Anoxia-Related Biogeochemistry of North Indian Ocean

Abstract

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.