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Above and below water view at the edge of the mangal.
Mangroves (generally) are trees and shrubs that grow in saline coastal habitats in the tropics and subtropics. The word is used in at least three senses, (1) most broadly to refer to the habitat and entire plant assemblage or mangal Hogarth, Peter J. (1999). The Biology of Mangroves Oxford University Press, Oxford. , for which the terms mangrove swamp and mangrove forest are also used, (2) to refer to all trees and large shrubs in the mangal, and (3) narrowly to refer to the mangrove family of plants, the Rhizophoraceae, or even more specifically just to mangrove trees of the genus Rhizophora. Mangals are found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action.
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A mangal is a plant community and habitat where mangroves thrive. They are found in tropical and sub-tropical tidal areas, and as such have a high degree of salinity. Areas where mangals occur include estuaries and marine shorelines.
Plants in mangals are diverse, but all are able to exploit their habitat (the intertidal zone) by developing physiological adaptations to overcome the problems of anoxia, high salinity and frequent tidal inundation. About 110 species have been identified as belonging to the mangal.Mangal (Mangrove). World Vegetation. Mildred E. Mathias Botanical Garden, University of California at Los Angeles Each species has its own capabilities and solutions to these problems; this may be the primary reason why, on some shorelines, mangrove tree species show distinct zonation. Small environmental variantions within a mangal may lead to greatly differing methods of coping with the environment. 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, but may also be influenced by other factors such as predation of plant seedlings by crabs.
A cluster of mangroves on the banks of Vellikeel river in Kannur District of Kerala, India
Once established, roots of mangrove plants provide a habitat for oysters and help to impede water flow, thereby enhancing the deposition of sediment in areas where it is already occurring. Usually, the fine, anoxic sediments under mangroves act as sinks for a variety of heavy (trace) metals which are scavenged from the overlying seawater by colloidal particles in the sediments. In areas of the world where mangroves have been removed for development purposes, the disturbance of these underlying sediments often creates problems of trace metal contamination of seawater and biota.
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Mangroves protect the coast from erosion, surge storms, especially during hurricanes, and tsunamis.Mazda, Y.; Kobashi, D. and Okada, S. (2005) "Tidal-Scale Hydrodynamics within Mangrove Swamps" Wetlands Ecology and Management 13(6): pp. 647-655Danielsen, F. et al. (2005) "The Asian tsunami: a protective role for coastal vegetation" Science 310: p. 643. Their massive root system is efficient at dissipating wave energy.Massel, S. R.; Furukawa, K.and Brinkman R. M. (1999) "Surface wave propagation in mangrove forests" Fluid Dynamics Research 24(4): pp.219-249 Likewise, they slow down tidal water enough that its sediment is deposited as the tide comes in and is not re-suspended when the tide leaves, except for fine particles.Mazda, Yoshihiro et al. (1997) "Drag force due to vegetation in mangrove swamps" Mangroves and Salt Marshes 1: pp.193-199 As a result, mangroves build their own environment. Because of the uniqueness of the mangrove ecosystems and their protection against erosion, they are often the object of conservation programs including national Biodiversity Action Plans.
Despite their benefits, the protective value of mangroves is sometimes overstated. Wave energy is typically low in areas where mangroves grow,Baird, Andrew (26 December 2006) "False Hopes and Natural Disasters" New York Times editorial so their effect on erosion can only be measured in the long-term. Their capacity to limit high-energy wave erosion is limited to events like storm surges and tsunamis.Dahdouh-Guebas, F. et al. (2005) "How effective were mangroves as a defence against the recent tsunami?" Current Biology 15(12): pp. 443-447 Erosion often still occurs on the outer sides of bends in river channels that wind through mangroves, just as new stands of mangroves are appearing on the inner sides where sediment is accreting.[citation needed]
Mangroves support unique ecosystems, especially on their intricate root systems. In areas where roots are permanently submerged, they may host a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Mangrove crabs improve the nutritional quality of the mangal muds for other bottom feeders by mulching the mangrove leaves. Skov, Martin W. and Hartnoll, Richard G. (March 2002). Paradoxical selective feeding on a low-nutrient diet: why do mangrove crabs eat leaves? Oecologia 131(1): pp. 1-7. In at least some cases, export of carbon fixed in mangroves is important in coastal food webs. The habitats also host several commercially important species of fish and crustacea. In Vietnam, Thailand, the Philippines, and India, mangrove plantations are grown in coastal regions for the benefits they provide to coastal fisheries and other uses. Despite replanting programs, over half the world\'s mangroves have been lost in recent times.
A red mangrove, Rhizophora sp.
