Wetlands, Swamps and Mangroves
WETLANDS: A wetland, as the name implies, is an area of land that is inundated, wet or moist throughout the year and a separate ecosystem as well. As mentioned earlier in this series of articles, an ecosystem is a collection of a number of species of fauna and flora.
It is also called communities, interacting together with their abiotic (non-living) environment. A wetland is an important part of the land that is between a terrestrial ecosystem and a real marine ecosystem, forming an interface between land and water.
Wetlands are an integral part of Sri Lanka’s unique ecological and biological diversity, and are vital habitats for a large variety of fauna and flora.
The Asian Wetland Directory records that there are 41 wetland sites of international importance in Sri Lanka covering a total of 274,000 hectares. They could be divided into three broad categories:
- Inland freshwater wetlands – rivers, streams, marshes, swamp forests, and villus.
- Salt water wetlands – lagoons, estuaries, mangroves, sea grass beds and coral reefs.
- Man-made wetlands – tanks, reservoirs, rice fileds and salterns.
Wetlands, found along the coast, are among the most productive ecosystems in the world. They play an important role serving not only as flood retention areas, but also as filters for sediments, nutrients and pollutants released to the environment, especially from the factories and other manufacturing processes.
Further, a wetland provides refuge for many species of vetebrates and invetebrates, and is a source of raw material for various industries.
Saltwater or coastal wetlands have two unique features. First, they occupy the interface between the land and the sea. Next, they are dependent on both fresh water and salt water for their existence.
Coastal wetlands are subject to the influence of the tides and when the tides rise there is an intrusion of salt water to these wetlands. With the tide receeding the toxic material that has collected, especially round the roots etc, are flushed out to sea. These wetlands get their fresh water from the rivers, streams and rainfall.
Along the 1,700 kilometer long coast line bordering Sri Lanka, mangroves, salt marshes, beds of sea grass, estuaries and coral reefs occur in abundance.
They are located in areas that are not exposed completely without water when the tide has gone out. Sea Grass beds are found mainly along the coast of Mannar. As mentioned in a previous article, the Dugong feeds mainly of this type of grass. Coral reefs are not exposed at low tide.
Sri Lanka is a signatory to the Ramsar Convention on Wetlands. Ramsar Convention is an international treaty with nearly 130 countries as signatories. The mission of Ramsar is ‘the conservation and wise use of wetlands by national action and international cooperation as a means to achieving sustainable development throughout the world’.
The Bundala National Park and the Annaiwilundawa Tank have been included as Ramsar sites in the Ramsar List of Wetlands of National Importance. Ramsar is the city in Iran where the first international wetland meeting was held.
Many wetaland ecosystems in Sri Lanka are being indiscriminately exploited at an alarming rate, for commercial, agricultural, residential and industrial purposes, and for dumping garbage and waste.
Although the proper management of these ecosystems is vital and urgent, it is a diffucult task due largely to the lack of specific legislation and there being no single responsible government agency or department.
Mangroves are woody trees or shrubs that grow in inundated coastal habitats.These areas are also called mangrove swamps.
Mangrove plants are found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action. A Mangal is an area covered by mangroves.
Mangrove plants are a diverse group which have been able to exploit this habitat because they have developed a set of physiological adaptations to overcome the problems of low levels of oxygen in the water (anoxia), salinity (the saltiness or the amount of salt dissolved in the water) and frequent tidal covering of the land and the roots of the plant.
Each species has its own capabilities and solutions to these problems. This may be the primary reason why, on some shorelines, certain mangrove tree species grow in groups due to variations in the range of environmental conditions across the intertidal zone.
Therefore, the mix of species at any location within the intertidal zone is partly determined by the tolerances of individual species to physical conditions, like tidal inundation and salinity.
Plants, called hydropahytes or just wetland plants, specifically adapted to the reducing conditions presented by such soils can survive in wetlands, whereas species intolerant of the absence of soil oxygen, called “upland” plants, cannot survive.
