This article provides a brief account of my early life and career, and a more detailed description of the contributions of the groups with which I have been associated to the biogeochemistry of the North Indian Ocean, especially nitrogen cycling in oxygen deficient waters.Some of the most intense oxygen depletion in the water column in the open ocean occurs at mid-depths in the two northern basins of the Indian Ocean – the Arabian Sea and the Bay of Bengal. This pattern, arising from the presence of land masses that restrict the northern expanse of the Indian Ocean, contrasts with the oxygen distribution in the Atlantic Ocean and the Pacific Ocean, where the oxygen minima are the most intense along their eastern boundaries. Moreover, the two open ocean oxygen-depleted systems in in the north Indian Ocean are quite different: while the oxygen minimum layer in the Arabian Sea is functionally anoxic and contains a prominent nitrite maximum (called the secondary nitrite maximum or SNM), oxygen in traces (sub-micromolar levels) is almost always present within its minimum layer in the Bay of Bengal, where the SNM is conspicuously absent. Nitrate concentration within the nitrite-bearing oxygen deficient zone (ODZ) of the Arabian Sea is about half of the corresponding value in the Bay of Bengal, indicating its loss to molecular nitrogen (N2) through denitrification and/or anaerobic ammonium oxidation (anammox). Estimates of N2 production rates in the Arabian Sea range between ~12 and 41 Tg N yr-1, comparable to the published estimates for each of the two Pacific Ocean’s ODZs. Other characteristic features of the Arabian Sea ODZ, not observed in the Bay of Bengal, are as follows. (1) Low (minimum) concentrations of nitrous oxide (N2O) sandwiched between two maxima located at the boundaries of the ODZ. (2) Large enrichment of 15N relative 14N in nitrate and N2O, resulting from preferential reduction of 14NO3- and 14N14NO, respectively. (3) Elevated N2/Ar ratio relative to the region outside the ODZ. (4) Maxima in respiration rates, as inferred from the activity of the respiratory electron transport system (ETS), in particulate protein, in total bacterial counts and in suspended particles, as determined by light transmission. In addition to nitrogen, oxidised forms of some other polyvalent elements (such as iodine, manganese and iron) are also reduced within the ODZ, as evident from elevated concentrations of their reduced species (I-, Fe(II) and Mn(II)). Lateral advection from the highly reducing continental margin sediments is another potential source of these species. As in the case of the ODZs in the eastern tropical Pacific, the intermediate particle maximum/nepheloid layer, is overlain by a relative sterile (low bacterial counts) and remarkably clear (low suspended particles) zone that defines the transition from oxic to anoxic conditions in the upper water column.Unlike the ODZs of the eastern Pacific, the Arabian Sea ODZ is geographically separated from centres of upwelling in the western Arabian Sea, in part because the relatively more oxygenated waters advecting from the south and from the Persian Gulf in the northwestern region prevent the development of anoxic conditions. Because they are slightly oxygenated, waters upwelling in the western Arabian Sea have a high nitrate to iron ratio, such that toward the end of the upwelling season primary productivity becomes iron limited. Under iron stress, diatoms consume more silicate when normalised to nitrate, leading to a community shift to smaller taxa offshore. The consequent offshore shoaling of the depth of organic matter re-mineralisation is the other possible reason for