The Ocean Can't Breathe
Scientists call it deoxygenation, but it can be understood more simply: the oceans are losing their breath. Just like animals on land, marine life—from the smallest plankton to the largest whales—depends on dissolved oxygen to survive. Since the mid-20th
century, the global ocean has lost about 2% of its oxygen, and the volume of water with zero oxygen has quadrupled since the 1960s. This isn’t a distant problem for future generations. A recent review led by the Scripps Institution of Oceanography warns that this trend is pushing aquatic ecosystems toward an “unsafe space,” with some changes potentially being irreversible within our lifetimes. The scientists argue this issue is so critical it should be considered a core planetary boundary, on par with climate change and biodiversity loss.
What is Causing This Crisis?
There are two primary culprits, both driven by human activity. The first is global warming. Basic physics dictates that warmer water simply cannot hold as much dissolved oxygen as colder water. The oceans have absorbed over 90% of the excess heat from climate change, leading to widespread oxygen loss. This warming also creates a layering effect, or stratification, in the ocean, which prevents oxygen from the surface from mixing into deeper waters where it's needed. The second major cause is nutrient pollution. Runoff from agricultural fertilizers and sewage from cities pours excess nutrients like nitrogen and phosphorus into coastal waters. This fuels massive blooms of algae. When these algae die and sink, they are decomposed by bacteria that consume vast amounts of oxygen in the process, creating hypoxic areas, or “dead zones,” where most marine life cannot survive.
India's Coastlines on the Frontline
This global problem has severe local consequences for India. The Arabian Sea, off India's west coast, is home to one of the world's largest and most intense natural Oxygen Minimum Zones (OMZs). Scientists warn that this zone is expanding, fuelled by both warming and nutrient runoff from the densely populated subcontinent. This is a direct threat to the livelihoods of millions in coastal states like Gujarat, Maharashtra, and Kerala who depend on fishing. As oxygen levels drop, fish either suffocate, show slower growth, or are forced to move, disrupting the entire food web and threatening valuable commercial fisheries. The Bay of Bengal is also experiencing similar pressures, with nutrient discharge from major rivers contributing to deoxygenation.
More Than Just Fish at Risk
The impacts of deoxygenation ripple far beyond fisheries. Low-oxygen zones can alter the ocean's fundamental biogeochemical cycles. For instance, in these conditions, some microbes start producing nitrous oxide, a greenhouse gas that is 300 times more potent than carbon dioxide. This creates a dangerous feedback loop, where a warming ocean loses oxygen, which in turn causes the ocean to release more gases that accelerate warming. The loss of oxygen also shrinks the available habitat for larger species like tuna and sharks, compressing them into smaller surface areas where they are more vulnerable to overfishing. Even air-breathing marine mammals like dolphins are affected, as the distribution and availability of their prey is dramatically altered.
A Call for Urgent Action
While the outlook is serious, the Scripps-led review and other scientific bodies emphasize that solutions are within reach. The most critical step is to aggressively reduce global greenhouse gas emissions to slow ocean warming. This is a global effort, but local actions are equally vital. India can make a significant impact by modernizing its wastewater treatment infrastructure and promoting more sustainable agricultural practices to reduce nutrient runoff into its rivers and coastal seas. Protecting and restoring coastal habitats like mangroves and wetlands can also improve water quality and create refuges for marine life. Effective management, such as creating marine protected areas and adapting fishing regulations to account for low-oxygen seasons, will be essential for building resilience.
