Dead Zone in the Bay of Bengal

A Dead Zone in the Bay of Bengal (BoB), nearly half the size of Bangladesh and at depths 70m and below, has been discovered in recent years by a group of multinational scientists, Bristow et al., (2016). This oxygen-depleted zone, called hypoxia, is the third largest Oxygen Minimum Zone (OMZ) in the world after the OMZs in Eastern Tropical Pacific Ocean and the Arabian Sea. Unlike its neighbouring Arabian Sea, the Bay of Bengal OMZ is somewhat different in nature. It still shows a trace of oxygen at 70m depth and below, ranging from .032-.064 mg/l, albeit way below the Oxygen level (5 mg/l) needed to support aquatic life and other uses. On the other hand, the Arabian Sea has no oxygen, causing devastating effects on its ecosystem and losing its Nitrogen balance unlike any other ocean in the world. The Arabian Sea OMZ contributes 20 per cent of the global ocean denitrification (loss of nitrogen) budget and has been identified as a hotspot of oceanic efflux of N2O, a greenhouse gas. Toxic algal bloom and massive fish kills are rampant in both the flanks of the Arabian Sea (Indian and Omani coasts) caused by the southwesterly summer monsoon winds which trigger upwelling circulations and bring the nutrient-rich and oxygen-depleted bottom ocean water into the shore. Fortunately, the Bay of Bengal OMZ hasn't gone to that level yet, but it is at the tipping point. Evidence shows that the Bay of Bengal OMZ is alarmingly expanding as reflected by its intensification. The trend is set to continue due to global warming, changing river runoff and circulation patterns in the Bay of Bengal. Anthropogenic inputs of nutrients, such as Nitrogen and Phosphorus, and organic Carbon, discharged into the Bay of Bengal by the rivers across the region, have adverse effects on the OMZs. The nutrients inputs to the Bay are increasing due to elevated level of fertilizers being used in modern agricultural practices to meet the demand of foods for 400m people in the region. The potential ramifications of this OMZ are large and it has thus been the subject of multiple international research efforts. However, a conspicuous absence of awareness among Bangladeshi policy makers has become a growing concern for the scientific communities.
ECOLOGICAL AND SOCIOECONOMIC RAMIFICATIONS: The Bay of Bengal has in excess of 400 million people living immediately around its rim, many depending on the Bay for their livelihoods and food security. Extreme OMZ would have dramatic impacts on the marine ecosystems, including commercial species that are already over-exploited. A slight drop in oxygen levels could lead to devastating effects on the ecosystem across the Bay of Bengal and intense denitrification can occur as seen in the Arabian Sea, where the oxygen level has gone down to zero (below detection limit).
Coastal hypoxia (water with little oxygen), as often seen on the Indian shelf of the Arabian Sea and to a limited scale in the Bay of Bengal, due to upwelling oceanic circulation, lead to massive fish killings. In addition the nutrient-rich bottom water, when brought to surface by upwelling circulation, may trigger massive algal bloom including toxic algal species (Noctiluca Scintillans) seen both in the Arabian Sea and in the Bay of Bengal. This has knock-on effects on food webs, carbon export and burial, and nutrient cycling in the coastal waters.
A remarkable feature of the Bay of Bengal, however, is the massive influence of the Ganges-Brahmaputra-Meghna (GBM) system and Irrawaddy river, which sets it apart from the Arabian Sea. River waters, emanating from the GBM river system stays on the surface while the heavier ocean water stays at the bottom, giving rise to a stable and stratified water column, which makes it difficult for monsoon winds to trigger upwelling circulation and prevent it from bringing oxygen-depleted and nutrient-rich bottom ocean water into the shore. Unfortunately, that is not the case for Arabian Sea as there is no freshwater flows in the region, as a result massive fish killing and algal bloom occur in the coastal waters around the Arabian Sea. However, the GBM river system brings millions of tons of sediments into the Bay which carry organic materials and nutrients that demand dissolved oxygen from the ocean water. Fortunately, in the Bay of Bengal these sediments with organic carbon quickly settle to the sea bottom and do not get sufficient time to consume oxygen from the water column, which helps make a dynamic balance of oxygen in the Bay keeping at least a trace of oxygen in the water column. However, a small reduction of oxygen could have a devastating effect on the ecosystem across the Bay of Bengal. Further, the Bay of Bengal, being connected to the Indian Ocean, is particularly susceptible to sea level rise, ocean acidification and extreme weather events, and the Indian Ocean is warming faster than any other ocean in the world.
