Mangroves are highly specialised ecosystems characterised by salt-resistant plants thriving in the intertidal areas along sheltered coasts and estuaries in tropical and sub-tropical regions. They are commonly found in the latitudes between 24ºN and 38ºS and cover up to 75 per cent of coastlines worldwide. Mangroves, often termed as the tropical rainforests of the sea, are the second highest source of primary production next to rainforests and have a substantial impact on the global carbon budget (Dittmar et.al., 2006). These forests are capable of fixing and storing significant amounts of carbon under extreme conditions—a point of growing interest. Yet, there remains little realisation of their importance, hence accounting for their destruction in the name of urbanisation and development.
Mangrove forests occupy less than 1 per cent of tropical forested areas, and yet account for approximately 3 per cent of the global carbon sequestration by tropical forests (Bhomia et. al., 2016). They function as carbon sinks, removing carbon dioxide from the atmosphere and storing it in the form of plant biomass and soil organic matter. Some recent studies claim that mangrove management may provide an attractive low-cost alternative to reduce atmospheric carbon dioxide. There is, hence, considerable impetus in developing mangrove conservation projects.
Besides this, mangroves also provide a variety of other ecological functions. They are a habitat for diverse aquatic life and act as an intricate part of the food web. Mangroves comprise nurseries and feeding grounds for fish, shrimps, crabs, and house a variety of endangered and rare species such as the Royal Bengal Tiger and the estuarine crocodile.
Ecological functions of mangroves
Mangrove forests act as the first line of defence in protecting shorelines from cyclones and their associated impacts (Kathiresan and Rajendran, 2005). They act as a barrier to slow down the ingress of water from storm surges and tsunamis, by interfering with incoming waves and reducing their height and speed as they move inland (Mazda et. al., 2006). They also serve as an important source of livelihood for coastal populations, providing several direct and indirect benefits. They are a good source of medicine, honey, sugar, vinegar, alcohol, fuel and local beverages. In addition, they are harvested for products like timber for construction, poles for fish traps, wood pulp for paper manufacture and wood chip production.
Mangrove ecosystems can easily endure adverse environmental conditions. Prolonged water-logging, anaerobic environment, harsh intertidal currents, strong winds, and high salinity have seen mangroves evolve through adaptations such as salt glands that can excrete excess salts and filter fresh water from salt water, roots that enable them breathe in waterlogged conditions, and stilt roots that help gather mechanical support by spreading out for firm anchorage under hostile wind and intertidal conditions.
Mangroves in India
India’s mangrove cover accounts for 3 per cent of the world’s mangrove vegetation and is spread over 4740 sq km along the Indian coastline. The gently sloping eastern coast of India is comparatively rich in mangroves as compared to the steeply sloping western counterpart (Fig. 1). West Bengal is home to the world’s largest mangrove patch—the Sundarban, having a total mangrove cover of 2,106 sq km, accounting for 44.4 per cent, the highest in the country. It is followed by Gujarat which has about 1,107 sq km and the union territory of Andaman and Nicobar which has 617 sq km under mangroves (FSI, 2015).
However, anthropogenic pressures and frequent natural calamities have degraded India’s precious mangrove cover. Discharge from industries along our coastline and consequent discharge of domestic and industrial sewage have also adversely affected these ecosystems.
Tropical storms and cyclones common to the Bay of Bengal have caused massive destruction to mangroves in Pichavaram (Cauvery) and Bhitarkanika (Mahanadi). An obvious environmental impact of these natural disasters is the physical damage to the forest cover. However, significant modifications of the water and soil properties have also been documented. For instance, a study by Ranjan et. al. in 2008 on the Pichavaram mangroves highlighted the incidence of increased metal levels and a possible risk of heavy metal pollution due to the backwash or receding storm water from the hinterlands after the 2004 Indian Ocean Tsunami.
Increased urbanisation is another important cause of rapid degradation of mangroves. A study by Cho et. al. in 2004, found that even though no significant increase in agriculture was observed around the Pichavaram mangroves during past decades (1970-1996), change in land use patterns with the increase in aquaculture ponds has increased the stress on the mangrove ecosystem, especially since the 1990’s. In fact, there were no aquaculture ponds in the area around the ‘70s. In 1984, aquaculture ponds covered 3.99 sq km and by 1996, the coverage had nearly doubled to 6.99 sq km.
Another stark example of the impact of urbanisation can be seen in the reclamation and linking of the original seven islands that made Mumbai, by the destruction of mangroves (Kathiresan,2005).
