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INVITED ARTICLE

Environmental Sedimentary Biogeochemistry of the mangroves

AL. Ramanathan
Department of Geology
Annamalai University, Annamalai Nagar
Chidhambaram, Tamil Nadu

A mangrove environment is like no other ecosystem on earth, except perhaps a closed tropical rainforest. In both the cases the soils are generally poor, with most of the nutrients close to the surface. Because of this tree species are very shallow rooted. Many mangrove species are related to rain forest flora and in some cases to tropical dry land vegetation.

Mangrove Mud/Silt

They are colonies of new sand and mud bars, and as the mangrove trees becomes more established in the area, wave energy is reduced and the fine particles carried by the water are allowed to settle. The result is a layer of dark sticky mud. More sediment is trapped by the various breathing roots of the different species of mangroves as is decaying leaf litter and sea grasses trap more sediment.

As sunlight reaches the developing mud, a vast mat of algae starts to grow which holds and stabilised the fine sediments. The algae is also an essential part of the food web within the mangroves, especially for the numerous species of snail that live in this environment.

In the mud, the most important group of organisation is the bacteria. The bacteria must overcome a number of difficulties living in the environment, the most pressing being the lack of oxygen in the mud. Once the oxygen is used in the decay of leaf litter on the surface, little penetrates deeper into the mud. Because

1. The spaces between the sediment parti-cles are full of water and so the mud is water logged. So the oxygen diffuses at a very slow rate, it never penetrates more than a few centimetres below the surface of this water logged mud.

2. Mudís here lack of oxygen due to the lack of soil structure. The sodium salts break down any crumbs or lumpy structures in the mud to form a homogenous texture of cream like consistency.

Bacteria on the surface of the mud will decay plant material in the presence of oxygen. However, there is no oxygen deep in mud and this is indicated by the black color of the mud, often associated with the strong smell of H2S.

The hydrogen sulfide results from sulfate reducing bacteria, which in the absence of oxygen use chemosynthesis to obtain energy. This is achieved by breaking down the sulfates as a waste product. Other bacteria release iron compounds as a result of their respiratory activity within the mud. This iron reacts with the H2S to produce the intense black iron sulfide, which is a characteristic of the mangrove mud.

Life and Mangrove Mud

Living on the surface of the mud, organisation are exposed to constantly changing conditions throughout the day, such as temperature, light intensity, pH, salinity, oxygen levels and H2S concentrations. The concentrations of hydrogen sulfide quickly reach a level, fatal to most organisms.

The bacteria living on the surface of the mud have a ready food supply of plant material, which consists of large and complexes such as cellulose. These bacteria have the ability to breakdown complex molecules and must have a ready supply of oxygen to be able to complete its reaction. A range of flagellates and ciliates are also found on the surface, feeding on the bacteria and algae present. Below the surface, the black oxygen-deficient mud is very stable.

The mud is full of nutrients, consists of party digested smaller matter left after decay on the surface and percolates deep into the mud. Anaerobic bacteria use the smaller particles as a food source. They have the ability to use sulfate from the seawater as their main energy source creating an environment rich in H2S. There H2S is either poisonous to the organisms or can be used as an energy source by other through chemosynthesis.

The areas with the high levels, of H2S are usually barren of larger organisms, usually due to the poisonous nature of the gas. In areas where there is no H2S and sediments are coarser, crabs, shrimps, worms and molluses are found, because this place is more suited for digging and burrowing and take oxygen through their breathing root. But small size population (bacteria) which can tolerate H2S and not get poisoned and further larger organisms that feed on them cannot on them cannot survive, be able to develop and grow in theis environment. Sulfur bacteria (Beggiatoa) requires O2, CO2, H2O and H2S the primary energy source for growing. This process occurs at or near the surface where these bacteriaís moves to the surface when the oxygen is used at night and move down into the mud during the day when oxygen is produced by photosynthesis. The area where the sulfur bacteria are present, (generally white to pale yellow color) in an area that is devoid of the distinctive odor of H2S.

Another essential groups of microorganisms are those involved in the denitrification within ghe flooded anaerobic soils and mud of the mangrove environment. The microbes can cause massive losses of N from the system, with the starting point being ammonia. NH4 located in the anaerobic zone of the soil will diffuse up toward the aerobic zone. At this stage the NH4 is oxidized by bacteria to NO2 and N2 gas. The gases are then able to rapidly diffuse out the mud and into the atmosphere.

Mangrove food chain

Mangroves, like all other plants, use photosysnthesis to convert sunlight to plant tissues, and so become a food source for animals. However, the basis of the mangrove food chain is not the mangrove themselves, but the leaf litter. The mangrove leaves together with other vegetable matter such as bark, fruit, etc decompose under the attack of bacteria and fungi soluble nutrients are released into the tides and the soil to become nutrients of algae, plankton and for the mangrove themselves. The decomposed leaf particles (detritus) become the direct food source for crabs and both Juvenile and adult prawns. Numerous species of zooplankton depend upon this vegetable protein as a food source also. 80% of the mangrove either is recycled through detrital break down.

The crop of mangrove litter averages about 17 tons/ha/yr. (Bunt, 1981). The conversion of this to usable protein makes the mangrove forest the richest zone of natural agriculture in the world, richer by far than rainforest, and richer than coral reef.

Environmental status

Mangroves and wetland ecosystem are very important for their biological productivity and that preservation and management of such places is vital if they are to continue to supply their many benefits to society and environment.

Mangroves as flood/erosion control

Mangroves occur in the saline reaches of river and creeks, along sheltered shorelines, and in areas where sedimentary run off from river and creeks built up. As mud banks emerge, mangroves colonize them.

