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INTRODUCTION 1.1. Mangrove Ecosystem Mangrove Distribution in India Adaptations of mangrove plants Main Uses and Functions of Mangroves Present status 1.2. Organic carbon
INTRODUCTION 1.1. Mangrove Ecosystem Mangrove Distribution in India Adaptations of mangrove plants Main Uses and Functions of Mangroves Present status 1.2. Organic carbon Aquatic system In the mangrove ecosystem Nutrients in the mangrove ecosystem 1.3. Aim and Scope of the Present Study *The ongm of the name 'mangrove' is not certain. It could be a combination of the Portuguese 'mangue', meaning an individual mangrove tree, with the English 'grove', although early versions were 'mangrove' and 'mangrave'. The term 'mangrove' may have been derived from a combination of the Malay word. manggi-manggi', for a type of mangrove tree (A vicennia) and the Arabic' el gurm', for the same, af 'mang-gurm'. As a word, it can be used to refer to a species, plant, forest or community. Mangroves are the rainforests by the sea. These forests are comprised of taxonomically diverse, salt-tolerant trees and other plant species which thrive in intertidal zones of sheltered tropical shores, overwash islands, and estuaries. These trees have specially adapted aerial and salt-filtering roots and salt-excreting leaves that enable them to occupy the saline wetlands where other plant life cannot survive. These forests literally live in two worlds simultaneously, acting as the interface between land and sea. Mangroves help to protect coastlines from erosion, storm damage, and wave action. The stability mangroves provide is of immense importance. They prevent shoreline erosion by acting as buffers and catch alluvial materials, thus stabilizing land elevation by sediment accretion that balances sediment loss. Vital coral reefs and sea grass beds are also protected from damaging siltation. 1.1 Mangrove Ecosystems The shallow intertidal reaches that characterize the mangrove wetlands offer refuge and nursery grounds for juvenile fish, crabs, shrimps, and mollusks. They are also prime nesting and migratory sites for hundreds of bird species. In Belize, for instance, there are over 500 species of birds recorded in mangrove areas. Additionally, manatees, crab-eating monkeys, fishing cats, monitor lizards, sea turtles, and mud-skipper fish utilize the mangrove wetlands. There are two types of mangrove communities: fringe mangroves and riverine mangroves. The fringe communities are found right along the coast, while ri';erine mangroves are found in environments Introduction terl s ed by a larger and more continuous supplies of fresh water (Lugo charac & Snedaker 1974). The structural development, reproductive capacity, and roduction of mangroves are inversely related to the availability of fresh ~ater, e.g. tall, robust forests with high leaf litter production are present where fresh water availability is good, whereas stunted forests with smaller stem diameters and lower litter production are present where the availability of fresh water is limited. Od urn (1967) has described the mangrove ecosystem as a pass through type of system because of the large flow of organic matter transported through the mangroves to coastal zones by physical activities of tides, runoff and rainfall, and the intense biological activities of decomposition, uptake, and bioturbation occurring in the mangroves that are important downstream in the marine ecosystem. It is noted that mangrove forests differ from terrestrial ones in that the primary source of nutrients lies outside of the forests themselves, and nutrients falling as leaf litter are removed from the forest. These complex ecosystems are found between the latitudes of 32 degrees north and 38 degrees south, along the tropical coasts of Africa, Australia, Asia, and the Americas. There are varying scientific classifications of what constitutes a mangrove plant. According to two reputable scientific studies, mangroves include approximately families and species. The greatest diversity of mangrove species exists in Southeast Asia. For example, there are only twelve mangrove species in the New World and only four species of mangroves exist along portions of the coasts of the southern USA. However, over the past several decades, the global area in mangroves has increasingly diminished as a r~sult of a variety of human activities, such as overharvesting, freshwater diversion and conversion to other uses. Mangroves are estimated to extend over 15 million ha world-wide; there are about 6.9 million ha in the Indo-Pacific region 3.5 million ha in Atl nca and some 4.1 million ha in the Americas including the Caribbean. The most extensive and luxurious mangroves extend across the Indo-Pacific regions where they are estimated to cover an area of 22 million ha. They dominate majority of the subtropical and tropical coastline and are best developed in the delta systems of major rivers, e.g. the Ganges Brahmaputra, Irrawady, Mekong and along very sheltered shores protecte. by