the offshore location of the ODZ.Over the past few decades the oceans have been steadily losing oxygen globally due to ocean warming and increased anthropogenic nutrient loading. The available data from the North Indian Ocean, however, show much smaller decreasing trends than those reported for other regions, particularly the Pacific Ocean, with the exception of data from the western Arabian Sea. However, results of modelling studies reveal the likelihood for large changes occurring in the near future.The issue of why the oxygen minimum layer in the Bay of Bengal retains traces of oxygen and does not support vigorous combined nitrogen loss is examined in detail utilising both published and unpublished information. It is concluded that anoxic conditions do not develop in the Bay of Bengal mainly due to a low rate of upwelling, which is most likely linked to a greatly subdued exchange at intermediate depth with the rest of the Indian Ocean. A strong stratification of the upper water column may also contribute to a lower diffusive flux of nitrate into the surface layer. The persistent presence of oxygen in traces probably results in low organic matter degradation rates (a kinetic control) with the ballast-driven faster sedimentation of the particulate organic matter being another potentially important factor. Finally, the presence of oxygen, albeit in traces, prevents large scale nitrate reduction (a thermodynamic control), which provides nitrite for denitrification and anammox.The intense oxygen minimum layer in the Arabian Sea is the only one in the world that receives freshly formed and relatively oxygenated waters from the two Mediterranean-type marginal seas (the Red Sea and the Persian Gulf). Both of these seas, especially the Persian Gulf, are currently being subjected to significant human induced changes (warming, increase in salinity and anthropogenic loading of nutrients) that are projected to bring about significant modifications of the Arabian Sea ODZ. The increased nutrient supply appears to have led to a large increase in primary production in the Gulf, although zooplankton grazing does not allow the build-up of phytoplankton biomass. The increased availability of organic matter has led to development of large scale hypoxia in bottom waters of the Persian Gulf in summer–autumn. The resultant decrease in the pre-formed oxygen content and increase in the pre-formed total organic carbon content of the Persian Gulf Water (which advects directly into the core of the ODZ in the Arabian Sea) may cause significant expansion and intensification of the ODZ. Model simulations show that the intensity and volume of the ODZ are also highly sensitive to physico-chemical characteristics of the outflows from the marginal sea, especially the temperature of the Persian Gulf Water. The ongoing warming within the Persian Gulf, which will reduce the density of the Persian Gulf Water, is expected to make the greatest contribution to the ongoing/future expansion and intensification of the Arabian Sea ODZ.Anoxic conditions also develop seasonally along the Indian west coast, over a shelf area that is the largest in the world, during the Southwest Monsoon when this region behaves like a mini-eastern boundary upwelling system. The upwelled water is generally capped by a warm, low salinity lens formed by the enormous precipitation in the coastal zone, which results in strong thermohaline stratification very close to the surface and a strong oxygen depletion, culminating in sulfidic conditions in near bottom waters. One distinguishing feature of this system is huge accumulation of N2O (to several hundreds of nM), mostly due to incomplete