‘Project mangroves’ takes root at Botanic as changing climate pushes up river salinity

Kolkata:

Mangroves

are retuning to Kolkata after centuries, taking root along the

Hooghly

riverbank, and in such numbers that

Botanic Garden

has taken up a plantation project along a 1.6km stretch. And the changing climate has a role to play in this.

The Hooghly riverbank has slowly been turning into a natural habitat for mangroves because of rising salinity of river water. “The Farakka treaty and resultant low downflow are pushing the salinity up. Because of the rising sea level, more saline water is flowing in during high tide. Over the past decade or so, we have noticed natural regeneration of mangroves in city’s vicinity,” said Basant Kumar Singh, botanical assistant, Acharya Jagadish Chandra Bose Indian Botanic Garden.

The project — ‘Ex situ Conservation of Mangroves’ — had been on the drawing board for the past three years, and finally, on July 6, a small ceremony was organized to start the plantation drive on a 4-acre patch. The ceremony was attended by, among others, Howrah police commissioner Gaurav Sharma. “Our objective is to stop soil erosion and conservation of some mangrove species to create a new biodiversity zone,” said A A Mao, director, Botanical Survey of India.

“In the last 50 years, no mangrove plantation drive has been successful at Botanic Garden since it is difficult to grow mangroves unless the water salinity is high,” said garden in-charge M U Sharief. “But over the past 10 years or so, mangrove seeds that flow up to Kolkata from the Sunderbans during high tide have started regenerating here. Now, you can find mangroves from Babughat to Belur,” said Singh.

The regeneration of mangrove seeds prompted botanists to check the salinity of the river here. And it was found that it was enough to start a mangrove planation.

“In 2018, the salinity during summer high tide here was around 0.022 ppt (parts per trillion). And salinity needed for most of these species is between 0.007 ppt and 0.012 ppt. So, we don’t have to provide any salt supplement,” said the botanist.

Tuhin Ghosh of Jadavpur University’s School of Oceanographic Studies, however, said, “The Farakka barrage project has reduced the river flow and triggered siltation in the lower course of the river. The riverbed is becoming shallower and water level is rising during high tides. This high-tide water is travelling more towards the north, and the brackish zone is shifting towards the north. Regarding the effect of

climate change

, I do not know if any study has established the correlation between rising sea level and the increase in salinity.”