The aerial roots slow water and wave movement, trapping the silt and the shallow root system binds the silt in a slow process of land reclamation to eventually turn the bank over to terrestrial plants. Many of the islands were also developed by this manner. In a similar manner, mangrove shorelines protect riverbank from erosion. When mangroves are removed, erosion increases, channels may change the effect of heavy rainfall, and flood runoff is more difficult to predict. If it is preserved, the cos-free erosion controls of a pristine mangrove area without the efforts of humans using rock and cement.

Pollution buffer

A mangrove forest provides a buffer between polluted areas and nearby marine environments. The mangrove reduces the speed and current and tidal flow within the forest allowing the deposition of finely grounded sediments, which have a greater capacity for the binding of metal than coarse- grained material. Further these sediments from an excellent habitat for the anaerobic bacteria, which generate the sulfide, which reacts with the metal leachate to from insoluble metal sulfides which are trapped in the sediments. Mangrove plants may directly adsorb quantities of copper, zinc and lead. Metals sulfides are trapped within the mangrove sediments. Those absorbed into the plant may later be removed from the forest as litter.

A storm buffers and land builder

Mangrove protect coastal land by absorbing the energy of storm-driven wave and wind action-creating in effect a natural breakwater which can result in significant savings to property in the event of a storm. Mangroves are important builders of land, helping in the formation of islands and the extension of shore lines, crop roots and pneumatophores reduce tidal flow, trapping silt and mud which results in extension deposition. So mangrove can be seen as the pioneering species moving further and further out, converting sea to land and our proper land have been formed, handing over the land to other species. Mangrove act as a stabilizing influence once the land has formed. Certainly as mudflats are builds up, mangrove tend not to appear until the height of the flat has reached above the low tide level. Whatever their exact role, mangrove play an important part in land building.

Further the list of recreational activities which may be undertaken in mangrove areas is quite extensive- boating, fishing, wild life observation and photography to name but a few. Coastal aborigines have close ecological and cultural limits with mangroves, which provide them with many useful resources. A full list is quite extensive and could be divide into 3 groups: Food (Source of honey, fruits can be eaten after careful preparation). Medicinal (eg. avicennai used to treat skin disorder including leprosy) and weapons/implements (spears, basket weaving, boomeranges, stupefy fish, etc). Mangrove timbers have a multiplicity of use including boat building, house construction, fish traps and oyster rocks.

Threats to mangrove

Mangroves had been cleared to make way for aqua-culture/fish ponds, agriculture and recreational activity. Numerous human activities are threat to mangroves, and the combined effects of some of these may well be disastrous to this ecosystem. Further more enormous distribution of mangroves and tidal wetlands has occurred as a result of filling to create new lands for airports, harbors, industrial development housing etc. Mostly the fill comes from dredging of nearby tidal or inter-tidal areas.

The effects of reclamation on surrounding areas are three folds physical, chemical and hydrological. There is nearly always an increase in sedimentation, which leads to a decrease in water quality and lower dissolved oxygen levels. Where inter-tidal vegetation is lost, there is a reduction in productivity and alteration to the fauna of the area. Estuary landfills modifies tidal range and pattern of tidal inundation. Because of changes to current pattern there may be a calculated erosion or sedimentation in other parts of the system. Landfills causes other problems like storm water runoff carrying silt and pollutants from residential areas, factories, and vehicles into streams/mangroves.

Development of canal estates may create a problem of flooding, erosion and poor water circulation, stagnation and modify the tidal pattern and even biting midges. It is rather ironic that creating sandy beaches and canals has actually optimum breeding conditions for biting insects.

Bund walls are constructed along the coast by developer or farmers to prevent tidal flooding. The mangroves on this flood land can be cleared, and the land then can be used for other purposes. One such use it to provide effluent ponds for industry. Bund wells are vulnerable to flooding. Effluent may overflow the walls or even in normal periods, leach through the substrate.

And even spoil surrounding shallow groundwater quality. Little is that mangrove themselves are able to survive reasonably at moderate levels of pollution (Ramanathan et al., 1999).

Mangroves management issues and solution

Management of mangrove ecosystem depends on the respective regional view of their value. In Asia, management means mostly controlling exploitation for fire wood, timber, or wood chip production, or clear felling for either agriculture or mariculture, or late recognizing their food value for local people/fauna and flora and pollution control were included under management.

So the goal of management of mangroves should include:-

    (i) Preservation of the ecosystem audits natural processes (including the prevention of endangered and representation species).

    (ii) Preservation of area for scientific and educational study.

    (iii) Protection of recreational and commercial fishing.

    (iv) Protection of against erosion, siltation and flood damage.

    (v)Reservation of aesthetic and recreational qualities of natural shore lines.

    (vi) Otection again over exploitation and pollution.

    (vii) Detailed interdisciplinary research into mangrove ecosystem. (research programs, ought to be focussed on pollution effects, sewage, farm runoff, eutrophication, etc.).

Mangrove destruction does occur naturally and is not always the result of human activities. So the protection of mangrove is activated through creating awareness about the natural process of mangroves, their values and environmental importance through public and school educational project using the visual media. Then EIA on the site and the surrounding areas should be investigated to come out with EIS (Environmental Investigation statement) for regeneration process. Good practical legislation to protect mangroves is to be developed. After these coordinated approaches, mangrove authority will be formed, funded and staffed with necessary power and responsibility to manage and protect mangroves in future.

BIBLIOGRAPHY:

Bunt,JS (1981): Mangrove forests- a national asset to be guarded. Australian Fisheries, 1(81): 14-17.

Ramanathan, AL; Subramanian, V; Ramesh, R; Chidambaram, S; James, A (1999): Environmental geochemistry of the Pichavaram mangroves ecosystem (tropical), southeast coast of India. Environmental Geology, 37(3): 223-233.


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