The Breathless Water
Across the globe, from coastal estuaries to the deep ocean, water bodies are losing their breath. This phenomenon, known as deoxygenation or hypoxia, is the decline of dissolved oxygen levels in water. The primary culprits are linked to human activity:
global warming and nutrient pollution. As water temperatures rise, the water’s capacity to hold oxygen decreases. At the same time, runoff from agriculture and sewage systems pours excess nutrients like nitrogen and phosphorus into our rivers and seas. This triggers massive algal blooms, and when this algae dies and decomposes, the process consumes vast amounts of oxygen, creating vast low-oxygen 'dead zones'. While these dead zones are the most extreme outcome, the problem begins with much subtler drops in oxygen that affect marine life in profound, non-lethal ways.
An Underwater Behaviour Shift
Long before oxygen levels drop low enough to cause mass death, fish and other mobile creatures begin to change their behaviour. The first response is avoidance. Species that can swim away from low-oxygen zones will do so, forcing them into smaller, more crowded areas of water. This habitat compression leads to increased competition for food and resources. It’s like an entire neighbourhood being forced into a single apartment building; tensions are bound to rise. More dramatically, some fish are forced to the surface to perform what is called aquatic surface respiration (ASR)—essentially gasping for air at the boundary where water meets the atmosphere. This provides them with life-saving oxygen but comes at a great cost: it makes them incredibly vulnerable to predators from above, like birds.
Rewriting the Predator-Prey Playbook
These behavioural changes have a cascading effect on the entire food web. A predator might find its usual hunting grounds empty because its prey has migrated. Some fish that typically feed on organisms living in the sediment at the bottom of a lake or bay are forced to find new food sources when the bottom becomes hypoxic. This can lead to unexpected dietary shifts. Furthermore, crucial social behaviours like schooling can be disrupted. Studies have shown that hypoxia can cause fish schools to break apart or change their structure, weakening their primary defence against predators. Even communication is affected; some fish species that use sounds to court and spawn become quieter in low-oxygen water, impacting their ability to reproduce. These subtle shifts dismantle the established predator-prey dynamics that have evolved over millennia.
The New Winners and Losers
In this reshuffled ecosystem, there are clear winners and losers. Less mobile bottom-dwellers like crabs, mussels, and clams are often the first casualties, as they cannot escape the encroaching low-oxygen water. Mobile fish that are more sensitive to oxygen loss are also put at a severe disadvantage. On the other hand, some organisms are uniquely adapted to thrive. Certain species of jellyfish and microbes have a much higher tolerance for hypoxic conditions. With their competitors and predators weakened or gone, their populations can explode, fundamentally altering the character of the ecosystem. What was once a diverse environment can become dominated by just a few resilient species, reducing overall biodiversity and destabilizing the food web.
A Growing Concern for India
This is not a distant problem. The Indian Ocean is particularly vulnerable, hosting both naturally occurring Oxygen Minimum Zones (OMZs) and rapidly growing coastal dead zones driven by human activity. India's west coast is home to one of the world's largest shallow hypoxic zones, posing a direct threat to the nation's fisheries. A severe hypoxic event off the west coast in 2001, for instance, nearly caused a collapse of the shrimp fisheries, a vital part of the local economy. The Bay of Bengal is also at high risk, with its massive freshwater and nutrient inputs from major rivers creating conditions ripe for deoxygenation. The health of these marine ecosystems is directly tied to the livelihoods of millions in coastal communities.