In the Sundarban, large tracts of mangrove forests were documented to be destroyed between the 1770s, and the 1990s, which saw the conversion of forest land into farms and human habitat. The rising sea level and increased salinity has severely impacted mangrove vegetation in the Sundarban and Pichavaram (Gopal and Chauhan,2006). Studies have shown that the increasing water salinity is altering the species abundance and structure of the mangrove ecosystems herein. The loss of Heritiera and Nypa species in most parts of the Sundarban has been well documented (Badve and Sakurkar, 2003). In Pichavaram, freshwater-loving mangrove species are being gradually replaced by the salt-loving species like Avicennia and Suaeda (Ranjan et. al., 2015).
Mangrove conservation initiatives
It is clear that there is a vital need for systematic conservation efforts to sustainably manage the sensitive ecosystem. The Government of India has already initiated several such efforts in the past. The Environment Protection Act (1986) is a crucial piece of legislation for the conservation and management of mangrove ecosystems, and earmarks a coastal regulation zone (CRZ), wherein all industrial activities such as the discharge of untreated effluents and sewage, deforestation, dumping of waste, land conversion are restricted to protect the coastal environment. Since its inception in 1991, several amendments have been made to the CRZ keeping in mind the sustainable use and management of mangroves.
Besides this, there have been various attempts towards mangrove restoration through people’s participation in mangrove management. The M S Swaminathan Research Foundation (MSSRF) has initiated several projects in Pichavaram since 1992 to conserve mangroves by developing and demonstrating an effective mangrove restoration technique, which could be replicated in all other Indian mangrove ecosystems. This four year MSSRF project (1993-1996) revealed that grazing, land use changes, and reduced fresh water flow were not the only causes of degradation. Instead, it found, coupe felling was the main cause. Coupe felling laid barren large swathes of mangrove wetland to sunlight leading to evaporation of soil water. As a result, soil in the coupe-felled area shrunk, changing the flat topography into a trough. This made this barren area prone to the piling up of tidal water, thus increasing soil salinity to a level which can prove fatal to mangrove survival (MSSRF, 2002).
In order to overcome this problem, a fish bone/canal method for mangrove re-plantation and restoration was developed. Degraded/barren patches were restored by ensuring free flow of tidal water in and out of the degraded area during high tide and low tide by digging canals and linking them to the nearby natural creeks. These man-made canals also facilitated flooding of the degraded area with less saline or near-freshwater during the monsoon season. Successful regeneration of mangrove species was thus achieved.
Following the successful demonstration of this restoration technique in Pichavaram, similar efforts were later undertaken in mangroves in Andhra Pradesh, Odisha and West Bengal under a joint mangrove management plan. From the early 1990’s till the 2000’s, the work of the MSSRF and other government and non-government organisations, has led to an upsurge of national and international interest in conservation and management of mangroves. These efforts have helped in a net increase of 112 sq km of mangrove forests, as assessed by the Forest Survey of India in 2015 indicating that we have learnt well from our mistakes and experiences. In fact, the statistics provides us with a benchmark for further conservation activity.
Mangroves are not only important but also crucial for the protection and survival of our coastal areas. Even if the benefits of mangroves as forests be ignored, their role as a bulwark against extreme coastal events triggered by climate change is reason enough for conserving them.
Badve R. M. and C. V. Sakurkar. 2003. ‘On the disappearance of palm genus Nypa from the west coast with its present status in the Indian subcontinent’, Current Science, 85: 1407-1409.
Bhomia R. K., R. A. MacKenzie, D. Murdiyarso, S. D. Sasmito and J. Purbopuspito. 2016. ‘Impacts of Land Use on Indian Mangrove Forest Carbon Stocks: Implications for Conservation and Management’, Ecological Applications. doi:10.1890/15-2143.
Dittmar T., N. Hertkorn, G. Kattner and R. Lara. 2006. Mangroves, major source of dissolved organic carbon to the oceans. Global Biogeochemical Cycle, 20:1-7.
Forest Survey of India, 2015. Report of the National Forest Commission. Ministry of Environment and Forests, New Delhi, India.
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Ranjan R., A. Ramanathan and G. Singh. 2008. Evaluation of geochemical impact of tsunami on Pichavaram mangrove ecosystem, southeast coast of India. Environmental Geology, 55: 687-697.
Ranjan R. K., J. Routh, J. V. Klump and A. L. Ramanathan. 2015. Sediment biomarker profiles trace organic matter input in the Pichavaram mangrove complex, southeastern India. Marine Chemistry, 171: 44-57.