The Bay of Bengal hosts a ‘dead zone’ of an estimated 60,000 square kilometres – an area that contains little or no oxygen and supports microbial processes that remove vast amounts of nitrogen from the ocean, scientists including those from India have found.
Dead zones are well known off the western coasts of North and South America, off the coast of Namibia and off the west coast of India in the Arabian Sea.
“The Bay of Bengal has long stood as an enigma because standard techniques suggest no oxygen in the waters, but, despite this, there has been no indication of nitrogen loss as in other ‘dead zones’ of the global ocean,” said Laura Bristow, a former postdoc at University of Southern Denmark.
Researchers, including those from the Council of Scientific and Industrial Research’s National Institute of Oceanography (NIO) in Goa, demonstrated that some oxygen does exist in the Bay of Bengal waters, but at concentrations much less than standard techniques could detect and some 10,000 times less than that found in the air-saturated surface waters.
The researchers also discovered that the Bay of Bengal hosts microbial communities that can remove nitrogen, as in other well-known dead zones and even some evidence that they do remove nitrogen, but at really slow rates.
“We have this crazy situation in the Bay of Bengal where the microbes are poised and ready to remove lots more nitrogen than they do, but the trace amounts of oxygen keep them from doing so,” said Bristow, now a scientist at the Max Planck Institute (MPI) in Germany.
“Remove the last amounts of oxygen, and the Bay of Bengal could become a major global player in nitrogen removal from the oceans,” said Wajih Naqvi, former director of NIO and a co-author of the study published in the journal Nature Geosciences.
Removing more nitrogen from the oceans could affect the marine nitrogen balance and rates of marine productivity.
Globally, warming of the atmosphere through climate change is predicted to lead to an expansion of ‘dead zones’ in the ocean.
It is currently unclear whether climate change would lead to the removal of these last traces of oxygen from the Bay of Bengal waters.
However, the Bay of Bengal is also surrounded by a heavy population density, and expected increases in fertiliser input to the Bay may increase its productivity, contributing to oxygen depletion at depth.
“Time will tell, but the Bay of Bengal is at a ‘tipping point’, and we currently need models to illuminate how human activities will impact the nitrogen cycle in the Bay of Bengal, and also globally,” said Bristow.