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Mangrove

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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. The word is used in at least three senses, (1) most broadly to refer to the habitat and entire plant assemblage or mangal [1], 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. Mangal is found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action.

General description

Plants in mangrove swamps are a diverse group which have been able to exploit a habitat (the intertidal zone) because they have developed a set of physiological adaptations to overcome the problems of anoxia, salinity and frequent tidal inundation. About 110 species have been identified as belonging to the mangal.[2] 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 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, but also may be influenced by other factors such as predation of their seedlings by crabs.

A cluster of mangroves on the banks of Vellikeel river in Kannur District of Kerala, India

Once established, the roots of the 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. It is usually the case that 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.

It is often stated that mangroves provide significant value in the coastal zone as a buffer against erosion, storm surge and tsunamis. While there is some attenuation of wave heights and energy as seawater passes through mangrove stands, it must be recognised that these trees typically inhabit areas of coastline where low wave energies are the norm. Therefore their capacity to ameliorate high energy events like storm surge and tsunamis is limited. Their long term impact on rates of erosion is also likely to be limited. Many river channels that wind through mangrove areas are actively eroding stands of mangroves on the outer sides of all the river bends, just as new stands of mangroves are appearing on the inner sides of these same bends where sediment is accreting.

They 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. 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.

Biology of mangroves

A red mangrove, Rhizophora sp..

The word mangal describes the plant community and habitat where mangroves thrive[2]; it is exclusively subtropical and tropical and tidal, and therefore having soil or sediment that is water-logged and saline or of variable salinity. Areas where mangal occurs includes estuaries and marine shorelines. 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[1]. Evolutionary convergence 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 the mangal, with more than 20 species in a given area being uncommon.[2] In higher latitudes, and in the Americas, this is particularly true. The greatest biodiversity occurs in the mangal of New Guinea, Indonesia and Malaysia.[3]

Adaptations to low oxygen

Red mangroves, which can live in the most inundated areas, prop themseles 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, though some species have ones that reach over 3 meters high. 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.

Limiting salt intake

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).

Limiting water loss

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 midday sun, mangrove plants can reduce evaporation from their leaf surfaces. Anthony Calfo, a noted aquarium author, has observed, anecdotally, that a red mangrove in captivity will not grow unless its leaves are misted with fresh water several times a week, simulating the frequent rainstorms in the tropics.[citation needed]

Nutrient uptake

The biggest problem that mangroves face is nutrient uptake. Because 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, which help contribute to a mangrove's particularly pungent odor 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.

Increasing survival of their offspring

In this harsh environment mangroves have evolved a special mechnanism to help their offspring to survive. All mangroves have buoyant seeds that are suited to dispersal in water. Unlike most plants, whose seeds germinate in the 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 what is called 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 a propagule does not root, it can alter its density so that it floats off again in search of more favorable conditions.

Mangrove ecosystems

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 substratum for anchoring while they filter feed. Mangrove crabs are important animals in the habitat. Mangroves are excellent buffers between the violent ocean and the fragile coast, especially during hurricanes, which can bring powerful storm surges onto shores. The massive mangrove root system is quite efficient at dissipating wave energy. 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. Because of the uniqueness of the mangrove ecosystems, they are frequently the object of conservation programs including national Biodiversity Action Plans.

Species of mangroves

The following listing (modified from Tomlinson, 1986) gives the number of species of mangroves in each listed plant genus and family.

Major components

Family Genus, number of species Common name
Acanthaceae (syn. Avicenniaceae or Verbenaceae) Avicennia, 9 Black mangrove
Combretaceae Laguncularia, 11; Lumnitzera, 2 White mangrove
Arecaceae Nypa, 1 Mangrove palm
Rhizophoraceae   Bruguiera, 6; Ceriops, 2; Kandelia, 1; Rhizophora, 8 Red mangrove
Sonneratiaceae Sonneratia, 5

Minor components

Family Genus, number of species
Acanthaceae Acanthus, 1; Bravaisia, 2
Bombacaceae Camptostemon, 2
Cyperaceae Fimbristylis, 1
Euphorbiaceae Excoecaria, 2
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 in geographical regions

Mangroves occur in numerous areas worldwide. See List of mangrove ecoregions for a full listing.

Africa

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.

Americas

Mangroves occur on the west coast of Costa Rica and on many Caribbean Islands, such as Antigua and St. Lucia.

United States

Because of their sensitivity to sub-freezing temperatures, mangroves in the continental United States are limited to the coastal Florida Peninsula from Cape Canaveral on the east around the keys and up to Tampa Bay on the west. The mangal of the Banana and Indian rivers of Brevard County and within the confines of the Kennedy Space Center is significant. The most northern Rhizophora mangle populations in Florida are around Cedar Key, although only a few specimens there are above 2 m tall, they develop extensive prop roots, but are prone to freezing down to the ground during a particularly cold winter [1]. Some Avicennia germinans also survive in cultivation in Florida in freshwater.

Around the entrance to Port Everglades in Fort Lauderdale are several stands of a formerly much more extensive covering of mangroves. Miami-Dade County's upper Biscayne Bay was formerly extensively fringed in mangroves. Most of these have been attrited by development down to isolated stands, however the Oleta River, an estuary in northern Miami-Dade County has a rather large mangal that is still relatively intact and is now a state recreation area. Southern Biscayne Bay and Card Sound have extensive fringing mangroves intact, as do the lee side of most of the Florida Keys.

The southern tip of the Florida Peninsula is the largest intact mangal in the continental United States. It comprises the whole southern part of the Everglades National Park. This habitat extends from Card Sound in the east across the southern part of Miami-Dade County into Monroe and Collier counties, including the Cape Sable area and the Ten Thousand Islands in the west. Included in this community are several almost exclusively mangrove habitat islets scattered across Florida Bay.

The Keys and Everglades mangrove community is critical as a nursery for the commercially important Keys shrimp industry. Other important species that breed or live part of their life cycle in this habitat are the tarpon, snook, lemon shark, nurse shark, snapper, spiny lobster, sea trout, and bonefish. It is also the exclusive habitat of the American Crocodile.

There are several scattered stands of mangrove on the west coast of Florida, particularly in the estuaries of the Caloosahatchee River and Charlotte Harbor. Like their counterparts on the east coast, they were formerly much more extensive, but have been attritted by development. Sarasota Bay, Lemon Bay, Anna Maria Bay, and the estuary of the Manatee River also possess sigificant mangrove growth. Tampa Bay's mangroves have been likewise relegated into small isolated stands.

The mangrove species Sonneratia, showing abundant pneumatophores, growing on the landward margin of the reef flat on Yap.

Asia

Mangroves occur on the coasts of Pakistan, India, some of the south-east asian countries (e.g. Malasia), and Bangladesh. The Sundarbans is the largest mangrove forest in the world, located in the Ganges delta in Bangladesh and West Bengal, India.

Australasia

Mangroves occur on the eastern and northern coasts of Australia and round much of New Guinea.

Mangroves in other media

  • The mangrove is used as a symbol in Annie Dillard's essay Sojourner due to its significance as a self-sustaining biome.

Notes

  1. ^ a b Hogarth, Peter J. (1999) The Biology of Mangroves Oxford University Press, Oxford
  2. ^ a b c "Mangal (Mangrove)" World Vegetation Mildred E. Mathias Botanical Garden, University of California at Los Angeles;
  3. ^ UN Report on mangrove diversity;

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