large ~and masses, notably Madagascar, the Malacca Straits, Kalimantan, the Indonesian Archipelago and Papua New Guinea. The largest intact area of mangroves occurs in Bangladesh, where there is almost 600,000 ha of the Sundarbans ecosystem and a mangrove forest area estimated in 1985 to cover 401,600 ha. Mangroves also penetrate some temperate zones, but there is a rapid decrease in the diversity of species with increasing latitude. At their latitudinal extremes: 31 N in southern Japan; 31 N on the Pacific coast of Mexico; 32 S in Brazil and 38 S in southern Australia the mangrove vegetation is in each case restricted to a single species. About 80 species of true mangrove trees/shrubs are recognised, of which around species make a significant contribution to the structure of mangrove forests. Species diversity is much higher in the Southeast Asian region, where approximately two-thirds of all species are found, while approximately 15 species occur in Africa and loin the America. Top Eight Nations with Remaining Mangrove Forests Nation Sq km (% World Total) Indonesia 42,550 (23.5) Brazil 13,400 (7.3) Australia 11,500 (6.3) Nigeria 10,515 (5.8) Cuba 7,848 (4.3) Tnrlia 6,700 (3.7) Malaysia 6,424 (3.5) Bangladesh 5,767 (3.2) 4 Introduction Mangrove Distribution in India India has a coastline of 7,516 km. It has an Exclusive Economic Zone of 2.02 million km 2 Out of its 1 billion populati~~, ~eariy 20% live in the coastal areas. Along the Indian coastline, the brackish water areas including marshes, backwaters, mangroves, inter- and sub-tidal measure about 14,16,300 ha. These areas act as feeding and nursery grounds for a variety of commercially important fish, prawn and crabs, media for inland transportation, fishing etc. Along the Indian coast mangroves are found along the islands, major deltas, estuaries and backwaters of the East Coast of India. They also exist along the oceanic island groups of the Andaman and Nicobar. The total mangrove area is estimated to be 670,000 ha. While the mangroves along the West Coast of India are dense, they are scattered and comparatively small in area along the West Coast. Gangetic Sunderbans (418,888 ha), Andaman-Nicobar Islands (115,000 ha), Krishna, Kaveri and Godavari deltas and Mahanadi delta are some of the best mangrove formations of India. There are about 45 mangrove species along the Indian coast. The dominant genera are Rhizophora, Avicennia, Bruguiera, Sonneratia, Canocarpus, Heretiera, Xy/ocarpus, Ceriops, and Exoecaria. Apart from the captive and culture fisheries, mangroves are also important as coastal stabilizers and shelter belt areas . These formations protect the coasts and the landward areas from erosion and cyclonic destructions to some extent. Apart from these the mangrove forests of India have importance from a wildlife, recreation and education point of view. Project Tiger of Sunderbans and Crocodile Sanctuary in the Mahanadi delta are examples of such activities. Large areas of the inland mangroves of southern Asia have been conv~rted to agriculture (mainly paddy fields) or salt production. Shrimp fannmg represents a relatively new form of coastal land use which is a further thre a t T ra d l110na II y the mangroves of India and Bangladesh have - 5 been exploited for timber and fuelwood, bark tannin, al1imal fodder, native medicines and food (fish, shellfish, honey, wild animals). Population pressure has greatly increased the rate of exploitation, leading to degradation of the remaining foi t,;~t~ at an alarming rate. In Bangladesh, where an estimated 300,000 wood and thatch cutters, honey collectors, and fishermen are directly dependent on the Sundarbans, the area of pure sundri (Heretiera forms the main economic timber species) is reported to have shrunk from 31.6 to 21.0% between 1959 and 1983 Heavy exploitation of mangroves in India for firewood and animal fodder has depleted the resource significantly, except in the Indian Sunderbans and the Andaman Islands where selective systems of rotational felling have been practiced. In addition to firewood, strip felling was carried out in the Andamans to extract poles of Bruguiera gymnorrhiza, with successful replanting ofbruguiera seedlings reported Adaptations of Mangrove Plants Mangrove plants live in an environment where there are too many limiting factors and too many odds to cope with. In such a hostile environment it is mandatory for such plants to undergo morphological and anatomical changes. A lot of such adaptive changes have been recognised and documented in mangrove plants. A few of these adaptive changes have been described in the following section. Mangroves are facultative halophytes, i.e., the presence of salt in the environment is not necessary for the growth of mangroves and they can grow very well in freshwater, on the contrary, the growth of mangroves is better in freshwater. B...:t to avoid competition, the mangroves chose a saline environment and successfully became: llophytes. To cope with salinity, the mangroves had to develop mechanisms