Ocean Deoxygenation – A Breathless Sea

We look at the causes and effects on marine life of ocean oxygen deprivation (hypoxia), a process which is spreading across the open sea. We look at 'dead zones' - hypoxic areas of oceans and large lakes - and ask: will we run out of breathable air? As usual the solution to lack of oxygen in the hydrosphere is simple: stop burning fossil fuels.
Oxygen depletion kills fish
Lack of oxygen in the ocean can cause suffocation of fish. Photo: US Fish & Wildlife Services/Public Domain

The Ocean is Losing Oxygen

In this article we explain everything you need to know about ocean deoxygenation, a process which could make the planet uninhabitable.

Okay, take a breath. And then remember this: roughly half the oxygen you breathe in, comes from trees and the other the half from the ocean.

Drifting plants in the sea called phytoplankton produce much of the oxygen we require through the process of photosynthesis. This oxygen-producing activity and the marine ecosystem that supports it, is vital to the health of every human being on the planet. Yet marine plants continue to be overshadowed by their more famous oxygenating cousins, the terrestrial plants of the Amazon Rainforest.

Unfortunately, while the world focuses its attention on the Amazon, a silent and potentially far more serious problem is unfolding under water. The ocean is losing oxygen.

In the past decade oxygen levels in the ocean have taken a big dive – a trend linked to climate change and other human activities. This poses a threat to marine animals who need oxygen to breathe, and to people who depend on fish for nutrition and jobs. Ultimately, it could even impact the air we breathe.

Note: Oceanographers are debating the best way to describe the phenomenon of declining levels of oceanic oxygen. The term ‘Ocean Deoxygenation’ is used increasingly by scientists, but alternatives include ‘ocean oxygen deprivation‘, and ‘ocean hypoxia’.

The Oxygen Cycle: Controlled by Plants.
10 Reasons Why Plants are Important.

What Is Ocean Deoxygenation?

Ocean deoxygenation refers to the widespread loss of oxygen being suffered by the hydrosphere due to global warming and other climate-related processes. It’s a perfect illustration of the Andropocene epoch at work.

Recent studies show that oxygen levels in some tropical regions have dropped by a startling 40 percent in the last 50 years. Levels have dropped more subtly elsewhere, with an average loss of 2 percent globally since 1960. 1

Climate models have routinely underestimated oxygen losses. 2 – But scientists are now forecasting that by the year 2100, the ocean will lose about 3 to 4 percent of its oxygen inventory under a ‘business-as-usual’ greenhouse gases emission scenario (RCP8.5).

Most of this loss is expected to occur in the upper 1,000 meters (3,500 feet) where marine life is most abundant. 3

Climate Variability in the Oceans
Regional weather cycles – such as the Indian Ocean Dipole (IOD) between East Africa and Southeast Asia, and the El Nino-Southern Oscillation (ENSO) in the equatorial Pacific – account for much of the climate variability in terms of temperature and heat in the upper ocean. So these cycles may well influence the ocean’s oxygen content.

Do Fish Breathe Oxygen?

Yes they do. Land animals have lungs to take in oxygen from the air, and fish have gills to breathe in oxygen contained in water. This process of breathing begins when the fish gulps a mouthful of water.

The water passes through their gills which are rich in blood. The gill filaments absorb oxygen from the water and move it into the fish’s bloodstream. The heart then pumps the blood to distribute the oxygen around the fish’s body.

All creatures in the marine food web respond to even the slightest change in oxygen levels – just as we land animals do. Their immediate reaction is to search for a safe location that has enough oxygen. Unfortunately, this action can expose them to new predators, like fishermen who know how to target areas that marine creatures might use as a refuge, or it can lead them into food-scarce regions.

Escaping from predators, digestion, cellular growth and repair – all these things require oxygen. Unfortunately this essential ingredient is no longer so easy for fish and other sea creatures to obtain, and they are literally suffocating. Even the smallest fall in oxygen levels, can create huge stress for marine life and can lead to far reaching consequences.

