Bay of Bengal: An Oxygen Minimum Zone at the Tipping Point

Laura A Bristow1, Cameron M Callbeck2, Morten Larsen3, Mark A Altabet4, Julien Dekaezemacker2, Michael Forth3, Ronnie N Glud3, Marcel Martinus Maria Kuypers2, Gaute Lavik5, Mangesh Gauns6, Jana Milucka2, Syed Wajih Ahmad Naqvi6, Anil Pratihary6, Bo Thamdrup3, Alexander Treusch3 and Donald E Canfield3, (1)University of Southern Denmark, Nordic Center of Earth Evolution, Department of Biology, Odense M, Denmark, (2)Max Planck Institute for Marine Microbiology, Department of Biogeochemistry, Bremen, Germany, (3)University of Southern Denmark, Nordic Center of Earth Evolution, Department of Biology, Odense, Denmark, (4)University of Massachusetts Dartmouth, New Bedford, MA, United States, (5)Max Planck Institute for Marine Microbiology (MPI), Bremen, Germany, (6)National Institute of Oceanography, Goa, India
Abstract:
Oxygen minimum zones (OMZs) host N2 producing microbial populations responsible for a major percentage of the fixed nitrogen (N) loss from the oceans. While, the Bay of Bengal (BoB) contains regions with oxygen (O2) concentrations below detection of standard methods, there have been no prior reports of N2 production. We sampled the BoB in January 2014 during the winter monsoon, measuring in situ O2 with a highly sensitive STOX (switchable trace oxygen) sensor. Sampling was carried out for natural abundance NO3- isotopes, molecular characterization of microbial populations and experiments exploring the aerobic and anaerobic microbial turnover of N species. Unlike recognized OMZs, the BoB has minimum O2 concentrations in the 10 to 100 nM range. Despite this measurable O2, the BoB OMZ houses microbial populations similar to other OMZs, capable of N2 production through both anammox and denitrification. Isotope labeling experiments with 15NO2- detected anammox up to 6.2 nM N d-1, but were below detection with 15NH4+ at all stations, indicating the great potential for anammox in the BoB OMZ waters, but the process is limited by NO2- availability. Denitrification was also detected in 15NO2- experiments, up to 0.9 nM N d-1. Active N cycling with very minor N loss was also indicated by NO3- isotopes. The general lack of NO2- accumulation (< 180 nM) and low in situ N2 production rates are likely a result of limited organic matter availability, which in turn limits the drawdown of O2 and allows a close coupling of NO3- reduction and NO2- oxidation. The potential for N2 production, is however high, and the BoB would become an important player in the marine N cycle if these last traces of O2 were removed, for example, by anthropogenic forcing or climate change.