The biogeochemistry of the Bay's water and especially its sediments, remain very poorly studied relative to those of the Arabian Sea and many other ocean regions. Consequently, while the Bay of Bengal phenomena are widely recognised to be of major, far-reaching importance, large uncertainties remain about how the system functions currently and how it will respond to environmental change (IPCC).
Finally, the Bay of Bengal is in downstream of large human populations and rapidly developing nations and is thus also subject to major direct anthropogenic influences. For example, as well as large projected increases in nutrient discharge, the recent launch of major engineering projects to dam and/or divert the Ganges and Brahmaputra, have major implications on future discharge of water and sediment, and add to uncertainty.
THE BAY OF BENGAL AT A TIPPING POINT: Even a trace of oxygen present in the OMZ reflects a dynamic balance between the mixing of atmospheric oxygen into OMZ waters and the oxygen uptake by the biological processes that degrade organic materials. Therefore, increase in the flux of organic matters in the OMZ waters would break this delicate balance and make the OMZ waters anoxic (no oxygen) and accelerate denitrification process (loss of nitrogen) in Bay of Bengal. Moreover, increase in the flux of anthropogenic nutrients (Nitrogen and Phosphorus) into the Bay, as projected for the coming decades with changing intensity of summer monsoon, would increase in primary production (algal growth) which would demand additional oxygen from the already oxygen-depleted OMZ waters.
High intensity southwesterly monsoon summer winds may trigger upwelling circulation that brings nutrient-rich hypoxic (very little to no oxygen) bottom water in the surface coastal waters. This phenomenon will trigger production of high concentration algae and further enhance oxygen depletion in coastal waters that may potentially kill, like in Arabian Sea, aquatic life including commercial fish and destroy the coastal ecosystem functions.
It is also true that an enhanced summer monsoon would increase river runoff and the flux of sediments to the Bay. Increased river runoff helps develop a strong stratification (lighter river water on the top and heavier saline water in the bottom) of the Bay of Bengal water column, preventing the southwesterly monsoon from causing upwelling circulation which may have a devastating effect on the coastal ecosystem. On the other hand, the enhanced flux of organic material flux in to the Bay water may cause removal of a small trace of oxygen in the OMZ waters and accelerate denitrification (loss of nitrogen) processes in the water column, causing an efflux of greenhouse gas (NOx) into the atmosphere, like in the Arabian Sea.
Therefore, the Bay of Bengal OMZ is sitting on a biogeochemical "tipping point" where any process of removing the last trace of oxygen, either anthropogenic nutrients input, enhanced flux in organic materials due to climate change or upwelling circulation, would put the Bay of Bengal ecosystem and the livelihood of the people at severe risks.
WHAT IS NEXT: It is important that we identify the current controls of the physical and biogeochemical state of the Bay of Bengal and how they regulate its OMZ. The global warming and human interventions particularly in the Ganges-Brahmaputra-Meghna (GBM) basin in the upper riparian countries have enormous influence on the physical and biogeochemical state of the Bay of Bengal. It is therefore, important to know how the Bay of Bengal will evolve in the decades to come? Extensive field observations not only in the Bay of Bengal but also in the GBM basin would shed lights on the ecological and biogeochemical health of the Bay of Bengal. A high resolution model incorporating the physical and biogeochemical processes of the Bay of Bengal and the upstream river basins would be required to provide both hindcast and forecast simulations of the hypoxia (OMZ) and wider Bay of Bengal system functions.
The Institute of Water and Flood Management (IWFM), Bangladesh University of Engineering and Technology (BUET) has developed an integrated model called Bangladesh Delta Model (BDM) which brings the inland and coastal waters and the Bay of Bengal into a single modeling framework. The BDM modeling analysis could help develop a clear understanding of physical factors that may trigger the upwelling effects, a tipping point, that may have disastrous consequences as in case of Arabian Sea. In addition, the biogeochemical processes are being implemented in the BDM model which would be a key to address the oxygen and nutrient cycling and the organic carbon cycling across the Bay of Bengal.
The Ministry of Water Resources, Government of Bangladesh, may look into this broad idea discussed here and support cross-disciplinary studies (physical, biological and geochemical, and through observation, experiments and modeling) in the coastal and open Bay of Bengal to assess the current state of hypoxia and marine ecosystem functions, and to identify the factors controlling these. It is important to know how hypoxia, ecosystem functions and key biogeochemical phenomena will respond to projected environmental change in future (IPCC) and fit in the Bangladesh Delta Plan 2100 and the Blue Economy ambition of Bangladesh.
Dr Quamrul Ahsan is former Assistant Professor and
Dr Anisul Haque is Professor, Institute of Water and Flood Management (IWFM), Bangladesh University of Engineering and Technology (BUET), Dhaka
[email protected]