Present past and future of the OMZ in the northern Indian Ocean

Abstract

<jats:p>Abstract. Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~ 59 % of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21 % of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N2) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean. </jats:p>

Causes of uncertainties in the representation of the Arabian Sea oxygen minimum zone in CMIP5 models

“…4a). In contrast, Rixen et al (2020) and our own analysis here show that two models (CESM1-BGC, MPI-ESM-LR) overestimate the OMZ volume when considering hypoxic conditions (<20 µmol l −1 ), with the maximum simulated OMZ volume being more than twice as large as in observations. However, the other eight models still underestimate the OMZ volume for the lower threshold.…”

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

“…This mechanism may occur at the northern end of the Leg 2 transect. Although there is no direct evidence that denitrification occurs north of the Equator, there is a region where the DO concentration is less than 20 μmol kg –1 ( Figures 6I,J ), below which anaerobic microbial processes start ( Rixen et al, 2020 , in review), and a nitrate deficiency based on N ∗ (nitrate – 16 ∗ phosphate +2.9) is observed ( Gruber and Sarmiento, 1997 ; Figure 8B ). In the eastern Pacific, where an OMZ exists, similar decoupling of FOM from AOU was found by Catalá et al (2015b) .…”

Emerging Global Ocean Deoxygenation Across the 21st Century

“…Note that the spatial coverage of deoxygenation in the mesopelagic North Indian is less than 50%. A possible reason for this is that a slight increase in the dissolved oxygen concentration is easily detected because historical oxygen concentrations were low (Liu et al., 2020; Rixen et al., 2020).…”