The mangroves of Kolkata are a sign that the city’s environment is changing

Mangroves growing by the Hooghly river in Kolkata. (Express photo: Neha Banka)

“The mangroves are an indication that salinity has increased in the Hooghly,” said Dr. Ghosh. But environmental changes in the Kolkata metropolitan area that are likely to have created ideal conditions for mangrove growth can be traced back to the building of the Farakka Barrage upstream in 1975, which was established to increase the navigability of the Hooghly River. 

After the barrage was commissioned, however, the river flow declined significantly and the diversion of the water by the barrage resulted in significant changes in the hydrology of the Ganges river system. A rise in the sea level has also contributed to creating environmental conditions for these mangroves, said Dr. Himadri Sekhar Debnath, chairperson of the West Bengal Biodiversity Board who has conducted extensive research on mangroves in the region. “The river flow has been blocked due to dams and fisheries. The mangroves grow in an ecosystem created in the confluence where the fresh water empties into the sea and where the brackish water enters the rivers. Now, the freshwater flow has reduced, but the amount of salt water entering into estuaries through tides has not,” explained Debnath.

During his research, Ghosh identified at least 14 different species of mangroves growing on the banks of the Hooghly river, from Kolkata all the way up to Barrackpore, approximately 160 kilometres away from the Sundarbans. Ghosh found that the Sonneratia caseolaris, a mangrove species that thrives in less saline waters than what is typically found in the Sundarbans, was growing by the banks of the Hooghly in Kolkata faster than any other mangrove species. 

A lone Sonneratia caseolaris mangrove growing along the eastern bank of the Hooghly, near central Kolkata. (Express photo: Neha Banka)

A higher number of the Sonneratia caseolaris mangroves in the Kolkata metropolitan area have been found along the eastern bank of the Hooghly, near central Kolkata, because the western bank of the river towards Howrah is largely covered with brick or concrete that prevents the deposition of soil, he said.

The mangroves that have been growing on the Hooghly river bank are a result of natural dispersion and are a species that thrive in freshwater. That means that unlike the mangroves in the Sundarbans, these grow in less saline environments, but still require certain levels of salinity which is present in the Hooghly river’s waters. 

A cluster of mangroves growing by the river in Kolkata. (Photo credit: Dr. Somdeep Ghosh)

“These mangroves are an indicator that some change to the environment has been coming. The mangroves are migrating upstream. The fact that they are growing here is an indicator that salinity is slowly rising in the Ganga River,” said Dr. Punarbasu Chaudhuri, assistant professor of Environmental Science, University Of Calcutta.

While the mangroves have not been growing in overly large numbers, for scientists, their growth in the city’s region is a sign that Kolkata’s environmental conditions are rapidly changing, primarily due to anthropogenic activities. “Mangrove migration is an indicator of climate change. But the global climate is changing and this migration is one expression of it,” said Dr. Chaudhuri.

Archaeological evidence indicates the presence of mangrove flora in the Holocene period in and around Kolkata some 7030 years  ago and in Howrah some 2,040 years ago. Scientific study points to the presence of diverse mangrove flora approximately 6,530 years ago in Rajarhat, now a part of the Kolkata metropolitan area.

The mangroves that have been growing on the Hooghly river bank are a result of natural dispersion and are a species that thrive in freshwater. (Photo credit: Dr. Somdeep Ghosh)

Colonial records of the city mention how thick jungle land in deltaic Bengal was slowly cleared over the centuries to create urban dwellings as towns and cities in the region grew.  In his book ‘Calcutta During Last Century’, Henry Ferdinand Blochmann, a German scholar who studied the Indian subcontinent and taught Persian during the 1860s in Calcutta, writes that in 1717, the area that is now called the Maidan and surrounding areas including Chowringhee, was mostly a dense “forest”, in the midst of which the structure of Fort William was established.

He writes that Chowringhee during the 18th century was a rural area, with small “puddles of water”. A creek ran from Chandpal Ghat by the Hooghly River, all the way up to Wellington Square, passing the Government House and Dharmatolla. Over the decades, the forest was gradually cleared to allow for the establishment of the villages that occupied Chowringhee.

Some of these “relic” mangroves have managed to survive, and can be found in the National Library and the Raj Bhawan, as well in small clusters scattered across the city.

Mangroves in its early stage of growth by the banks of the Hooghly river in Kolkata. (Express photo: Neha Banka)

“When the Chowringhee area was being excavated for metro lines, there used to be mounds of excavated soil that was dumped in the Maidan. When the soil was being excavated, the top of the mound contained soil from the very bottom of the excavation. Back then, we had found partial fossils of mangrove species. So the Chowringhee area did have mangroves a few centuries ago,” said Dr. Debnath. 

He pointed to the Salt Lake area of the city and its nomenclature. “This area had water bodies with brackish water and mangroves used to grow here just like in the Sundarbans,” Dr. Debnath explained. Over the past four decades, rapid urbanisation has changed the landscape of the Salt Lake area, transforming it from its origins as a salty marsh. “The mangroves in and around Kolkata grew because salt water would come into the city through creeks,” said Dr. Chaudhuri. 

These creeks that are spread across the city and have survived urbanisation, are a part of the Bidyadhari river system that empties into the Sundarbans. The mangroves that grew along these creeks thrived because of the abundance of saline water. “We can still see some mangroves in the East Kolkata Wetlands in Rajarhat,” said Dr. Chaudhuri. But these have historically been a part of the city’s ecosystem, unlike those growing by the Hooghly. 

The rate at which the salinity is increasing in the Hooghly, however, does not mean that the river’s water is going to turn brackish overnight, scientists interviewed for this report said. But the city of Kolkata is likely looking at a future where the river’s water may not be fit for human consumption, adding to the city’s water stress because a large percentage of the population relies on the river’s water which reaches millions of homes across the city after treatment in municipal plants. “The available ions in the water are increasing very slowly, so while it may not be significant to human health immediately, it is important for the survivability of these plants. In 30 to 40 years, the water may become harmful for human health,” said Dr. Chaudhuri.