Scientists raise caution as mangroves erode faster along the Bengal coastline

India’s mangroves along the Bengal coastline, already at risk due to climate change, sea-level rise and land-use changes, are beginning to fast erode, new research shows.

West Bengal contributes the maximum – 43% — of India’s mangroves. A team of scientists from the department of geography at the Delhi School of Economics in the University of Delhi, and the West Bengal Biodiversity Board in Kolkata, divided West Bengal state’s coast and mangrove areas into grids and found that about one-third of the coastal grids, and around 40% of the mangrove grids, are highly vulnerable to climate change. The mangrove-dominated islands such as Thakuran Reserve Forest, Bulcherry Island, Dulibhasani Reserve Forest, Dalhousie Island, Bhangaduni, and Gosaba are under severe threat, their report in Remote Sensing Applications: Society and Environment says.

The findings add to those of a second recent study in the Journal of Cleaner Production on changes in the shoreline of three islands in the estuaries of the Sundarbans delta that is both a Ramsar Site and a World Natural Heritage Site (UNESCO). The Sundarbans study reports a very dynamic shoreline, with the erosion of mangrove forests which, the authors say, are under severe stress more due to shoreline ingression and sea-level rise, and not climatic alterations.

Mangroves and climate change

The findings come against a backdrop of increasing concerns over the state of mangroves that not only harbour rich biodiversity but also help in climate change mitigation. In 2021, the United Nations Environment Programme (UNEP) observed that mangroves extract five times more carbon from the atmosphere than forests on land. The UNEP also said that protecting mangroves is 1000 times less expensive than building sea walls to combat sea-level rise. In 2020, the UNEP said, that although mangroves make up less than 1% of all tropical forests around the world, “they have a critical role in mitigating climate change.

The study in Sundarbans, by scientists from Kolkata and Ranchi in Jharkhand, used Landsat satellite images from 1975 to 2017 to analyse the shoreline changes of three estuarine Islands in the Sundarbans delta, and the impact on mangrove forests around the Islands. The researchers also prepared mangrove degradation maps from the data and combined them with evidence collected from field works.

The scientists, in the second study, have reported “a very dynamic shoreline ensuing in the erosion of mangrove forests”. While some areas showed encouraging trends due to sustained accretion, especially in the southern and eastern parts, the “overall erosion is higher than accretion in the islands,” they report. Their results also show that vegetation has been decreasing along patches that are near erosion hotspots, irrespective of climatic trends. So, they argue, more than climatic trends, it is shoreline ingression and sea-level rise that have put mangrove forests “under severe stress”.

Going forward this work could provide significant information on the nature of shoreline changes and could assist in sustainable development for Sundarbans biodiversity niche management. Among the various climate change-linked factors such as sea-level rise, high-water events, storminess, precipitation and temperature that impact mangroves, sea-level rise may have the most significant impact on coastal mangroves, the authors say.

The new Sundarbans report adds to previous similar findings on the area by a Delhi School of Economics team of Ashis Saha and Biswajit Mondal, who in 2018 they reported that the “mangroves are gradually reducing over the last 40 years and about 4% mangrove area has been converted into water.” In Bhangaduni, Bulchery, Dalhousie and Halliday islands, more than one-third of both the land area as well as mangroves have been destroyed. It is a major indication of an increase in seawater level, making many islands vulnerable, their report says.

“Sundarbans is certainly under threat,” Kathiresan Kandaswamy, professor of marine biology at the Annamalai University, Chennai. Shoreline changes will reduce the mangrove habitat and associated organisms, besides increasing soil salinity, flood risk and coastal erosion. In general, the east coast of India has a smooth slope and is, hence, vulnerable to sea-level rise and flooding.

The natural threat to the Sundarbans is increasingly more important than the man-made threat, Kandaswamy says. “Basically the Sundarbans is located in the most stressful environment with rough tides 3.5-5.0 m high, erosion, salinity, natural disasters, and human pressure, in the long past since 18th century, and (now) climate change.”