There are just over 25 species of mangroves recorded in Sri Lanka. Some of the common mangrove plants are Rhizophora mucronata S. Kadol, Bruguiera gymnorhiza, S. Malkadol, Sirikanda, Ceriops tagal S. Punkanda or Rathugas, Sonneraata caseolaris S. Kirilla, Avicennia marina S Manda or Mandagas, Aegiceras corniculatum S. Heenkadol or Averi kadol, Acanthus ilicifolius S. Mulli or Katu Ikikili, Excoecaria aggalocha S. Thela, Xylocarpus grantum S. Mutti Kadol, Nypa fruticans, S, Gin Pol, Lumnitzera racemosa, S. Bariya. There do not seem to be any popular English names for these mangroves.
There are some other species of plants called mangrove associates. Mangrove associates are those plants that are found in mangroves as well as in fresh water swamps. An example of this is the popular Kaduru (Cerbera mangha).
Mangroves also provide habitats for wildlife, including several commercially important species of fish and crustacea and, in at least some cases, export of carbon fixed in mangroves is important in coastal foodwebs.
Amongst the animals found in the mangroves are crabs, lobsters, clams and prawns. Most of the crabs belong to the family Grapsidae or Ocypodidae. They are semi-terrestrial and live in burrows which invariably terminate below the water table.
When they come out of their burrows, they can remain on land for a long time because their branchial chamber can be kept moist for a long time. Burrowing is an adaptation of many mangrove animals.
The Mudskipper is a fish unique to mangroves. Its eyes, gills, fins and the tail are modified for a terrestrial life. The mudskipper uses its pelvic and pectoral fins to skip along the mud and to climb the mangrove plants.
The mangroves have many uses. Several species of fish, prawns, crabs and molluscs are harvested from mangroves.
The timber from many species of mangroves are used for fuelwood, beams and poles for structures, for the construction of fish and prawn traps, materials for boat building, furniture and as handles for tools.
The bark of some species is used to make dyes for fishing nets. Even some of the leaves are used as green manure.
A large extent of mangroves in this country have been lost or degraded due to prawn (shrimp) farming, construction of salterns, expansion of villages and urban areas.
At one stage prawn farming along the Puttalam coastline expanded in an uncontrolled manner, mainly due to political interference and the non-implementation of the law by the Police.
Mention is made regularly that mangroves act as a buffer against erosion, the surge of storms and tsunamis. While there is some buffer effect on the height of waves and energy as seawater passes through mangroves, their capacity to ameliorate high energy events like storm surges and tsunamis is limited, since these trees typically inhabit areas of coastline where the wave energies are low.
If you look at a river running through mangroves, it will be noticed that they wind through mangrove areas and actively erode the outer sides of all the river bends, just as new stands of mangroves appear on the inner sides of these same bends.
Because of the limited availability of freshwater in the salty soils of the intertidal zone, mangrove plants have developed ways of limiting the amount of water that they lose through their leaves.
They can restrict the opening of their stomata (small pores on their leaf surfaces which exchange carbon dioxide gas and water vapour during photosynthesis) and also have the ability to vary the orientation of their leaves.
By orienting their leaves to avoid the harsh mid-day sun, mangrove plants can reduce evaporation from their leaf surfaces.
The biggest problem that mangroves face is the uptake or absorbption of necessary nutrients. Since the soil that mangroves live in is perpetually waterlogged, there is not much free oxygen available.
At these low oxygen levels, anaerobic bacteria proceed to liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane.
This is what gives mangrove’s their particularly pungent odour and also make it a hostile environment to most plants. Since the soil is not particularly nutritious, mangroves have adapted by modifying their roots.
Prop root systems allow mangroves to take up gasses directly from the atmosphere and various other nutrients, like iron, from the otherwise inhospitable soil. They, quite often, store gasses directly inside the roots so that they can be processed even when the roots are submerged during high tide.
In this harsh environment mangroves have evolved a special mechnanism to help their seeds and seedlings to survive. All mangroves have buoyant seeds that are suited to dispersal by water.
Unlike most plants, whose seeds germinate in the soil, many mangrove plants are viviparous, i.e., their seeds germinate while still attached to the parent tree.
