Microbiology, Bose Institute, P‐1/12 CIT Scheme VIIM, Kolkata, 700054, West Bengal, India
3 Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
* Corresponding Author: Aninda Mazumdar ([email protected]) ORCiD code: 0000‐0002‐7897‐1646
† Equal contribution
ABSTRACT
Oxygen minimum zones (OMZs) enclose O2 depleted subsurface water masses in the global ocean extending approximately 150 to 1200 m below sea level. The most pronounced OMZs occur in Eastern Tropical North Pacific off Mexico and California (ETNP), Eastern Tropical South Pacific off Peru and Chile (ETSP), and the Arabian Sea (AS) defined by secondary nitrite maxima attributed to intense denitrification in the water column. These OMZs sites are critical for biogeochemical processes that control the biodiversity and primary productivity of the ocean. The preservation of organic carbon is efficient within the sediments underlying oxygen‐depleted waters as a result of incomplete decomposition as it sinks through the water column and diminished bioturbation activity. The partially degraded (reactive) organic matter fuels microbe‐mediated biogeochemical processes in the anoxic marine sediments where sulfate reduction is a significant remineralization pathway. The OMZs exert a strong influence on the abundance, diversity, and composition of microbial communities. Recent geochemical and environmental genomic studies identified the prevalence of C, N, and S cycles in the OMZs. Here, we review the progress and current understanding of the C–S–N cycle in the OMZ sediments with regard to its biogeochemistry and microbial ecology, and present a brief account of the mechanism of the formation of OMZs in the northern Indian Ocean.
1.1. INTRODUCTION
Oxygen minimum zones (OMZs) are oxygen‐depleted intermediate‐depth water masses in the global ocean, usually between 150 and 1200 m below sea level (mbsl). The upper threshold of dissolved oxygen (DO) concentrations defining the OMZs is ~20 μM (Figure 1.1; Paulmier and Ruiz‐Pino, 2009; Ruvalcaba Baroni et al., 2020), while the lower concentration of DO detected in an OMZ may be <2 nM O2 (Revsbech et al., 2009), amounting to functionally anoxic conditions. Oceanic regions generally classified as OMZs include (1) the Eastern Tropical North Pacific off Mexico and California (ETNP), (2) the Eastern Tropical South Pacific off Peru and Chile (ETSP), (3) the Bay of Bengal (BoB), and (4) the Arabian Sea (AS). Of these OMZs, the ETNP, ETSP, and the AS have also been classified as anoxic marine zones (Ulloa et al., 2012), owing to the buildup of secondary nitrite maxima (SNM) within the water column. The SNM forms because of denitrification in the water column, which occurs when DO concentrations drop to <5 mM (Anderson et al., 2007; Banse et al., 2017). The functionally anoxic parts of these OMZs occupy only ~0.8% of the world ocean but are responsible for ~35% of the production of N2 through denitrification (Ward et al., 2009).
Figure 1.1 Global annual oxygen concentration (ml l–1) at 150 mbsl.
Source: World Ocean Atlas 2009.
The buildup of OMZs is controlled by the interplay of physical and biological processes coupled with regional geography. Inadequate ventilation of the oxygenated water masses (e.g. the northern Indian Ocean), upwelling of nutrient‐rich deep waters (leading to high productivity in the euphotic zone), and subsequent microbial respiration of organic particulates (phytodetritus, fecal pellets, dead organisms, etc.) lead to a depletion of DO in the water column, resulting in the formation of OMZs (Behrenfeld et al., 2006; Regaudie‐de‐Gioux and Duarte, 2012). OMZs are high in nutrient concentrations and support highly productive fishing regions (Garçon et al., 2019).
In the northern Indian Ocean, OMZs occur in both the AS and the BoB (Figure 1.2) because of their closed northern geographical boundaries and monsoon‐driven seasonal upwelling (Naqvi, 1991; Stramma et al., 2008). The DO concentration within the Arabian Sea (ASOMZ) reflects a balance between biological O2 consumption and O2 replenishment. The primary source of intermediate water (Indian Ocean Central Water: ICW) (Stramma et al., 1996; Schott and McCreary, 2001; Rixen et al., 2020) in the northern Indian Ocean include contributions from the Antarctic Intermediate Water (AAIW), Subantarctic Mode Water (SAMW) and the Indonesian intermediate waters (Lachkar et al., 2019). The dense and highly saline Persian Gulf water (PGW) and relatively less saline but denser Red Sea water (RSW) are the lateral sources of intermediate water in the AS (Bower et al., 2005).
1.1.1. The Arabian Sea Oxygen Minimum Zone
The AS is believed to contain the thickest and most intense of the OMZs (Agnihotri et al., 2003; Naqvi et al., 2010a; Prakash et al., 2012; Banse et al., 2014; Acharya and Panigrahi, 2016), extending over the entire sea, with its upper boundary occurring at 100–150 m, and its lower boundary at 1000‐1200 m (Wyrtki, 1971; Bange et al., 2005; Banse et al., 2014), impinging upon a large area of the continental slope (Naqvi et al., 2006). The benthic area of the upper slope underlying hypoxic water in the AS is computed to be 230 440 km2, making it responsible for over one‐quarter of the world’s naturally hypoxic seafloor (Global estimate = 764 000 km2; Helly and Levin, 2004). The ASOMZ arises from its landlocked geography, mainly owing to the presence of the Asian landmass that restricts the flow of oxygenated water from the north. Monsoonal upwelling, wind‐driven mixing, Ekman pumping during summer (Kumar et al., 2009), convective mixing during winter (Madhupratap et al., 1996; Prasanna Kumar et al., 2001; Kumar et al., 2009), and advection of NO3 – rich upwelled water mass off the Oman, Yemen, and Somalia margins leads to large‐scale fertilization of the euphotic zone (Naqvi et al., 2006; 2010b). The productivity in this region is controlled by the NO3 – and Fe (aeolian) limitations which also show seasonal variation (Naqvi et al., 2010b; Banerjee and Kumar, 2014).
Figure 1.2 Annual oxygen concentration (μmol l–1) in the water column of the Indian Ocean. The data is obtained from the World Ocean Atlas 2013 (Boyer et al., 2013). The map was produced using generic mapping tool software.
The ASOMZ shows an east–west variation in structure, where the upper part (400 m) is located in the central/eastern basin, and the lower part (below 400 m) extends to the Omani coast, indicating a northward intensification of the ASOMZ (McCreary et al., 2013). The PGW water enters the ASOMZ from the northwest at shallow depths of – 300 m and spreads around the perimeter of the basin and southward along the Omani coast (Prasad et al., 2001), and the denser RSW encroaches to depths of 600–1000 m and spreads across the basin (Shankar et al., 2005; Shenoi et al., 2005).
A poleward undercurrent (West India Undercurrent, WIUC) carries the ICW northwards into the eastern Arabian Sea (EAS) up to 16°N at a water depth of – 500 m (Shenoy et al., 2020; Schmidt et al., 2020a). In the EAS, the OMZ expands southwards during the SW monsoon (~ 9°N) and retreats northwards during the NE monsoon (11°N) (Shenoy et al., 2020). Although the productivity across the EAS is significantly lower compared with that of the Western Arabian Sea (WAS), the OMZ is more intense in the central and eastern AS (Naqvi, 1991). This observation may be attributed to one or more hypotheses, including:
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