The term mangrove refers to salt-tolerant species of tree or shrub which grow on sheltered shores and in estuaries in the tropics and some sub-tropical regions. There are about 60 species which occur exclusively in this habitat, and many non-exclusive species. Mangroves are outstandingly adapted to growing in sea water, which they desalinate by an ultrafiltration process. Mangrove roots typically grow in anaerobic sediment and receive oxygen through aerating tissue which communicates to the air through small pores (lenticels) on the aerial roots and trunks.
The calm waters in the forests are ideal breeding and nursery grounds for young fish and shrimps, while the aerial roots, lower trunks and mud surface usually support a varied fauna of oysters, snails, barnacles, crabs and other invertebrates. The upper part of the mangrove trees is an essentially terrestrial environment with a fauna of birds, mammals and insects. Mangroves are affected by the freshwater and nutrient supply which they receive from their catchment area, and on the other hand have a strong influence on the adjoining coastal waters and associated ecosystems such as coral reefs, seagrass beds and tidal marshes. For example, they trap and stabilize sediment which might otherwise limit the growth of corals.
HUMAN USE OF MANGROVES
The mangrove ecosystem has a utilitarian value based on both ‘goods’ (products) and ‘services’ of benefit to humans.
Products from mangrove trees include logs, fuelwood, charcoal, wood-chips, paper pulp, scaffold poles, piling and construction material, stakes for fish traps and fishing platforms, railway sleepers, wood for furniture making and carvings, material for roof thatching, bark for tannin, medicinal products, sugar, alcohol, acetic acid and dyes. Exploitation to obtain products for domestic use has occurred since the beginning of history. Modern exploitation has increased to industrial levels, and the application of proper forest management practices based on cutting cycles and specially designed tree-extraction systems has been necessary in order to maintain a sustainable yield.
In addition to plant products, the mangrove ecosystem can provide a sustainable yield of fish and shellfish if it is not over-exploited.
++ stabilization and protection of shorelines;
++ filtering and trapping of water-borne pollutants;
++ provision of nursery and feeding grounds for numerous species of finfish and prawns, and habitat for crabs and molluscs;
++ provision of nesting sites for sea and shore birds; and l provision of resources for tourism and recreation.
Non-sustainable uses lead to loss of the mangrove habitat, and associated losses of shoreline, organic matter production and species dependent on the habitat and mangrove-based food chains. Mangrove forests may be felled for uses such as aquaculture ponds, salt pans, agricultural use including rice fields, airport and road construction, port and industrial development, resettlement and village development. Canalization and changes in drainage associated with these uses modify the natural water supply and may be detrimental to remaining mangroves.
From the viewpoint of oil spill response, some of the above activities are vulnerable to oil and need to be considered along with the mangrove habitats.
Aquaculture and the use of salt ponds might be particularly affected. Appropriate responses are also needed for port and harbour facilities and associated navigation channels. Aquaculture ponds for fish and crustaceans (prawns and shrimps) have traditionally been excavated in mangrove areas in some parts of the world, notably south-east Asia and South America.
During recent decades, increased aquaculture of crustaceans has been an economic success worldwide, representing, nevertheless, a major problem for mangrove conservation. This is because of the large scale of habitat conversion, with associated changes in the natural tidal flow patterns and the generation of acid conditions (by oxidation of the disturbed sulphide-rich soils) which eventually affect the larvae. These problems can lead to a vicious cycle of mangrove clearing and abandonment called ‘shifting aquaculture’.
Brine evaporation ponds required for salt crystallization are developed mainly by clearcutting mangroves on arid or semi-arid coasts, and significant areas have been destroyed in this way.
In very dry regions, solar evaporation is sufficient to cause crystallization, but under more humid climatic conditions the brine has to be boiled over fires using mostly mangrove wood as fuel, which increases the pressure on the local mangrove resource.
The establishment and expansion of deep-water ports and associated waterways has led to direct loss of mangroves, and erosion. In particular, port facilities located upstream on tropical rivers can have a strong influence on downstream mangrove areas through the permanent dredging activities (and associated dumping of dredge spoil) required to maintain the navigation channels. Dredge spoil may physically smother mangroves, and there may also be an increase of acidity due to oxidation of the sulphide-rich sediment.