to regulate the salt concentration in the body. One such mechanism is for the retention of water in the leaves giving rise to leaf succulence in many species, viz., Sonneratia apetala, S. alba, Lumnitzera recemosa, Salvadora persica etc. Also, these species show Introduction remarkably high concentration of salts stored in their tissue. Another mechanism is extrusion of salt from the root tissue as in Rhizophora, Ceriops, Bruguiera and Kandelia. This is achieved by an in-built ultrafiltration mechanisui. Only fresh water is allowed to enter the body and the entry of salt is arrested by the semi- permeable membrane of the roots. In case the plants are not equipped with a mechanism for ultrafiltration, yet another mechanism is seen where leaves develop a special tissue called the salt glands which help in expulsion of salts from the body as in Avicennia, Laguncu/aria, Aegiceras, Acanthus, etc. Another method of coping with salt is to concentrate it in bark or in older leaves which carry it with them when they drop. (Lumnitzera, Avicennia, Ceriops and Sonneratia species all use this.). In addition, a number of features serve to prevent water loss from the plant. These include a thick waxy cuticle (skin on the leaf) or dense hairs to reduce transpiration. Most evaporation loss occurs through stomata so these are often sunken below the leaf surface where they are protected from drying winds. Apart from coping with salt, mangroves also face common problems of water-logged, unstable and oxygen deficient soils. Roots perfonn a number of functions for a plant. They support it and they obtain essential nutrients and oxygen. In unstable, sometimes semi-fluid, soil an extensive root system is necessary simply to keep the trees upright. As a result, most mangroves have more living matter below the ground than above it. The main mass of roots, however, is generally within the top two meters and mangroves do not seem to grow deep taproots, probably because of the poor oxygen supply below the surface. Little oxygen is available in fine, often waterlogged mud. The solution which many mangroves have come up with, is to raise part of their roots above the mud. These roots are covered with special breathing cells called lenticels, which draw in air. They are connected to spongy tissue within the r~ots. When the roots are submerged in water, the pressure within these tissues falls as the plant uses up the internal oxygen. The resulting negative pressure mtans that when the root is re-exposed as the tide drops, more air is drawn in through the lenticels. The fruits and/or seed (ling)s of all mangrove plants can float, which is, of course, an excellent sea dispersal mechanism for plants which live along coastal waters. Certain mangrove species can propagate successfully in a marine environment because of special adaptations. Through viviparity, embryo germination begins on the tree itself; the tree later drops its developed embryos, called seedlings, which may take root in the soil beneath. Viviparity may have evolved as an adaptive mechanism to prepare the seedlings for long-distance dispersal, and survival and growth within a harsh saline environment. During this viviparous development, the propagules are nourished on the parent tree, thus accumulating the carbohydrates and other compounds required for later autonomous growth. The structural complexity achieved by the seedlings at this early stage of plant development helps acclimate the seedlings to extreme physical conditions which otherwise might preclude normal seed germination. Another special adaptation is the dispersal of certain mangroves' propagules which hang from the branches of mature trees. These fall off and eventually take root in the soil surrounding the parent tree or are carried to distant shorelines. Depending on the species, these propagules may float for extended periods, up to a year, and still remain viable. Viviparity and the long-lived propagules allow these mangrove species to disperse over wide areas. Some species of these floating seedlings (Rhizophora) can survive in a state of suspended animation for up to a year in the water. Once lodged in the mud they quickly produce additional roots and begin to grow. Some other species (AviLennia, Aegialitis and Aegiceras) also produce live seedlings but these are still contained within the se j coat when it drops from the plant. The seed of Avicennia floats until this coat drops away Interestingly the speed with which this happens depends on the temperature and salinity of the water. In water of high or low salinity the seed coat is slow to drop off but in brackish water it is shed quickly allowing the Introduction seedling to lodge in the favoured habitat of this species. Higher temperatures also favour faster action. Avicennia seeds can stay alive in the water for only three to four days Main Uses and Functions of Mangroves Mangroves have long functioned as a storehouse of materials providing food, medicines, shelter and tools. Fish, crabs, shellfish, prawns and edible