Ocean Deoxygenation – Yet Another Man-Made Mess-up

Ocean deoxygenation is one of many ocean problems that are causing serious concern among scientists. Others include: (1) Ocean acidification from an overdose of carbon dioxide (CO2); (2) Marine heatwaves from elevated temperatures; rising sea levels caused by polar ice melt; (3) Declining blue carbon reserves due to coastal redevelopment; (4) The replacement of vital mangrove forests with shrimp farms and other inappropriate installations; and (5) Microplastics pollution; to name but a few.

All these growing problems are major stressors on marine biodiversity and share a common cause – climate change. Scientists warn that the effects of global warming on oceans may reach a tipping point of no-return in our lifetime.

What Causes Ocean Deoxygenation?

1. Climate Change

Before we began burning huge amounts of fossil fuels, oceanic oxygen came from two sources:

• Atmospheric oxygen dissolving in surface water.
• From photosynthesis, performed by phytoplankton.

But now after 250 years of greenhouse gas emissions, rising ocean temperatures have caused a decline in oceanic oxygen, for two reasons:

Firstly, the warmer a liquid becomes, the less gas it can hold. This is why a carbonated beverage like a Coca-Cola, will go flat faster when left in the sun. So the warmer the ocean becomes, the less oxygen it can retain.

Secondly, as polar sea ice melts, it forms a layer of water on the sea surface, above colder, more saline waters. This layer forms a lid which prevents oxygen in the atmosphere dissolving in the sea. It is also interferes with the important thermohaline circulation, which takes oxygen produced on the surface by phytoplankton, down to the depths.

Also, any surface oxygen gets used up more quickly because fish and other marine life use more oxygen when temperatures are warmer. 2

2. Human Activities

Nutrient pollution – the increasing use of agricultural phosphorus-rich and nitrogen-rich fertilizers which leak into our waterways and eventually the sea, has led to a gradual process of eutrophication. This occurs when water ends up with too many nutrients which causes algae and plants to go into a growth spurt. When the plants die, the decomposition process uses up all the available oxygen. This depletes the surrounding water of oxygen, with disastrous consequences for marine life and the ecosystem.

The Nitrogen Cycle: How Does It Work?
The Phosphorus Cycle: The Slow Pathway.

What Impact Does Ocean Deoxygenation Have On Marine Life?

Hypoxia – Breathing Problems

When oceanic oxygen levels are low, marine life become deprived of oxygen at a tissue level. This condition is called hypoxia. Some creatures, like jellyfish, are more tolerant of low oxygen conditions than others. But all sea creatures will eventually suffer the impact of deoxygenation.

Hypoxia leads to major loss of biodiversity through reduced growth and reproduction, disruption to life cycles, physiological stress, forced migration, reduction of liveable habitat and increased vulnerability to predators. 4

For example, Chinese shrimp flip their tails less vigorously in order to conserve energy in lower oxygen environments, and as a result become less agile 5. Some male fishes produce less sperm as oxygen levels decline — a fault which is not corrected in future generations, even when oxygen levels return to normal.

Zooplankton – Destruction of The Food Web

Zooplankton are animals at the base of the food web. By eating lots of phytoplankton, they turn themselves into convenient snack for larger species like krill, the all-important food resource for most larger marine animals.

Zooplankton are very sensitive to lack of oxygen. Some species swim deeper into cooler water to find more oxygen, but at some point going deep becomes counterproductive, because it gets harder to find food or reproduce in lower temperatures. If zooplankton populations suffer, it will have ramifications all the way up the food web, even for sharks and whales.

Sight And Hearing Problems

Oxygen-depleted oceans can also cause sight and hearing problems in marine life (this is also true of humans). Numerous species, like zooplankton rely on visual cues to escape predators, so loss of any sensory capability can be a serious problem.

As the field of study on ocean deoxygenation is so recent, it is still too difficult to predict exactly how marine species will adapt, if at all. In the long term, adaptation through natural selection may occur with species which have short generation lifespans. However, most commercial fish species that we eat, tend to have long generation times, making it too hard for them to adapt in such fast-changing ocean conditions.

What Are Dead Zones?

A number of areas in the ocean contain little or no oxygen, where fish cannot breathe. These areas are described as hypoxic, low in oxygen, and are lethal to most marine life forms.