BIOLOGICAL IMPACTS OF OIL POLLUTION: MANGROVES
International Petroleum Industry Environmental Conservation Association
MANGROVE FLORA AND FAUNA
Mangrove fauna is generally represented by acquatic, semi-acquatic and terrestrial communities adapted at stress conditions. As many as 8 species of mammals, 53 species of birds, 7 species of Reptiles 3 species of Amphibian, 253 species of fish, 13 species of polychaetes, 410 species of Arthropods and 53 species of Meiofauna are reported from the mangroves of Andaman & Nicobar Islands
In general, the following three conspicuous zones are identified.
1. Proximal Zone
This zone is towards water front, subject to regular tidal effect where intensity of soil accumulation and inundation is a continuous process. The mangrove species in this zone are specially adapted with stilt roots, prop roots for stability and anchorage. Main species with these features are Rhizophora apiculata and Rhizophora mucronata. On rocky and coral reef substrata, Avicennia, S. Caseolaris are also found. Both Avicennia and Sonneratia produce pneumatophores.
2. Middle Zones
Above the Rhizophora/ Avicennia line luxuriant group of Bruquiera gymnorrhiza B. Cylindrica, Lumnitzera racemosa, L. littoralis, Ceriops tagal and Aeguiceras corniculatum occur. Soil formation in the Core Zone is congenial for mangrove growth wherein the trees attain a height of 10-15 metres in compact blocks. Ceriops and Bruguiera develop a strong hold fast in the form of knee roots or bent roots as a special adoption for supporting the erect bole.
3. Distal Zone
Towards island area mangroves like Excoecaris agallocha, trees like Heritiera littoralis and xylocarnus spp. In association of Phoenix paludosa, Nypa fruticans, Acanthus ilicifolius and ferns like Acrostichum aureum occur, the latter occurring precariously in thick patches. Both Heritiera and Xylocarpus produce buttresses and help in containing soil in their cavities. Generally the salinity is on lower side in this Zone occurring towards hill sides where run off of fresh water is for a prolonged period. The duration of tidal submersion is low in this Zone compared to water front mangroves.
SIGNIFICANCE OF MANGROVES :
- Being living resources, mangroves are self maintaining and renewable. For example as a coastal protection barrier, Mangroves maintain themselves at no cost and in the vent of tropical storm, the damages sustained will be self repaired without cost. Similarly both the direct and indirect harvests of products from mangroves are renewable yet, the mangroves resource is renewable only if the ecological processes governing the systm are maintained.
- The leaf fall from the mangrove trees also contributes substantially to formation of detritus which supports coastal fisheries.
- Mangroves constitute a unique habitat for wild animals and birds. They provide nesting breeding places for birds.
- Mangroves provide breeding grounds for fish and other marine animals.
- Mangrove serve as potential recreation site for fishing, boating, bird watching, sight seeing and photography. This has special significance for the Andaman & Nicobar Islands having tourism potential.
- Mangroves provide vast scope for scientific and socio-economic studies.
Extraction of mangrove fuel and land development including agriculture has contributed to extensive damage of mangroves in some areas. Although damage to mangrove ecosystem from insecticides and pesticides used in agriculture and their run off, deliberate and operational discharges from ships, oil spills due to accidents and industrial outflows are not reported in the island, but these have caused extensive damage to mangroves ecosystem in other areas. Mangroves are traditionally used for aquaculture. There is tremendous demand for mangrove fish in international market. The prospectus of potentially quick economic games have caused a rapid rate of clearing of mangroves for development of brackish waterfish products. Excessive exploitation of mangroves could result in lower litter production and consequently could effect productivity of coastal fisheries.
