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Coastal Wetlands

Wetlands are one of the Most Significant Ecosystems on the Planet

Wetlands make up 6% of the earth's surface.  They are lands which are periodically flooded and they are found on every continent except Antarctica  and in every climatic zone.  Wetlands can be classified under four main types:

  • Marine - around the coastline

  • Estuarine - in river estuaries

  • Riverine - in low lying river valleys

  • Palustrine - Inland freshwater marshes including those around the edges of lakes

Mangrove swamps are marine or estuarine wetlands found in subtropical or tropical areas.  They are tidal forests found in the zone between high an low tide called the inter-tidal zone, in bays or estuaries.

Mangrove Ecosystem and the Food Web

Mangroves convert sunlight to food energy by photosynthesis and hence provide a food source.  However, most of the animals do not graze directly on the mangroves.  Instead, they eat decomposed plant material produced in a slow process called the 'detrital food chain'.Litter (fragments of dead leaves, bark and fruits) fall from the mangroves.  After the soluble nutrients are leached out it is colonised by microscopic fungi and bacteria.

The resulting detritus (decomposed material) is eaten by small animals such as prawns and crabs. These animals excrete the undigested plant material which is then recolonised by fungi and bacteria.  The cycle continues until even the most resistant tissues are broken down.

Crabs are scavengers and detritus feeders. Mud whelks feed on surface films of algae and other micro-organisms. Mangrove snails feed on algae and other plant material. The detritus feeders, mud whelks and snails are then eaten by the larger fish and birds of the wetlands, including the egret, ibis, and spoonbill.

One square kilometre of grey mangrove forest contributes about 600 tonnes of leaf litter each year to the detrital food chain.

Mangrove Mud

The mangrove mud is rich and high in nutrients but is essentially anaerobic which means lacking in Oxygen. Only the first few centimetres of the surface mud are oxygenated by the aeration of burrowing animals.

In the black, smelly, oxygen-poor, lower layer of mud is found sulphur reducing bacteria which produce the sulphur smell of the mangroves. The sulphur contained in organic debris is reduced to hydrogen sulphide (rotten egg gas). The combination of hydrogen sulphide with iron compounds give the mud its characteristically black colour.

Mangroves grow in the inter-tidal zone. Most plants cannot survive in this environment because the water is too salty, the ground is too muddy and there isn’t enough oxygen.

How do Mangroves Survive?

Mangroves have adapted to the environment in which they live. These include:

  • Roots: Pneumatophores Aerial roots that stick out of the mud at low tide and allows the tree to breathe. They have the ability to change salt water into fresh water.

  • Leaves: The leaves provide protection against the heat of the sun and the damaging salt spray by their shiny and leathery nature.

  • Seeds: The essential feature of the seeds is that they have the ability to float and as they germinate on the tree, wherever they stop they will grow.

Value of Mangroves


  1. Form important barriers and buffer zones preventing erosion of coastal areas

  2. Places where silt is trapped and new land is built up and coastlines extend outwards

  3. Stabilise shorelines

  4. Act as sediment filters and help maintain coastal an estuarine water quality

  5. Are breeding and spawning grounds for many coastal species

  6. Alleviate flooding

  7. Provide essential wildlife habitats.

  8. Are a productive ecosystem

  9. Regulate Climate

  10. Trap Sediments and Nutrients

2. Place Where Sediment is Traped

The wetlands catch the sediments and nutrients as they wash along the surface.  As the runoff reaches the swamp, it slows down and the sediments, which includes nutrients, settle to the bottom.

4. Act as a Filter and Reduce Water Pollution

Litter in the water is trapped by the vegetation so the clean water can run into the rivers, thus, cleaner waterways result.  It acts as a water purifier.

6. Stopping Floods

The water is soaked into the ground and also used by plants and animals, thus reducing flooding.  They act as sponges to absorb runoff.

7. Essential Habitats

Wetlands provide habitats for a variety of plants and animals, particularly wild fowl.  They are important breeding grounds and wintering areas for migratory birds.

8. Productive Ecosystem

The biological yield of wetlands is very high.  There is no lack of moisture for growth.  In some areas of wetlands life is as abundant as in the same area of tropical rainforest.  Overall, 24% of the world's primary productivity of biomass comes from wetlands.

9. Regulating Carbon Dioxide in the Atmosphere

If areas of wetlands are not disturbed hey perform an important role in regulating climate.  Most wetland plants are very leafy, therefore, they are efficient users of sunlight.  They take carbon dioxide from the atmosphere and use it to make biomass.  This assists in reducing the amount of carbon dioxide in the atmosphere.

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