“It is quite unusual for the mangrove to grow at a place far upstream from the Bay of Bengal. It indicates increased salinity in the river water caused by global warming and consequent sea level rise,” Pranabes Sanyal, ocean scientist and eastern India representative of National Coastal Zone Management Authority, said here.

Sanyal and his team of researchers spotted the 25-ft tall plant on the banks of Hooghly river, which is usually found in the high salt, low oxygen soil of the Sundarbans.

They were also found growing at the Kidderpore docks, Judges Ghat, Prinsep Ghat and Shibpur Botanical Garden, besides Millennium Park.

The plant, with its distinctive oval-shaped, leathery leaves with a thick upper coat of wax and pneumatophores sticking out of the mud near its base at Millennium Park drew the attention of members of the nearby government-run Sea Explorers Institute (SEI).

When brought to Sanyal’s notice, a survey was carried out with members of SEI and the plants identified, Tapas Chowdhury of SEI said.

Sanyal said, “The mangrove plants have grown here for the past eight years corroborating the reports of International Panel for Climate Change (IPCC) that the global average temperature has risen steeply since 2000, causing rapid sea-level rise.”

Sea level in some parts of Sunderbans has been rising at 3.14 mm annually against the global average of 2 mm, experts say.

A river water salinity measurement conducted by West Bengal Pollution Control Board too showed a sudden rise in salinity level in the Hooghly river water.

The average salinity of river water which varies with the tide, addition of upstream water during monsoon and release from Farraka Barrage was noticed to rise from 16 ppt in 1994 to 20 ppt in 2006 at Sagar Island. The germinated seeds of mangroves from Sunderbans are likely to have floated upstream in high tide and had taken root on the banks of the river, Sanyal said.

In previous studies, mangroves were seen to grow downstream of Hooghly river at Diamond Harbour, 50-km from the Bay of Bengal.

“But, their growth here about 120-km upstream and at a place 14-ft above the sea level is quite unusual,” Sanyal said.

However, the development has cheered environmentalists who suggest that the plants could be planted on the river bank after proper survey.