Kandaswamy, however, does not agree with the second study’s conclusion that mangrove forests are under severe stress due to shoreline ingression and sea-level rise and not climatic alterations. “Climate alterations cannot be ignored for the vulnerability for the reason that the sea surface temperature increases by 0.5o Celsius per decade in the Sundarbans, as against global the average of 0.060 C,” he says. Also, other climate change-related factors do prevail in the region, such as erratic monsoon with heavy rainfall or longer periods of drier monsoon, and 29% increase in severe storms in the past 120 years in the region, he adds.

Other mangroves

Other Indian mangroves too are under threat due to increased coastal erosion due to natural disasters such as cyclones and tsunamis; sea-level rise; human interventions such as building ports, fishing harbours, and jetties; and reduced inflow of sediments from the river, says R Ramasubramanian, director of coastal system research programme at the MS Swaminathan Research Foundation (MSSRF), Chennai. The rate of erosion has increased due to the lack of sediment supply as the construction of dams in these areas affects the sediment supply.

For example, scientists have observed erosion in the Godavari and Krishna mangroves along the Bay of Bengal. However, in these areas, the rate of erosion is less due to mangrove vegetation. In the Godavari coastal region, for example, mangroves erosion and accretion are more or less the same, says Ramasubramanian.  Erosion is occurring along the shore and accretion is occurring in sheltered coastlines such as the lagoons and the Kakinada bay, he adds.

Similarly, in Muthupet mangroves in Tamil Nadu, large-scale mangrove destruction occurred after the 2019 Gaja cyclone that uprooted a large number of trees and led to the loss of nearly three square km of mangroves, according to data from the 2019 Forest Survey of India.

Mangroves help in stemming the coast erosion, as the mangrove roots trap sediments. If there were dense mangroves in Muthupet, for example, the impact would have been less, says Ramasubramanian. Freshwater supply in the mangrove wetlands of Muthupet and Pichavaram also impacts these forests, he adds. The increase in temperature and lesser freshwater flow into the mangroves has resulted in the death of mangroves.

Mitigating the problem

Kandaswamy points out that the construction of an embankment, about 3500 km long, in the Sundarbans to prevent seawater from entering agriculture farms and human dwellings during high tide, poses a “serious threat” to the mangroves though.  “This construction has reduced the mangrove and shoreline vegetation in the periphery of islands.”

Instead, he suggests, the embankment should be pushed towards the interior and in a ring-like fashion. He also recommends growing trees such as Calophyllum inophyllum, and Casuarina inside the embankments and mangroves outside the embankments to prevent erosion.

Other suggested measures include first developing the eroded areas with grasses such as Porteresia coarctata to stabilise the soil, and then raising the mangroves. People’s participation in such programmes, as well as climate mitigation and adaptation measures also help in managing the erosion, he adds.

The Indian government has taken various measures such as hard engineering structures to reduce erosion which has yielded good results. “However, these structures require huge sum for the establishment and their maintenance,” points out Kandaswamy. “Now people are suggesting hybrid models with the combination of hard and soft structure like mangroves to act as a barrier for shoreline erosion.”

Sediment supply through river water or beach nourishment of sand are possible solutions to reverse the trend and it will be difficult to achieve, he adds. Some of the measures seem to be working. Asish Saha’s team attempted to map land-use changes and land cover in mangrove areas, using remote sensing data from 1989 to 2018. In 2021, they reported that “the overall mangrove area increased to 0.48%, which is about 68.06 sq. km.” They attributed the increase in mangrove cover in some parts to conservation strategies, including plantation, raising awareness of mangroves’ importance in the local communities, and declaring Sundarbans first as a mangrove sanctuary, and later as a World Heritage Site in 1987, as a Biosphere Reserve in 1989, and Ramsar wetland in 2019.

Banner image: Bengal’s mangroves are eroding due to multiple factors. Photo by Snigdhendu Bhattacharya.