denitrification. Such conditions do not develop along the shores of the Bay of Bengal because the Bay of Bengal experiences much weaker upwelling than the Arabian Sea. On the other hand, an enormous amount of reactive nitrogen is released to the environment in South Asia due to human activities. However, the quantity of combined nitrogen transported to the ocean by the South Asian rivers is much lower than model predictions, indicating loss in/retention by the terrestrial systems of a large fraction of the reactive nitrogen being anthropogenically released. Recent work has demonstrated two pathways of such losses – methanotrophy – denitrification coupling in the hypolimnia of freshwater reservoirs that turn anoxic in summer, and heterotrophic denitrification in groundwaters of the Indo-Gangetic Plain. Such processes probably also operate in other aquatic ecosystems such as lakes, ponds, rice paddies and soils/sediments of river beds and wetlands. The consequently lower inputs of nutrients (especially nitrogen) to coastal waters by rivers may explain the absence of human-induced formation of large coastal hypoxic zones in the northeastern Indian Ocean unlike other coastal areas (e.g., the Gulf of Mexico and the Black Sea) that also receive large land runoff.

中文翻译：

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北印度洋缺氧相关的生物地球化学

这篇文章简要介绍了我的早年生活和职业生涯，并更详细地描述了与我有关的团体对北印度洋生物地球化学的贡献，尤其是缺氧水域中的氮循环。一些开阔海域水柱中最严重的缺氧发生在印度洋北部两个盆地——阿拉伯海和孟加拉湾的中层深度。这种模式是由限制印度洋北部广阔的陆地块的存在引起的，与大西洋和太平洋的氧气分布形成鲜明对比，大西洋和太平洋的氧气最小值在其东部边界最为强烈。此外，北印度洋的两个开放海洋缺氧系统也有很大不同：虽然阿拉伯海的最低氧层在功能上是缺氧的并且含有显着的亚硝酸盐最大值（称为次级亚硝酸盐最大值或 SNM），但痕量氧气（亚微摩尔水平）几乎总是存在于孟加拉湾的最低层中，其中 SNM 明显不存在。阿拉伯海含亚硝酸盐的缺氧区 (ODZ) 内的硝酸盐浓度大约是孟加拉湾相应值的一半，这表明它通过反硝化作用和/或厌氧氨氧化（厌氧氨氧化）损失为分子氮 (N2) . 阿拉伯海 N2 生产率的估计值介于 ~12 和 41 Tg N yr-1 之间，与两个太平洋 ODZ 中每一个的已公布估计值相当。阿拉伯海 ODZ 的其他特征，在孟加拉湾没有观察到，如下面所述。(1) 氧化亚氮 (N2O) 的低（最低）浓度夹在位于 ODZ 边界的两个最大值之间。(2) 硝酸盐和 N2O 中 15N 相对于 14N 的大量富集，分别是 14NO3- 和 14N14NO 优先还原的结果。(3) 相对于 ODZ 以外的区域，N2/Ar 比率升高。(4) 从呼吸电子传输系统 (ETS) 的活动推断出的呼吸速率最大值，在颗粒蛋白质、总细菌计数和悬浮颗粒中，通过光传输确定。除氮外，一些其他多价元素（如碘、锰和铁）的氧化形式也在 ODZ 内减少，这从它们的还原物质（I-、Fe(II) 和 Mn(II)）浓度升高中可以明显看出). 来自高度还原的大陆边缘沉积物的横向平流是这些物种的另一个潜在来源。与热带太平洋东部的 ODZ 一样，中间颗粒最大/霞状层被相对无菌（低细菌计数）和非常清晰（低悬浮颗粒）区域覆盖，该区域定义了从有氧条件到缺氧条件的过渡与东太平洋的 ODZ 不同，阿拉伯海 ODZ 在地理上与西阿拉伯海的上升流中心分开，部分原因是相对较多的含氧水从南部和波斯湾平流西北地区防止缺氧条件的发展。因为它们略微含氧，所以阿拉伯海西部上升流的水域中硝酸盐与铁的比例很高，这样在上升流季节结束时，初级生产力变得有限。在铁胁迫下，当归一化为硝酸盐时，硅藻会消耗更多的硅酸盐，从而导致群落向离岸的较小类群转移。有机物再矿化深度的随之而来的近海浅滩是 O​​DZ 离岸位置的另一个可能原因。在过去的几十年中，由于海洋变暖和人为养分负荷增加，海洋在全球范围内一直在稳步失去氧气。然而，北印度洋的可用数据显示，除西阿拉伯海的数据外，其下降趋势比其他区域，特别是太平洋报告的下降趋势要小得多。然而，建模研究的结果揭示了在不久的将来发生巨大变化的可能性。利用已发表和未发表的信息详细研究了为什么孟加拉湾的最低含氧层保留了微量氧气并且不支持剧烈的复合氮损失的问题。得出的结论是，孟加拉湾没有形成缺氧条件，这主要是由于上升流率低，这很可能与中等深度与印度洋其余部分的交换大大减弱有关。上层水柱的强烈分层也可能有助于降低硝酸盐进入表层的扩散通量。痕量氧气的持续存在可能导致低有机物降解率（动力学控制），而镇流器驱动的颗粒有机物更快沉降是另一个潜在的重要因素。最后，氧气的存在，尽管存在痕迹，但可以防止大规模硝酸盐还原（热力学控制），从而为反硝化和厌氧氨氧化提供亚硝酸盐。阿拉伯海中的高氧最低层是世界上唯一一个从这两个区域接收新鲜形成和相对含氧水的区域地中海型边缘海（红海和波斯湾）。这两个海域，尤其是波斯湾，目前正受到人类引起的重大变化（变暖、盐度增加和人为营养物质负荷）的影响，预计这些变化将导致阿拉伯海 ODZ 发生重大变化。尽管浮游动物的放牧不允许浮游植物生物量的积累，但营养供应的增加似乎已导致海湾初级生产的大幅增加。有机物质可用性的增加导致夏秋季节波斯湾底部水域出现大规模缺氧。波斯湾水（直接平流进入阿拉伯海 ODZ 核心）的预形成氧含量降低和预形成总有机碳含量增加可能导致 ODZ 显着扩张和强化. 模型模拟表明，ODZ 的强度和体积对边缘海流出物的物理化学特性也高度敏感，尤其是波斯湾水的温度。波斯湾内持续变暖，这将降低波斯湾水的密度，预计将对阿拉伯海 ODZ 的持续/未来扩张和强化做出最大贡献。在西南季风期间，缺氧条件也会沿着印度西海岸季节性发展，覆盖世界上最大的陆架区域区域表现得像一个微型东部边界上升流系统。上升流的水通常被沿海地区大量降水形成的温暖、低盐度的透镜体所覆盖，这导致非常靠近地表的强烈温盐分层和强烈的氧气消耗，最终导致近底部水域的硫化物条件。该系统的一个显着特征是 N2O 的大量积累（达到数百 nM），这主要是由于反硝化不完全所致。这种情况不会在孟加拉湾沿岸形成，因为孟加拉湾的上升流比阿拉伯海要弱得多。另一方面，由于人类活动，大量的活性氮被释放到南亚的环境中。然而，南亚河流输送到海洋的复合氮量远低于模型预测，表明陆地系统损失/保留了人为释放的大部分活性氮。最近的研究表明，这种损失有两种途径——甲烷氧化作用——淡水水库低液层中的甲烷氧化-反硝化作用在夏季转为缺氧，以及印度-恒河平原地下水中的异养反硝化作用。这些过程可能也发生在其他水生生态系统中，例如湖泊、池塘、稻田以及河床和湿地的土壤/沉积物。因此，河流向沿海水域输入的养分（尤其是氮）较少，这可能解释了与其他沿海地区（例如墨西哥湾和黑海）不同，印度洋东北部没有人为形成大型沿海缺氧区的原因这也收到大量的土地径流。