A wide variety of plant species can be found in mangrove habitat, but of the recognized 110 species only about 54 species in 20 genera, from 16 families constitute the "true mangroves", species that occur almost exclusively in mangrove habitats and rarely elsewhere. Convergent evolution has resulted in many species of these plants finding similar solutions to the problems of variable salinity, tidal ranges (inundation), anaerobic soils and intense sunlight that come from living in the tropics. Plant biodiversity is generally low in a given mangal -- more than 20 species are uncommon. This is especially true in higher latitudes, and in the Americas. The greatest biodiversity occurs in the mangal of New Guinea, Indonesia and Malaysia.UN Report on mangrove diversity.
Red mangroves, which can live in the most inundated areas, prop themselves up above the water level with stilt roots, and can then take in air through pores in their bark (lenticels). Black mangroves live on higher ground, and make many pneumatophores (specialised root-like structures which stick up out of the soil like straws for breathing) which are covered in lenticels. These "breathing tubes" typically reach heights of up to 30 centimeters, and in some species over 3 meters. There are four types of pneumatophore - stilt or prop type, snorkel or peg type, knee type and ribbon or plank type. Knee and ribbon types may be combined with buttress roots at the base of the tree. The roots also contain wide aerenchyma to facilitate oxygen transport within the plant.
Red Mangroves exclude salt by having rather impermeable roots which are highly suberised, acting as an ultra-filtration mechanism to exclude sodium salts from the rest of the plant. Water inside the plant shows that 90%, and in some cases of high salinity, up to 97%, of the salt has been excluded at the roots. Any salt which does accumulate in the shoot is concentrated in old leaves which are then shed, as well as stored away safely in cell vacuoles. White (or Grey) Mangroves can secrete salts directly, they have two salt glands at each leaf base (hence their name - they are covered in white salt crystals).
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 (pores on the leaf surfaces, which exchange carbon dioxide gas and water vapour during photosynthesis). They also vary the orientation of their leaves to avoid the harsh midday sun, and so reduce evaporation from the leaves. Anthony Calfo, a noted aquarium author, has observed anecdotally, that a red mangrove in captivity only grows if its leaves are misted with fresh water several times a week, simulating the frequent rainstorms in the tropics.Calfo, Anthony (2006). Mangroves for the Marine Aquarium.
The biggest problem that mangroves face is nutrient uptake. Because the soil is perpetually waterlogged, there is little free oxygen. Thus anaerobic bacteria liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane, which makes the soil much less nutritious and contributes to a mangrove\'s pungent odor. Prop root systems allow mangroves to take up gasses directly from the atmosphere and various other nutrients, like iron, from the inhospitable soil. Gases are quite often stored directly inside the roots, and processed even when the roots are submerged during high tide.
In this harsh environment mangroves have evolved a special mechnanism to help their offspring survive. All mangroves have buoyant seeds suited to dispersal in water. Unlike most plants, whose seeds germinate in soil, many mangrove plants (e.g. Red Mangrove) 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. Aegialitis, Acanthus, Avicennia and Aegiceras), or out through the fruit (e.g. Rhizophora, Ceriops, Bruguiera and Nypa) to form a propagule (a seedling ready to go), which can produce its own food via photosynthesis. When the propagule is mature it drops into the water where it can then be transported 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 it does not root, it can alter its density so that it floats off again in search of more favorable conditions.
The following listing (modified from Tomlinson, 1986) gives the number of species of mangroves in each listed plant genus and family.
| Family | Genus, number of species | Common name |
|---|---|---|
| Acanthaceae, Avicenniaceae or Verbenaceae (family allocation disputed) | Avicennia, 9 | Black mangrove |
| Combretaceae | Conocarpus, 1; Laguncularia, 11; Lumnitzera, 2 | Buttonwood, White mangrove |
| Arecaceae | Nypa, 1 | Mangrove palm |
| Rhizophoraceae | Bruguiera, 6; Ceriops, 2; Kandelia, 1; Rhizophora, 8 | Red mangrove |
| Lythraceae | Sonneratia, 5 | Mangrove apple |
| Family | Genus, number of species |
|---|---|
| Acanthaceae | Acanthus, 1; Bravaisia, 2 |
| Bombacaceae | Camptostemon, 2 |
| Cyperaceae | Fimbristylis, 1 |
| Euphorbiaceae | Excoecaria, 2 |
| Lecythidaceae | Barringtonia, 6 |
| Lythraceae | Pemphis, 1 |
| Meliaceae | Xylocarpus, 2 |
| Myrsinaceae | Aegiceras, 2 |
| Myrtaceae | Osbornia, 1 |
| Pellicieraceae | Pelliciera, 1 |
| Plumbaginaceae | Aegialitis, 2 |
| Pteridaceae | Acrostichum, 3 |
| Rubiaceae | Scyphiphora, 1 |
| Sterculiaceae | Heritiera, 3 |
Mangroves occur in numerous areas worldwide. See List of mangrove ecoregions.
