Damage could last for centuries: Why oxygen loss in oceans and rivers is alarming researchers 

While climate change, biodiversity loss and plastic pollution dominate environmental discussions, scientists are warning that another planetary crisis is unfolding beneath the surface of Earth's waters. Oxygen levels in oceans, lakes, rivers and streams are steadily declining, threatening aquatic ecosystems, food security and even the planet's ability to regulate its climate.

A growing body of research now suggests that this phenomenon — known as aquatic deoxygenation — has become so widespread that it deserves recognition alongside climate change as one of the defining environmental threats facing humanity.

Researchers from the Scripps Institution of Oceanography at the University of California San Diego, along with an international team of scientists, argue that dissolved oxygen should be added to the Planetary Boundaries framework — a globally recognised scientific system used to measure whether Earth is operating within safe environmental limits. The proposal appears in the journal Limnology and Oceanography.

Why oxygen is disappearing 

Dissolved oxygen is essential for nearly every aquatic organism, from microscopic plankton and shellfish to large fish and sharks. Without sufficient oxygen, aquatic ecosystems begin to unravel.

Scientists say three major forces are driving the decline:

• Climate change: Warmer water naturally holds less oxygen than colder water. 
• Stronger water stratification: Rising temperatures reduce mixing between oxygen-rich surface waters and deeper layers, preventing oxygen from reaching the depths. 
• Nutrient pollution: Fertiliser runoff and wastewater trigger algal blooms. When these algae die and decompose, bacteria consume enormous amounts of oxygen, creating so-called "dead zones." 

These processes reinforce one another, making oxygen depletion increasingly difficult to reverse.

A problem decades in the making 

The decline is not new — but it is accelerating. Scientists estimate that the world's oceans have already lost roughly 2% of their dissolved oxygen since the mid-20th century. Climate projections suggest another 1% to 7% could disappear by the end of this century if warming continues.

Although those figures may appear modest, marine ecosystems often depend on finely balanced oxygen levels. Even relatively small reductions can force fish to migrate, reduce reproductive success, increase disease and shrink suitable habitats.

What happens when oxygen disappears? 

Low-oxygen waters affect ecosystems in multiple ways. Fish and marine animals are forced into increasingly smaller oxygen-rich areas, intensifying competition for food. Some species simply cannot survive prolonged oxygen shortages, leading to mass fish kills and biodiversity loss.

Scientists also warn that oxygen depletion disrupts nutrient cycling and alters the chemistry of water bodies. In severe cases, expanding "dead zones" become virtually incapable of supporting complex aquatic life.

The impacts extend beyond wildlife. Fisheries, aquaculture, tourism and communities dependent on healthy freshwater and marine ecosystems could all face mounting economic pressure.

Why scientists call it a planetary threat 

The researchers believe oxygen loss is more than just another consequence of climate change — it is a planetary process in its own right.

The Planetary Boundaries framework, introduced in 2009, identifies environmental systems that must remain within safe limits to keep Earth stable. It currently includes climate change, biodiversity, freshwater use, ocean acidification and several other global processes.

Adding aquatic deoxygenation would effectively create a tenth planetary boundary, acknowledging that declining oxygen has become a fundamental risk to Earth's life-support systems.

Why recovery could take centuries 

One of the study's starkest warnings is that reversing oxygen loss will not happen quickly. Deep oceans and many freshwater systems exchange water over extremely long timescales. Even if greenhouse gas emissions and pollution were drastically reduced, scientists say some oxygen-depleted regions may require centuries to recover, while certain ecological changes could prove irreversible on human timescales.

Unlike melting glaciers or rising sea levels, declining oxygen cannot be seen from space or photographed easily. Yet scientists argue it may be one of the clearest indicators that Earth's aquatic ecosystems are under mounting stress.