Sites include the tropical oceans off California, Peru and Namibia and the subsurface waters of the Arabian Sea. These so-called ‘oxygen minimum zones’ are a natural phenomenon, resulting from a combination of weak ocean currents and the decomposition of organic matter. These zones can also release nitrous oxide – a powerful greenhouse gas – into the lower atmosphere. Recent research shows that these low-oxygen regions are growing in size because of eutrophication. Paradoxically, the dead zones are expanding because of too much plant life.

In 2019 researchers identified 900 areas of the ocean around the world as experiencing the effects of eutrophication – and many are growing in size. Surveys of the dead zone off the coast of Louisiana, in the Gulf of Mexico for instance, show that since the 1980s it has grown from less than 10,000 sq kms (6,200 sq mi) to an average of 14,000 sq kms (8,700 sq mi) in 2015-2020.

Satellite image of phytoplankton blooms in the Gulf of Mexico
Phytoplankton blooms colour the ocean green and blue. Gulf of Mexico, 2009. Image: © NASA

In extreme cases, oxygen can become so completely depleted that the waters become anoxic (no oxygen at all) and even euxinic (anoxic with raised levels of hydrogen sulfide) in which only anaerobically respiring microbes can survive.

Marine Microbes Drive the Aquatic Food Web.
What is the Hydrosphere?

On the surface, dead zones look normal enough, but the bottom is eerily devoid of any living animal and is littered with the casualties of suffocation. A fish can swim of out hypoxic waters, but other marine animals like worms and mollusks, as well as coral reefs and aquatic plants cannot, and therefore die.

Will We Run Out of Breathable Air?

As phytoplankton produce an estimated half the planet’s atmospheric oxygen, a recent study 6 investigated what would happen to our oxygen supply if global warming continues unabated and the emission gap continues to grow. They ran computer prediction models to assess what would happen to phytoplankton’s ability to photosynthesize at different temperatures.

The study showed that at a rise of 6°C (10.8°F) – a realistic possibility if global emissions continue unchecked – ocean warming is likely to force phytoplankton to halt oxygen production.

By 2100, the report concluded that, at sea level, Planet Earth could have atmospheric oxygen levels comparable to the top of Mount Everest today. Climbers will tell you, it is not possible to stand on Everest without oxygen masks for more than a few minutes. Needless to say, this outcome would be catastrophic for all life forms, humans included.

Also worth reading: Why does half a degree rise in temperature make such a difference to the planet?

What Is The Solution To Ocean Deoxygenation?

If we are to solve earth’s climate crisis, and save the ocean at the same time – we need to decarbonize our energy system and switch to renewable energy wherever feasible. At the same time, we need to stop nutrient pollution from agriculture and other sources spilling into the sea. We can certainly improve farming practices, and we have the technology to prevent nutrients and raw sewage from entering rivers and streams.

However, climate change-driven hypoxia is another matter. It cannot be easily reversed. The carbon dioxide we emit today can remain active in the atmosphere for up to a thousand years. Unless we can develop a stunningly effective carbon capture and storage technology, we need nations around the world to commit to becoming carbon neutral as soon as they possibly can. Otherwise, the health of our oceans, and planet for that matter, does not look bright.

Remember the motto of the American Lung Association: if you can’t breathe, nothing else matters.


  1. “Manifestation, Drivers, and Emergence of Open Ocean Deoxygenation” Lisa A Levin. Annual Review of Marine Science, Vol. 10:229-260 – January 2018 []
  2. Drivers and mechanisms of ocean deoxygenation.” 2018 Andreas Oschlies et al. [][]
  3. Causes, impacts and consequences of ocean deoxygenation: Everyone’s problem. IUCN 2019
    https://www.unitgraphics.com/deox/IUCNDeoxreportBOOK15-11-2019.pdf []
  4. “Thresholds of hypoxia for marine biodiversity” – Raquel Vaquer-Sunyer and Carlos M. Duarte – 2008 []
  5. “Effects of acute and chronic hypoxia on the locomotion and enzyme of energy metabolism in Chinese shrimp Fenneropenaeus chinensis” []
  6. Mathematical Modelling of Plankton–Oxygen Dynamics Under the Climate Change – Yadigar Sekerci & Sergei Petrovskii 2015. []
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