Women walk across the East Calcutta Wetlands area in Kolkata. AFP

Wetlands just outside India's Kolkata have for generations provided tonnes of food daily and thousands of jobs as they filter sewage through fish ponds -- but rapid urbanisation is threatening the ecosystem.

Conservationists warn that pollution and strong-arm land grabs are putting a lifeline for the megacity's 14 million residents at risk.

"We are destroying the environment," said Tapan Kumar Mondal, who has spent his life farming fish in the ingenious system of canals and ponds stretching across about 125 square kilometres (48 square miles).

"The population... has increased, there is a pressure on nature, they are ruining it," 71-year-old Mondal added.

  A woman rides a bicycle along a path in the East Calcutta Wetlands area. AFP

Listed as a wetland of global importance under the United Nations Ramsar convention, the waters offer natural climate control by cooling sweltering temperatures -- and act as valuable flood defences for low-lying Kolkata.

But Dhruba Das Gupta, from the environmental group SCOPE, said that short-sighted building development was encroaching on the wetlands.

"The wetlands are shrinking," said the researcher, who is trying to finance a study of what is left of the waters.

Ecologically-subsidised city

Every day, 910 million litres of nutrient-rich sewage flow into the wetland, feeding a network of about 250 hyacinth-covered ponds.

 Fishermen prepare to catch fish in the East Calcutta Wetlands area in Kolkata.

"Sunlight and the sewage create a massive plankton boom," said K. Balamurugan, chief environment officer for West Bengal state, explaining that the microorganisms in the shallow fish ponds feed rapidly growing carp and tilapia.

Once the fish have had their fill, the water runoff irrigates surrounding rice paddies and the remaining organic waste fertilises vegetable fields.

"The sewage of the city is being naturally treated by the wetlands," Balamurugan said, giving them the nickname the "kidneys of Kolkata".

The late ecologist Dhrubajyoti Ghosh, who played a key role in the 2002 Ramsar submission, called Kolkata an "ecologically-subsidised city".

The wetlands system processes about 60 percent of Kolkata's sewage free of charge, saving the city over $64 million a year, according to a 2017 University of Calcutta study.

 Fishermen prepare to release fish in a larger waterbody for further growth in the East Calcutta Wetlands. AFP

For Kolkata, on the vast delta where the Ganges River meets the Indian Ocean, the wetlands also provide flood defences for a city facing rising sea levels due to climate change.

Land is being snatched

But the Ramsar listing notes that industrial effluent is tainting natural systems, threatening food production.

Fish farmer Sujit Mondal, 41, said that compared to last year "production has reduced" because of "murky water".

About 95 percent of the wetlands are in private hands.

 Environmental activist Dhruba Das Gupta talks with fisherman Sujit Mondal.  

As land prices surge, environment officials say they have pleaded with people not to fill in the fish ponds to create new building space.

"We asked them not to convert the wetlands, not to trade these wetlands to buildings, not to get them filled," Balamurugan said.

But residents say village councils are being bribed by land-hungry developers.

"They are often accused by residents of giving informal permission in return for money to real estate developers to build, while they look the other way," said Das Gupta.

Agence France-Presse