Why scientists are planting steel rods in India's mangroves

Perched on a wooden plank, ecologist Sahadev Sharma cautiously drills steel rods into the mud near the tangled roots of mangrove trees. In the fast-depleting Sundarbans forest, the rods will monitor whether the mangroves are building enough sediments in time to outpace the rising seas around them.

For Sharma, the sediments in these mangroves are vital. The Indian Sundarbans is a cyclone-battered collection of about 100 islands, home to 4.5 million people, and the world’s largest continuous mangrove stretch spanning 10,000 square kilometres across the Indo-Bangladesh border.

The aerial root systems of mangroves trap sediments in a process called vertical accretion, pushing the forest floor up as they continue to amass fresh sediment from the catchment. This way they store carbon dioxide at densities far greater than other forests and reduce erosion from water surges such as storms and tides. Mangroves protect the coastal villages from flooding and provide habitat for fish and species such as the endangered Bengal tiger and Gangetic dolphins in the Sundarbans.

Sharma is camping in the Sundarbans with a collaborative scientific team led by the Center for International Forestry Research (CIFOR-ICRAF) and the United States Forest Service. Researchers, Samakshi Tiwari, Utchimahali Mariappan and Sudip Debnath wade in the mangrove slush after parking their boat in one of the creeks. Over the past year, they have been planting steel rods across mangroves in India – from the Andaman islands, to Coringa in Andhra Pradesh, in collaboration with regional forest departments.

The steel rods, called surface elevation table marker horizon set or rSET-MH rods, are used internationally to measure estuary level changes.

“We can now compare data from the benchmark sites to build a comprehensive picture of what’s happening to our mangroves,” Sharma tells Nature India at an installation site in Lothian island, a boat ride from the Namkhana village in the South 24 Parganas district of West Bengal.

Forest guards watch the installations across the Sundarbans, especially looking out for Bengal tigers that swim across this maze of transboundary islands.

In the Sundarbans, land erosion and submergence have swept away up to 110 square km¹ of contiguous mangroves between 2000 and 2020. Part of the landscape also gained 81 sq km during this time through plantation and regeneration, partially compensating for the losses.

“In addition to climate change-induced sea level rise, gradual subsidence of the Bengal delta, a natural geological feature, deposits sediments in an uneven fashion, causing the land (and mangroves) to sink more rapidly in some parts, increasing the relative sea level,” says oceanographer, Sugata Hazra, former director at the School of Oceanographic Studies in Jadavpur University in Kolkata.

Dam construction and water diversions upstream of deltaic rivers also starve mangroves of sediment, a mix of silt, clay and fine sand grains.

Frequent cyclones this past decade have wreaked havoc on the fragile ecosystem. “Sea level rise is outpacing sediment deposition in the Indian Sundarbans. We can’t save them by planting saplings because there isn’t enough alluvium in the Ganga delta for them to bind and grow,” says Hazra.

Mounting human pressures are making mangrove shorelines behave differently. Coastal geographer and geospatial analyst, Radhika Bhargava, at the National University of Singapore, says the shape of eroding shores steer the cyclone to chip away more land. Bhargava shows the hotspots of mangrove damage across three recent cyclones² on a satellite image map.

“The highest mangrove losses were along shorelines that were eroding over the past 35 years which means the legacy of our actions has had a lasting impact,” she says.

Reduced freshwater supply has also affected the health of the mangroves. Pointing to the oblong leaves of freshwater-loving mangrove Sundari (Heritiera fomes), an iconic species that gives the island conglomerate its name, Anil Mistry, a local conservationist with the Wildlife Protection Society of India says, “You will see less and less of Sundari and other freshwater-loving species Nypa, and Sonneratia because there isn’t enough sweet water (freshwater) to keep them supple.”

Finding the ‘sweet spot’ for mangroves

In 2015, using rod installation data from 27 sites across the Indo-Pacific, marine ecologist, Catherine Lovelock, and colleagues concluded that sediment supply is important for mangrove resilience. This region, she says, holds most of the world's mangrove forests, but sediment supply is declining, due to infrastructure expansion.

“They (mangroves) don’t like to be in water all the time. And if they lose elevation, eventually they'll move out of that sweet spot into something terrible,” she says.

Low sediment supply and high rate of sea level rise is a recipe for disaster for coastal wetlands. Lovelock says these datasets provide information on exactly where mangroves need urgent protection and where they are they are doing well.

In China, USA, Australia, and Indonesia, rSET measurements, along with ground-truthing and remote sensing (Lidar or Light Detection and Ranging), offer big-picture data on mangrove and coastal wetland resilience. rSET measurements help pinpoint the inundation regime best suited for mangroves and other coastal ecosystems.

“Poorly managed and sick mangroves can become carbon sources instead. rSETs give us the data we need to make practical restoration and conservation strategies,” says CIFOR-ICRAF's Rupesh Bhomia, the project's lead scientist.

On a surf-sprayed, laminated map of the Indian Sundarbans, Tiwari pins islands where they have already installed the rods and the ones they will sail to next. In three weeks, the hardy team has navigated a dense maze of islands, creeks and estuaries installing 20 rSETs, 25% of what is being deployed across six mangrove sites in India.

Some rSETs are fitted with data loggers to measure salinity and depth of water that might indicate tidal flooding levels, says Samakshi Tiwari of the rSET-installing team. The scientists will return to these sites every few months for baseline measurements of mangrove forest floor height and other changes.

How much mud is enough?

Hazra says the landscape needs about 1 cm of sediment every year to remain alive. This figure factors in the established 6 mm per year sea level rise³ threshold that mangroves can tolerate before the sea drowns them, and the minimum regional deltaic subsidence rate of 2.5mm per year.

Even with this bare minimum sediment refill, 10% to 30% of mangroves will likely disappear by the year 2100, according to Hazra’s latest simulation⁴ that looks at sea-level rise due to climate change and natural geological subsidence. If we continue on a high-emissions path and fail to protect the forest-islands through elevation maintenance, the rate of sea-level rise will surpass the growth rate of Indian Sundarbans mangroves in 70 years.

In that case, as much as 42 to 80% of Indian Sundarbans mangroves will face a substantial risk of loss.

Safeguards, including reducing greenhouse gas emissions and restoring freshwater supply to the mangroves by revamping river basin management, could arrest further damage.