Data_Sheet_1_Massive Nitrogen Loss Over the Western Indian Continental Shelf During Seasonal Anoxia: Evidence From Isotope Pairing Technique.pdf

The western Indian continental shelf houses the world’s largest naturally formed coastal low-oxygen zone that develops seasonally during the summer monsoon. We investigated multiple reductive nitrogen transformation pathways and quantified their rates in this system through anaerobic incubations with additions of 15N-labeled substrates during the anoxic period for three consecutive years (2008–2010). Addition of 15N labeled ammonium (15NH4+) resulted in low to moderate anaerobic ammonia oxidation (Anammox) rates in about half of our incubations from the oxygen depleted waters. In contrast, incubations with labeled nitrite (15NO2–) led to large production of 30N2 over 29N2 in all incubation experiments, indicating denitrification to be the dominant N-loss pathway. Rates of dissimilatory nitrate/nitrite reduction to ammonium (DNRA) were found to be highly variable and were lower by an order of magnitude than the denitrification rates. Extrapolation of average rates over the sampling periods and volume of anoxic waters showed large nitrogen removal (3.70–11.1 Tg year–1) which is about three times as high as the previously reported estimate (1.3–3.8 Tg year–1). Despite the small area it occupies, this shallow seasonal anoxic zone may account for as much as 20–60% the of the total annual fixed nitrogen loss in the perennial oxygen minimum zone of the Arabian Sea.

印度西部大陆架拥有全球规模最大的自然形成的沿海低氧区，该区域于夏季季风期季节性发育。本研究于2008–2010年连续三年的缺氧期内，通过添加15N标记底物的厌氧培养实验，针对该系统中的多种还原性氮转化途径开展了研究并量化了各途径的反应速率。向培养体系中添加15N标记铵离子（15NH4+）后，在约半数取自缺氧水体的厌氧培养实验中，观测到了低至中等水平的厌氧氨氧化（Anammox）速率；与之相反，在所有添加标记亚硝酸盐（15NO2–）的培养实验中，30N2的生成量显著高于29N2，表明反硝化是该系统中占主导地位的氮流失途径。异化硝酸盐/亚硝酸盐还原为铵（DNRA）的反应速率波动幅度极大，且较反硝化速率低一个数量级。基于采样周期内的平均速率与缺氧水体体积进行外推，该区域的年氮移除量可达3.70–11.1 Tg·年⁻¹，约为此前报道估算值（1.3–3.8 Tg·年⁻¹）的三倍。尽管该季节性浅海缺氧区的覆盖面积较小，但其年度固氮流失量可占阿拉伯海常年性低氧区总年度固氮流失量的20%–60%。