snakes and worms are found there. Mangrove ecosystems have traditionally been sustainable managed by local populations for the production of food, medicines, tannins, fuel wood, and construction materials. For millions of indigenous coastal residents, mangrove forests offer dependable, basic livelihoods and sustain their traditional cultures. The fruit of certain species including the nypa palm can be eaten after preparation along with the nectar of some of the flowers. The best honey is considered to be that produced from mangroves, particularly the river mangrove Aegiceras corniculatum. Numerous medicines are derived from mangroves. Ashes or bark infusions of certain species can be applied to skin disorders and sores including leprosy. Headaches, rheumatism, snakebites, boils, ulcers, diarrhoea, haemorrhages and many more conditions are traditionally treated with mangrove plants. The latex from the leaf of the blind-your-eye mangrove Excoecaria agallocha can indeed cause blindness but the powerful chemicals in it can be used on sores and to treat marine stings. The leaves are also used for fishing; when crushed and dropped in water, fish are stupefied and float to the surface. This sap is currently being tested for its medical properties and may play a part in western medicine. Certain tree species, notably the cedar mangrove, cannonball ~a~grove (relatives of the red cedar) and the grey mangrove, are prized for heir hard wood and used for boat building and cabinet timber as well as for tools such d. as Iggmg sticks, spears and boomerangs. The fronds of the nypa --~ palm are used for thatching and basket weaving. Various barks are used for tanning, pneumatophores (peg roots) make good fishing floats while the wood from yellow mangroves (Ceriops species) has a reputation for burning even when wet. Mangrove tannin is used in India and Bangladesh fvi leather curing and there are some other traditional uses, e.g. for curing fishing nets in Sri Lanka. The gathering of mangrove leaves (Avicennia) for animal fodder remains widespread in the Middle East and Southern Asia, for feeding camels in Iran and India, for example; in fact grazing by domestic animals is a serious cause of mangrove degradation in parts of India. Mangrove ecosystems are important, transformative interfaces between land and sea. The mangrove ecosystem imports inorganic matter from terrestrial systems and exports organic matter as detritus (both dissolved and particulate organic matter) to marine ecosystems. Mangroves litter production has been measured at 896 grams dry wt.lm2/year adding 224 g C/m2/year to the soils and waters. Mangrove leaves are usually colonized by fungi bacteria, nematodes, etc. upon deposition into water. Mangrove leaf litter provides the most important nutrient base for food webs leading to commercially important fisheries. The leaves themselves are poor in nutrients when they fall from the trees but once in the aquatic ecosystem the organic matter is transformed by bacterial decomposition into nutrientrich, high protein detritus. About 75 percent of fish caught commercially spend some time in the mangroves or are dependent on food chains, which can be traced back to these coastal forests. Mangroves also protect the coast by absorbing the energy of storm driven waves and wind. The mangrove forests act as natural barrier to storm surge and cycle ae wind. The protective mangrove buffer wne helps minimize damage of property and losses of li ~e from hurricanes md storms. In regions where these coastal fringe forests iiave been cleared, remendous problems of erosion and siltation have arisen, and sometimes errible losses to human life and property have occurred due to destructive torms. Human destruction of mangroves was quoted as one of the reasons Introduction for the unprecedented flood havoc in Orissa. Mangroves have also been useful in treating effluent, as the plants absorb excess nitrates and phosphates thereby preventing contamination of nearshore waters. Sediments trapped by roots prevent silting of adjacent marine habitats where cloudy water might cause corals to die. In addition, mangrove plants and sediments have been shown to absorb pollution, including heavy metals. An increase in level of atmospheric C02 through the combustion of fossil fuels, or more precisely its likely effect on the global climate, is cause for concern. This concern has focused the interest of scientists in carbon fixation and storage in individual ecosystems and their overall performance as a sink or source for atmospheric C02. Today, mangrove forests cover an area of about less than I % of the total forest area on the earth. Here at first glance, it seems unlikely that mangrove ecosystems play an important role in the global carbon cycle. However, if we are to actively utilize ecosystems for the enhancement of CO2 sequestration, our focus should be place on their ability to produce and store organic carbon. It must be kept in mind that mangrove
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