Sundarbans are resilient in face of natural, manmade stress: A new study highlights how

First, what are mangroves, and why are they important?

Mangroves are the only kind of trees that can thrive in salty water near the sea. The Sunderbans, for example, are located in the delta formed by the Ganga, the Brahmaputra, and the Meghna rivers off the coast of Bengal, which means they have access to fresh water while also being washed by the tidal salty water of the Bay of Bengal.

According to a Global Mangrove Alliance report on the Sundarbans, “Threatened and endangered wildlife — like the Bengal tiger, Irrawaddy river dolphin, finless porpoise, and estuarine crocodile — live here. It is also a nursery for 90% of the aquatic species on the East coast of India and serves as a bio-shield for the fierce waves and winds of coastal storms.”

A tiger in Sundarban. (Express file photo)

Mangroves are an important agent of another critical process — carbon sequestration, which basically means the removal of carbon dioxide from the atmosphere.

Removing carbon from the atmosphere is crucial in the fight against global warming. While all plants use up carbon dioxide from the air for photosynthesis, mangroves “extract up to five times more carbon than forests on land, incorporating it in their leaves, branches, roots and the sediments beneath them,” says an article by the United Nations Environment Programme.

Mangroves also have high carbon density, which is the amount of carbon a plant can store.

The Indian Sundarbans were classified as endangered in a 2020 assessment under the IUCN Red List of Ecosystems, making a detailed study about their threats and fight-back mechanisms significant.

What is the study?

The study, ‘Resilience of Sundarban mangroves in South Asia to Weather Extremes and Anthropogenic Water Pollution’, was conducted by researchers from IIT Bombay, Indian Institute of Science Education and Research, Kolkata, and National Remote Sensing Centre (ISRO), Hyderabad.

A plant interacts with its atmosphere in various ways, and these interactions are an indication of its health. For example, a plant takes nutrients from soil and water, carbon dioxide from the air, and releases water vapour and oxygen through its leaves.

Thus, the study looked at the nutrient profile available to the mangroves in the soil from 2013 to 2015 and studied their response to it. It also looked at how mangroves reacted to natural events like a cyclone. This was done by studying the mangroves’ interactions with a variety of factors, including water, wind, water vapour, sunlight, etc.

One of the markers of a plant’s health is ‘vegetation productivity’, basically the rate at which it can convert solar energy into biomass through photosynthesis. To study this, researchers looked at the light being re-emitted (after absorption for photosynthesis) by the chlorophyll in the mangroves. They also looked at the data from a flux tower, which measures the rate at which gases like carbon dioxide and water vapour move between the earth (including plants) and the atmosphere.

What did the study find?

“Our results show that mangroves recover from physiological stresses caused by weather extremes quickly, within one to two weeks, and maintain stable productivity despite steeply declining nutrient composition due to human-induced water pollution. We demonstrate that mangroves maintain this stable productivity by increasing link strength and memory with the hydro-meteorological variables of the region,” the study says.

To establish “steeply declining nutrient composition”, the study considered the Redfield Ratio, the ratio of carbon to nitrogen to phosphorus, which has an optimum value of 106:16:1. “We found that the N/Ph ratio has constantly increased from being close to the Redfield ratio of around 16:1 in to an average value of around 75:1 in 2015. The increase is roughly five-fold, with a few months showing around 10-fold increases from 2013 to 2015. During the same period, the amount of Nitrates have increased, and Phosphates have decreased,” the study says.

And what is “link strength and memory”?

A large and complex vegetation system like mangroves has several links — intertwined roots, shared nutrients and soil composition, shared stress factors, etc. Memory in plants refers to “remembering” how they had reacted to a past stressful event, like a cyclone, and storing that response for the future.

Subimal Ghosh, Professor at IIT Bombay’s Centre for Climate Studies and one of the co-authors of the study, told The Indian Express, “We have found that depending on the stress they are facing, mangroves alter how they interact with the environment, in order to tide over the stressful event. For example, if water chemistry changes, mangroves adjust their response to hydrometeorological processes around them.”

Why are the findings significant?

To put it very simply, for now, the Sundarbans have shown resilience, but there is a limit to their ability to self-repair. The study sheds light on the challenges the Sundarbans are facing and is one step forward in looking for ways to protect and preserve them.

As the researchers write, “The mangrove resistance to natural and anthropogenic stressors observed reflects the resistance provided by highly connected networks. However, the limits of this resistance and the nature of associated critical transitions remain unknown. Our study advances the understanding of the dynamics of mangroves and calls for examining and quantifying the limits of mangrove resistance to anthropogenic stressors. The generated information will be essential for undertaking scientifically driven nature-based solutions interventions for restoring mangroves in South Asia.”