Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

Abstract

Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.

Reply on RC2

“…Qualitatively consistent with the short term trend of decreasing primary production between 1998 and 2015, a pronounced decrease of up to 20% in phytoplankton in the Western Indian Ocean over the past six decades has been ascribed to increasing ocean stratification as a consequence of rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers (Roxy et al 2016). This result is indeed consistent with a long term trend with decreasing productivity since the last glacial maximum (Contreras-Rosales et al 2016;Shetye et al 2014), therefore challenging the often discussed situation of the BoB at a tipping point (Bristow et al 2017;Canfield et al 2019;Rixen et al 2020) to developing severe anoxia.…”

Deoxygenation in Marginal Seas of the Indian Ocean

“…Interannual and decadal-scale variability in oxygen forced by regional climate modes (e.g., the Indian Ocean Dipole) has been reported to control the intensity of seasonal anoxia over the western Indian shelf (Vallivattathillam et al, 2017). Such a variability may also occur in the off shore OMZs of the northern Indian Ocean (Rixen et al, 2020), but convincing evidence for this is lacking so far. Nevertheless, even though it is unlikely, one cannot completely rule out such a variability to account for a part of the oxygen declines being reported here.…”

Ideas and perspectives: Ushering the Indian Ocean into the UN Decade of Ocean Science for Sustainable Development (UNDOSSD) through marine ecosystem research and operational services – an early career's take

“…The Arabian Sea also witnesses large-scale blooms of Noctiluca scintillans, and research in recent years may provide the foundation for the bloom forecasting (Xiang et al, 2019). Crashing of jellyfish swarms or algal blooms have implications to the deep sea ecology, particularly in regions such as the Arabian Sea, which has a prominent oxygen minimum zone (OMZ; Rixen et al, 2020). Stratified waters, such as those of the northern Indian Ocean basins, have implications for large pelagic fish, which may face additional fishing pressure with the expansion of the OMZs due to the shrinkage of the habitat (Nimit et al, 2020).…”

“…However, due to the limitation of cellular signal support at sea, it is imperative that a fully fledged dissemination system should be supported through satellite communication (Singh et al, 2016). This results in episodes such as Cyclone Ockhi, where a lack of timely alerts costs many lives (Roshan, 2018). Such dissemination systems are to be provided, along with feedback mechanisms, to validate the services in real or near-real time.…”