Mangroves, amazing sentinels of coastlines


MANGROVES are a species of trees that are unique to coastlines, deltas and estuaries.
According to Floyd (1977), Cragg (1983) and Pajimans (1976), there are 33 known species of mangroves in Papua New Guinea coastlines, estuaries and deltas, 31 of which are found in the Gulf of Papua.
Mangroves are very unique and peculiar in the sense that they are few coastline dwelling plants that thrive very well in highly saline conditions and in huge numbers often in loose soil substrate in anaerobic conditions. They are also exposed to severe battering by storms, violent waves and tropical cyclones but often in briny conditions with loose and muddy soil substrate but have withstood test of time and nature. One such species is Rhyzopora stylosa, which is the most abundant mangrove amongst coastlines, often found inhabiting the high tidal zones of coastlines.
How are mangroves able to do that? They have various adaptations that enable them to do that.

Rhyzophora stylosa, the most abundant mangrove species in coastlines of PNG, in full bloom.

Physiological adaptation (internal)
Their leaves are highly salt-tolerant and have salt secreting glands. They can take in so much salt through their roots but can also secrete (remove) so much. In other words, their leaves are salt-tolerant as too much salt in organs can be very detrimental to their survival.

Morphological adaptation (external)
a) Pneumatophores (breathing roots) and lenticels
Mangroves have a special breathing root system called pneumatophores or breathing roots and lenticels that can carry out gas exchange whist inundated in water. The pneumatophores are often found protruding out of water like stilts when there is low tide. The lenticels and pneumatophores close tightly during high tides just like the stomata closing off during intense sunlight to prevent excessive water loss. This is how they are able to survive in waterlogged environments under anaerobic (oxygen-deficient) conditions using the breathing roots for gas exchange
b) Prop root system
Unique to mangroves, the prop roots are like two persons hugging with the arms wrapped around each other to support each other. They grow around each other (entangling and twisting) without growing downwards to support each other and stand upright unlike terrestrial plants that have a tap root system for anchorage and resilience.
This is because the soil and the environment they grow in is muddy and loose so they have this unique adaptation to overcome that difficulty and to survive.
This will be the scene when you walk into a mangrove swamp which is often a hard to get through. In fact, this has a unique role in ecology which will be explained in the next paragraph on the role of mangroves.

Behavioral adaptation
This adaptation of mangroves is the most baffling one. Mangrove seedlings have a sensor like pheromones that will sense tides alternating between low and high. The mangrove seedling will only drop off from the parent plant when there is low tide and the sand bank is exposed and will germinate straight away. This was very evident with the seedling of the Rhyzopora species, which is sword shaped or arrow shaped and falls from the pointed end’ enabling the seed to germinate/sprout and regenerate.
Those seedlings that drop and lie flat are swept away by the in-coming tides – a case of survival of the fittest or adapting to overcome the challenges by nature and survive as proposed by the famous naturalist, Charlies Darwin. This author was so convinced and awed when this was explained by then resident scientist Thomas Manawabie at Motupore Island, University of PNG’s research island just before Loloata Island Resort.

Avicena marina or grey mangrove fruits.

Ecological and biological functions of mangroves
They act as wave breakers and strong wind buffers. Mangroves can be equated as wave breakers and buffers against effect of strong winds, tides and waves. In doing so, they prevent coastal erosion and protect villages and habitats from erosion and tidal waves. Where mangroves have been cleared, there is massive coastal erosion with collapsing side banks and over time.
Some of these islands and atolls might become submerged aided by the looming effects of climate change.
Some of main causes of such mangrove habitat destruction along coastlines are:
Coastal development like wharves, jetties, dockyards and piers like Motukea and NapaNapa
Road constructions like what’s happening towards Taurama Bay
Firewood and timber for housing by coastal villagers.

Breeding grounds for fisheries (prawns, lobsters, crabs, fish)
Mangroves serve a very important biological and ecological role in that most aquatic organisms spawn (breed) where the water/wave velocity is non-existing or minimal, especially for detritus feeders like crabs, lobsters and prawns. The mangrove swamps provide an ideal environment for breeding (reproduction) ground for such and food source.
This is why you there is an abundance of prawns and lobsters in the deltas of the Gulf of Papua with the extensive river network bringing so much nutrients (food) to the detritus (decomposed organic matter) or filter feeders in our big river systems.
They also serve as a haven against high sea surface temperatures during the day time for marine organisms to seek refuge amongst the mangrove.
However, of concern is the fast disappearance of mangroves either through cutting for timber, woodchips or firewood or inundation by sediments from land use changes (plantation agricultural activities, mining, logging) taking place upstream or within catchments.
Increased sedimentation has the tendency to block of light for underwater plants to photosynthesise and make food and also reduce dissolved oxygen in water as highly turbid and muddy water has less dissolved oxygen than clean water.

  • The author is an environmental scientist. This article is intended to be used by teachers, non-governmental organisations and environmentalists to educate people on the importance of mangroves and their preservation.

Leave a Reply