literally.

Mangroves live in shifting environments where tides and floods constantly move the mud in which they live, destabilising the trees. To counter this problem mangroves have developed extensive root systems to keep them firmly rooted in the mud. Some species such as Rhizophora use prop or stilt roots coming out of their trunks to provide additional stability.

The mud also creates another unique problem for mangrove roots: lack of air. Mangroves live in gooey sediments that are anoxic – pretty nasty stuff. As oxygen from the leaves cannot be quickly transported down into the roots, ‘peg roots’ or pneumatophores (Greek: pneuma = ‘air’ or ‘breath’: phoros = ‘bearer of’) are poked up through the surface of the mud. These snorkels absorb oxygen from the air and water and transfer it directly into the roots that stay close to the surface. You can trace the root systems of mangroves by following the lines of pneumatophores sticking out of the mud.

Seeding The Sea

How does a plant reproduce when the ground around it is flooded by salt water twice daily? Mangroves have got around this problem by developing a system of reproduction called viviparity. Seeds stay on the tree longer and store nutrients needed for rapid establishment and growth once they leave the ‘parent’ tree. Fully developed seedlings either drop from the parent tree and can swiftly take root in the mud below or can be transported long distances by currents and establish new colonies elsewhere. Some species have seedlings that can survive after a year of floating in the sea. The down side of viviparity is that parent trees must ‘invest’ a great deal more energy in producing fewer numbers of developed seedlings.

Mangroves Under Pressure

Mangroves are obviously a tough species, yet mangrove forests in Australia and overseas are under tremendous stress. There are an estimated 220,000 square km of mangrove forests worldwide, and about 110 square km in NSW, 41 square km in Victoria and 230 square km in South Australia. Despite this relatively small area NSW mangrove forests are under huge pressure.

Mangroves have often been seen as ‘cheap’ land and consequently been the target of many poorly designed developments. The rubbish tips of many coastal towns is either in or adjacent to the local mangrove swamp. Poorly planned and designed roads, flood mitigation works, storm water systems and other engineering structures can change the movement of water around the mangrove forests resulting in ‘environmental stress’ which eventually leads to the death of large areas of trees and subsequent collapse of estuarine food webs. Local and state governments are starting to plan civil engineering works better to protect mangrove forests but for many areas it is already too late.

You have an important role in protecting these valuable assets. Take care when moving around in mangrove forests – particularly if you are walking around. Pick up the rubbish left by other slobs. Report speeding jet or water skiers hooning around estuarine waterways. Teach others (especially children) about the importance of mangroves. Lobby your local council and government agencies that may be constructing roads or other infrastructure through your local area or making decisions about developments that may interfere with mangrove forests.

Ultimately we are all responsible for ensuring the future of these amazing ecosystems.