Once germinated the seedling grows either within the fruit (e.g. Acanthus, Avicennia and Aegiceras), or out through the fruit (e.g. Rhizophora, Ceriops, Bruguiera and Nypa) to form what is called a propagule (a seedling ready to go), which can produce its own food via photosynthesis.
When the propagule is matured it drops into the water where it can then be transported to great distances. Propagules can survive desiccation and remain dormant for weeks, months, or even over a year until they arrive in a suitable environment.
Once a propagule is ready to root, it will change its density so that the elongated shape now floats vertically rather than horizontally.
In this position, it is more likely to become lodged in the mud and root. If a propagule does not root, it can alter its density so that it floats off again in search of more favourable conditions.
Mangroves support unique ecosystems, especially on their intricate root systems. In areas where roots are permanently submerged, mangroves may be host to a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substrata for anchoring while they filter feed. This same root system also helps prevent coastal erosion.
As tidal water flows through the root system, it is slowed substantially enough so that it deposits its sediment as the tide comes in, and the return flow is kept slow as the tide goes out to prevent resuspension of some of the finer particles. As a result, mangroves can build their own environment.
A marsh is another type of wetland. A swamp has grasses, rushes, reeds, sedges and other herbaceous plants growing in shallow water. A marsh, which is an open area, is different from a swamp. A swamp is dominated by trees rather than grasses and low shrubs.
The water in a marsh can be fresh, brackish or saline. Marshes on the coast maybe estuaries of rivers and waterways. Estuarine marshes are often based on soils consisting of sandy or muddy bottoms. Marshes are very important as habitats for certain species of wildlife. They are also breeding grounds for a wide variety of animal life.
The point at which rivers enter the sea are known as estuaries. The tide ebbs and flows into these estuiaries bringing salt water which mixes with the fresh water of the river and creates what is known as brackish water.
These estuaries have their own ecosystems. The better known estuaries in Sri Lanka are the Kala Oya estuary, which is relatively unspoilt, and the Madu Ganga and Bentota estuaries.
Lagoons are basically bays along the coast, which have a low sand bank along its mouth with a seasonal or permanent outlet to the sea. There maybe more than one outlet in a lagoon. Lagoons have slat water or brackish water.
These lagoons also have within them mangroves, mud flats and sea grass beds. The better known lagoons in this country are the Negombo Lagoon, the Bundala Lagoon, the Mundel Lagoon and the Kalametiya Lagoon.
By absorbing the force of strong winds and tides, wetlands, to a certain extent, protect terrestrial areas adjoining them from storms, floods and tidal damage.The plants in wetlands help to filter pollutants in the water.
Fresh water marshes are often on flood plains of rivers. The Flood Plains National Park is a flood plain of the Mahaweli River. Intertidal wetlands provide an excellent example of invasion, modification and succession. The invasion and succession process is the establishment of sea grasses.
These help to stabilise the sediment and increase the rate of trapping sediment. The trapped sediment gradually develops into mud flats. Mud flat organisms then become established and encourage other life forms changing the organic composition of the soils.
The mangroves establish themselves in the shallower water upslope from the mudflats. Mangroves further stabilize sediment and over time increase the soil level. This results in less tidal movement and the development of salt marshes.
This changing process is called succession. The salty nature of the soil means it can only be tolerated by special types of grasses e.g. rushes and sedges. The species diversity also changes in each succession.
We have made large-scale efforts to drain or fill in wetlands for development. They are often used for everything from agriculture, fisheries and housing to parking lots. This is because the economic value of wetlands has only been recognised recently.
For instance the shrimp that breed in salt water marshes are generally harvested in deeper water. However since the 1970s, more focus has been put on preserving wetlands for their natural function-sometimes also at great expense.
We can maximize the area of healthy, functioning intertidal wetlands by minimising their impacts and by developing management strategies that protect and, where possible, rehabilitate those ecosystems that are at risk.
We should also remember that since coastal wetlands provide resources for both subsistence and commercial economies, their sustainable management becomes important to both man and the environment.
There must be a dynamic mangement of these coastal resources with the participation of the state and the communities that are dependent on the coastal wetlands.