Water, water everywhere but no oxygen to breathe.
In a paper published in Science magazine, two dozen marine scientists have sounded off the alarm that oxygen is depleting in water bodies all over the world, including the Bay of Bengal and the Arabian Sea. According to the paper, the concentration of oxygen in open oceans and coastal areas have been on a decline since the 1950s. Over the last six decades, the affected oceanic area has quadrupled to 32 million square kilometres.
Why is there deoxygenation?
Climate change resulting in rising temperatures and decreased CO2 levels from human activities have resulted in an imbalance of nutrients in global water bodies. According to the paper, organic, agricultural and sewage waste from the coastlines have increased the nutrient loading in the oceans, influencing the oxygen decline.
Map showing oxygen-minimum zones in oceans as well as coastal areas which have exacerbated oxygen decline (Photo Courtesy: Science magazine)
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As oceans are getting warmer, they can’t dissolve as much oxygen and as different layers in the oceans don’t mix much, the oxygen in deep waters is used up by marine life quickly. The warmer water also forces marine animals to breathe more quickly, further depleting oxygen levels.
How is India affected?
Low oxygen areas in both the Bay of Bengal and the Arabian Sea have been expanding since the middle of the 20th century. SWA Naqvi, a co-author of the paper and former director of the National Institute of Oceanography, told Hindustan Times,
“The seas around India experience the most intense oxygen depletion. During summer, you get anoxic (lack of oxygen) conditions for 4-5 months in the Arabian Sea, forming the ‘largest hypoxic (low oxygen) area in the world’ that is almost 20,000 sqkm.”
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Consequences
Expansion of low-oxygen zones (or dead zones) can increase the production of N2O — a greenhouse gas 300 times more potent than CO2. According to the paper, deoxygenation can result in reduced eukaryote and phytoplankton diversity which form the basis of the marine food chain. This can have a domino effect as most small fishes depend on these bacteria and algae for food and so do a considerable fraction of large sea animals. Hypoxic zones near the coastlines can reduce the population of marine life as well as fishes, resulting in depleted hunting grounds for fishermen and an imbalanced ecosystem.