There are important examples of mangrove swamps in Kenya and Madagascar, the latter even admixed at the coastal verge with the Madagascar dry deciduous forests.Nigeria has the largest concentration of mangroves in Africa, spanning an area of 36,000 sq km. Many of Nigeria\'s mangroves have been destroyed in the last 50 years due to oil spills and leaks, destroying local fishing economy and water quality.O\'Neill.T (2007). "Curse of the Black Gold". National Geographic: 88 to 117. .
Mangroves are found in many parts of the tropical and subtropical coastal parts of the Americas.
Because of their sensitivity to sub-freezing temperatures, mangroves in the continental United States are limited to the coastal Florida Peninsula (see Florida mangroves) and south Texas.
Mangroves also occur on the west coast of Costa Rica, on the Pacific and Caribbean coasts of Nicaragua, Belize, Guatemala, Honduras, and Panama and on many Caribbean Islands, such as Antigua and St. Lucia. Significant mangals include the Marismas Nacionales-San Blas mangroves in Mexico. Mangroves can also be found in Puerto Rico, Cuba, Dominican Republic, Haiti, Jamaica, and the Pacific coast of El Salvador.
Mangrove near the town of Cienaga, Magdalena in the Ciénaga Grande de Santa Marta swampy marshes.
Mangroves occur on the south coast of Asia, throughout the Indian subcontinent, in all the southeast Asian countries, and on islands in the Indian Ocean, Arabian Sea, Bay of Bengal, South China Sea and the Pacific. The mangal is particularly prevalent in the deltas of large Asian rivers.
The Sundarbans is the largest mangrove forest in the world, located in the Ganges delta in Bangladesh and West Bengal, India. There are major mangals in the Andaman and Nicobar Islands and the Gulf of Kutch in Gujarat.Mangroves of India - URL retrieved November 26, 2006 Other significant mangals include the Bhitarkanika Mangroves and Godavari-Krishna mangroves.
The Pichavaram Mangrove Forest in Chidambaram, South India is the second largest Mangrove forest in the world. It is home to a large variety of birds - local resident, migratory resident and the pure migratory birds, separated from the Bay of Bengal by a lovely beach. It is one of those rare mangrove forests which has actually increased by 90% between 1986 and 2002.[citation needed]
There are large areas of mangroves in Oman near Muscat, in particular at Shinas, Qurm Park and Mahout Island. In Arabic, mangrove trees are known as qurm, thus the mangrove area in Oman is known as Qurm Park.
Iranian mangrove forests occur between the northern orbits 25ْ, 11َ to 27ْ, 52َ. These forests exist in the north part of the Persian Gulf and Oman Sea, along three Maritime Provinces in the south of Iran. These provinces respectively from southwest to southeast of Iran, include Bushehr, Hormuzgan and Sistan & Balouchestan .
In Vietnam, mangrove forests grow along the southern coast, including two forests: the Can Gio Mangrove Forest biosphere reserve and the U Minh mangrove forest in the Sea and Coastal Region of Kien Giang Province biosphere reserve.
In Australasia, mangroves occur around much of New Guinea, Sulawesi and the surrounding islands. Australia has mangle primarily on the northern and eastern coasts of the continent. It has approximately 11,500km² of mangroves with occurrences as far south as Corner Inlet in Victoria (37°45\' S) and Barker Inlet in Adelaide, South Australia.Zann, Leon P. [1995]. "Mangrove ecosystems in Australia: structure, function and status", State of the Marine Environment Report for Australia. Australian Government, Dept of Environment and Heritage. ISBN 0-642-17399-0. Retrieved on 2006-11-25. New Zealand also has mangrove forests extending to around 38 degrees South (similar to Australia\'s southernmost mangrove incidence): the furthest geographical extent on the west coast is Raglan Harbour (37°48\' S); on the east coast, Ohiwa Harbour (near Opotiki) is the furthest south that mangroves are found (38°00\' S).http://www.treasuresofthesea.org.nz/mangroves-and-seagrasses
Twenty-five species of mangrove are found on various Pacific islands, with extensive mangals on some islands. Mangals on Guam, Palau, Kosrae and Yap have been badly affected by development.Hawaii and the Pacific Islands Mangroves are not native to Hawaii, but the Red mangrove, Rhizophora mangle, and Oriental mangrove, Bruguiera sexangula, have been introduced and are now naturalized.Allen, James A. and Krauss, Ken W. (2006) "Influence of Propagule Flotation Longevity and Light Availability on Establishment of Introduced Mangrove Species in Hawai\'i". Pacific Science 60:3, July 2006. Abstract at [1] - URL retrieved November 28 2006. Both species are classified as "Pest Plants of Hawaiian Native Ecosystems" by the University of Hawaii Botany Department.Hawaiian Alien Plant Studies - URL retrieved November 28 2006.
The United Nations Environment Program has estimated that a quarter of the destruction of mangrove forests stems from shrimp farming.Botkin, D. and E. Keller (2003) Enrivonmental Science: Earth as a living planet (p.2) John Wiley & Sons. ISBN 0-471-38914-5
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