High Salinity Destroys Mangrove Diversity
When Amphan hit West Bengal in May, thermal imaging showed the mangroves of Sundarbans absorbing most of the brutal force of the cyclone before it crossed into the land. This was similar to how the mangrove forests along the coast of Tamil Nadu mitigated cyclone Gaja’s impact in 2018.
These incidents have highlighted the importance of mangroves, with governments realising the urgency with which they must be preserved. But, while the focus is on protecting mangroves, not much attention is paid to its biodiversity.
For instance, five species of mangrove trees have become locally extinct owing to the rising soil and water salinity levels in coastal Tamil Nadu. The wetlands of Muthupet and Pichavaram — which about two decades ago had a thriving variety of mangroves — today have only one dominant species, Avicennia Marina.
According to experts, it is the hardwood mangroves which act as an effective barrier against strong winds. Avicennia Marina, however, is a softwood species with high tolerance to soil salinity which has taken over the entire region as other species are unable cope with the increasing salinity levels.
“Salinity is one of the most important factors affecting mangrove health,” says Dr R Ramasubramanian, Principal Co-ordinator-Coastal Systems Research, MS Swaminathan Research Foundation (MSSRF).
“If the water is highly saline, mangrove plants need to use a lot of their energy to overcome the salinity levels. Mangroves are salt-tolerant species but if it takes in more salt than it needs, the plant tries to send out the extra salt and to do that, it requires a lot of energy,” he added.
Ramasubramanian said that if the salinity levels remain high, the mangroves exhaust themselves and their growth is stunted. That is what happened to species like Sonneratia apetala and Rhizophora, the researcher said.
Currently, other low salt-tolerant species such as Xylocarpus granatum, Kandelia candel, Cynometra Irripa and Bruguiera gymnorhiza have become locally extinct.
“Now, you cannot find a single Sonneratia apetala in Pichavaram and Muthupet. Only Avicennia Marina, a high-salt tolerant mangrove species, can survive in such high salinity levels and they have taken over the wetlands. There isn’t much diversity in species there anymore,” said Ramasubramanian.
Elaborating on the risks of losing species diversity, researcher V Selvam, who has spent decades studying mangroves, said, “Species such as Avicennia Marina that have been spreading rapidly across the coast are structurally weak and vulnerable to strong winds. Unlike the hardwood varieties, they will not be able to protect the coastal ecosystems from storms.”
Here, Ramasubramanian pointed out that soil salinity is a problem specific to the east coast of the country. “The west coast receives almost double the rainfall and as a result, even though the mangrove population is sparse in the west coast, their biodiversity is very rich. The rainfall keeps the salinity levels in check, allowing other mangrove species to flourish,” Ramasubramanian said.
Another factor contributing to high salinity levels is the discharge of water into the sea. Selvam said, “Because Tamil Nadu often faces water shortage, the government tries to utilise as much water possible and letting it flow into the sea is seen as wastage. This has been aggravating problems related to soil salinity.”
He added that the loss of mangrove biodiversity may eventually affect the communities who depend on it for their livelihood.
A 2013 study published by Rajarshi Das Gupta and Rajib Shaw from Graduate School of Global Environmental Studies, Kyoto University, Japan, on Cumulative Impacts of Human Interventions and Climate Change on Mangrove Ecosystems of South and Southeast Asia also highlighted the connection between the loss of species biodiversity and livelihood.
It stated: “Potential consequences of the loss of mangroves in this region are multifaceted with distinct ecological, social, and economic dimensions. In many remote coastal areas of this region, mangrove-based livelihood opportunities strongly define the social and economical well-being of the coastal communities.”
“Over the years, mangroves have sustained more than 70 direct human activities. Of these, on-shore fishing and timber and firewood collection are worth mentioning in the backdrop of South and Southeast Asia, where rural poverty still forms a critical sociopolitical issue. Degradation of mangroves has adversely impacted traditional fishing activities. Many aquatic species including the commercially important fishes were lost or significantly reduced along with the mangroves.”
While both Selvam and Ramasubramanian agreed that loss of livelihood among coastal communities is not a threat right now, they insisted that it may develop into a major problem if salinity levels increase unchecked.
“Marine animals that depend on mangroves can withstand slight changes in the environment, however, if the salinity levels continue to go up it will definitely decrease fish productivity. It will also make the environment uninhabitable for marine animals such as mud crabs, oysters and barnacles that live among mangrove roots, and in turn affect the livelihood of coastal communities,” Ramasubramanian said.