doi:10.1038/nindia.2016.163 Published online 12 December 2016
The Bay of Bengal (BoB) hosts a 'dead zone' of around 60,000 square kilometers almost devoid of oxygen, a new multi-national study has shown1. The findings point to implications on global nitrogen balance.
Marine 'dead zones' contain no oxygen. Until now, there have been only three major identified dead zones – two in the eastern tropical Pacific (off Peru/Chile and Mexico) and one in the Arabian Sea. The newfound dead zone in BoB joins this list, according to a joint study by National Institute of Oceanography (NIO), Goa, India; University of Southern Denmark and the Max Planck Institute for Marine Microbiology in Bremen, Germany.
"Oxygen depletion in the oceans occurs both due to natural causes and human activities," Wajih Naqvi, former director of NIO, and a co-author of the study told Nature India. Lack of oxygen makes the ecosystem shift to anaerobic metabolism. This pushes microorganisms living within these oxygen minimum zones (OMZ) to get energy by degrading organic matter in a process that removes vast amounts of nitrogen — a key nutrient for life — from the oceans. This upsets the nitrogen balance of the planet.
According to Naqvi, the study demonstrates that oxygen is not 'completely' removed from the dead zone of the BoB, but is present in very small amounts. This is sufficient not to allow the system from operating fully in the anaerobic mode even though the microbial population is all set to do so.
If this tiny amount of oxygen also goes, the BoB could become a major global player in nitrogen removal from the oceans, the report says. Removing more nitrogen from the oceans could affect the marine nitrogen balance and rates of marine productivity.
"There have been reports that the OMZs of the world have been expanding and intensifying over the past few decades, although a clear cause for this is yet to be established," Naqvi says. "Should a similar global trend apply to the BoB, its OMZ will trip to anaerobic mode, like in the Arabian Sea."
The BoB receives enormous runoff from numerous major and minor rivers that carry increasingly large loads of fertilizers, especially nitrogen. This boosts plant productivity in surface waters. The additional organic matter so produced may lead to an increase in oxygen demand in the depths of the ocean, Naqvi says.
"If the BoB does turn completely anoxic, and I suspect it will happen in future, global biogeochemical fluxes and nitrogen cycle will be substantially affected, and change the community structure at depth," he says. "However, it will probably not have any implications for India because we know that Arabian Sea OMZ operates anaerobically but has little effect on surface processes."
1. L. A. Bristow et al. N2 production rates limited by nitrite availability in the Bay of Bengal oxygen minimum zone. Nat. Geoscience (2016) doi: 10.1038/ngeo2847