CONSERVATION AND MANAGEMENT OF
MANGROVES IN A & N ISLANDS
Chengappa, in 1951 had estimated that Bruguiera species constitute about 26 to 30% of mangrove area of South and Middle Andaman Division. He prescribed clear felling and selection- felling of Bruguiera trees along with other natural mangrove trees. Although no systematic working of mangroves was followed in the subsequent years a small area was worked in North and South Andaman under selection system by removal of marked trees under the shelter wood system by removal of marked trees under the selection wood system, in which 40 well formed Rhizophora and Bruguiera poles per hectare were left out as standards uniformly spaced. Precautionary measures were also taken to leave protection belt of 20 metres width along the main creek on either side and 10 metres width along the smaller creek on either side to guard against sea erosion. Brush wood barriers across the creeks the were also left to hold the seeds of Mangrove species from washing away along with the tides.
In the past the mangrove forests were worked for extraction of fuel wood around Port Blair to meet the domestic requirement of the people. The Andaman Timber Industry and Chatham Power House were also using mangrove fuel wood for running their boilers. However, the mangrove fuel wood extraction and sale of mangrove has been totally stopped in the islands.
The limited extraction in the past of mangroves for fuel wood and poles from Government forest has not caused any damage to the Mangroves. But in the revenue areas, the destruction of mangroves is conspicuous and at places the area has been reclaimed for agriculture as well for settlement. The extent and condition of the crop and the threat under which such mangrove area lie presently is required to be assessed.
THE MANAGEMENT ACTION PLAN DEVELOPED BY THE DEPARTMENT OF ENVIRONMENT & FORESTS INCLUDES :
1. Survey and demarcation of mangrove area including mapping of degraded mangroves areas using remote sensing as well as ground survey.
2. Regular patrolling in creeks to check possible destruction of existing mangroves and protecting rare species of mangroves found in these islands.
3. Ecological restoration of degraded mangroves by raising nurseries and replenishing degraded mangrove areas through artificial regeneration.
4. Publicity and awareness campaign through film shows, organizing seminars, nature camps, distributing publicity materials etc.
Text based on : ‘Mangroves of Andaman and Nicobar Islands’ by J.C.Dagar, A.D.Mongia and A.K.Bandhopadhya, 1991.·
Ecology of Mangrove Forests
The term mangrove refers to salt-tolerant species of tree or shrub which grow on sheltered shores and in estuaries in the tropics and some sub-tropical regions.
Human Use of Mangrove
Products from mangrove trees include logs, fuelwood, charcoal, wood-chips, paper pulp, scaffold poles, piling and construction material, stakes for fish traps
Non-sustainable uses lead to loss of the mangrove habitat, and associated losses of shoreline, organic matter production and species dependent on the habitat
maintenance of ‘reservoirs’ for natural restocking of adjacent exploited areas; protection of breeding and feeding areas important for fisheries;
Ecological Importance of Mangrove Trees The example of Bruguiera gymnorrhiza
Mangrove forests most important coastal ecosystems in the world in terms of primary production and coastal protection.
Natural Recovery Of Amazonian Mangrove Forest As Revealed By Brachyuran Crab Fauna
Due to the enormous influence of the Amazon River, accretion and erosion processes alternate in rapid succession on the North coast of South America.
Red Mangrove Eradication in Hawaiian Wetland,
Alien red mangrove (Rhizophora mangle) and pickleweed (Batis maritima) are major invasive plants
Rehabilitation Of Oil- Damaged Mangroves
When mangroves have been killed by oil there is often a great interest in rehabilitation of the forests based on a desire both to re-establish the important mangrove ecosystems
Fate and Effects of Oil
Oil slicks enter mangrove forests when the tide is high, and are deposited on the aerial roots and sediment surface as the tide recedes.
Oil Spill Response
It is generally agreed that mangroves are particularly sensitive to oil and that they are priority areas for protection.
The Restoration, Conservation And Management Of Mangrove
Mangrove wetlands in the coastal states of East Coast of India could be restored and better protected with the involvement of local user communities.
Mangrove carbon sequestration in the Florida Everglades
Mangrove forests represent one of the most geochemically and biologically active biomes yet at present no unified modeling framework exists to investigate processes governing carbon sequestration.
Mangroves of the Caribbean
Mangrove swamps are unique biological environments which occur where land and sea come together. The term “mangrove” refers to vascular plants that have developed physiological mechanisms for living in shallow seawater.