New Delhi

India is set to go ahead with plans to mine millions of tonnes of manganese nodules from the Indian Ocean's floor, despite warnings from an environmental assessment that the project will disrupt deep-sea ecology.

The potato-shaped nodules are rich in manganese, copper, nickel, cobalt and iron. India has a shortage of some of these elements, and has purchased exclusive rights from the International Seabed Authority to mine the nodules from 150,000 square kilometres of the Indian Ocean. The country estimates that the reserves could be worth as much as $200 billion.

But researchers at the National Institute of Oceanography (NIO) in Goa, who undertook a survey to assess the environmental impact of mining operations, say that discharges of unwanted sediment during mining will upset the area's ecological balance.

To simulate the mining process, the researchers mimicked the release of sediment over a small area 5,500 metres below the sea's surface.

During a nine-day experiment in 1997, they lifted about 580 tonnes of sediment from the sea floor and then released it five metres above the sea bed. They have since been tracking how the sediment plumes drift and what effect they have on the area's ecology.

The recently released report says that “noticeable changes were observed in the physical and biogeochemical properties” of the area. The sediment plumes migrated at least two kilometres from the drop site, and the abundance of fauna and bacteria decreased in the region, says marine geologist Anil Valsangkar, who led the study.

Valsangkar says that actual mining would cause much more disruption if, as is likely, the mining ships discharged the sediments at the sea's surface.

The Indian government will not say when it plans to begin the mining operation, but it has spent about $40 million surveying the site, has bought a ship and is developing machinery for the purpose. NIO sources say that the environmental study was conducted to comply with the law, rather than to obtain clearance to proceed.

But critics have questioned the plan's economics, given that the nodules are so far below the sea's surface and that the site is 2,000 km away from India.