The ocean is losing oxygen. This is how it will affect the future of the planet

Over 70% of the Earth’s surface is covered by water.

The deep sea's allure stems largely from the fact that we know far less about its vast, largely unexplored regions than we do about outer space, such as the Moon or Mars.This immense and mysterious environment, characterised by extreme pressure, darkness, and cold temperatures, harbours unique ecosystems and life forms that are still largely unknown to science. Oceans and marine life are suffering. The consistent decline of ocean oxygen will have long-term consequences in the future of the planet.

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Our oceans are running out of breath, literally. A new international study warns that declining oxygen levels in the ocean, driven by climate change, could have devastating effects on marine life, especially the deep-sea fish that live in the so-called “twilight zone.” And the consequences could ripple far beyond the deep, impacting fisheries, ocean health, and even the planet’s ability to store carbon.

Researchers from the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB) looked to the past to understand what’s happening now. They studied fossil remains of lanternfish, which are tiny, glowing fish that are among the most abundant creatures in the deep ocean, preserved in seabed sediments in the Eastern Mediterranean dating back to over 10,000 years. Their findings, recently published in Communications Earth & Environment (Nature Portfolio), tell a worrying story. You can read the full study here.

So, why lanternfish?

Despite their small size, these fish play a huge role in the ocean ecosystem. They’re masters of the daily commute: by day, they hang out in the dark mesopelagic zone (200-1,000 metres deep) to avoid predators, and at night, they swim up to the surface to feed.

This nightly migration moves massive amounts of carbon from the surface to the deep ocean, helping regulate Earth’s climate. With an estimated biomass of 600 million tons, lanternfish may even be the most abundant vertebrates on Earth by weight.

Here’s the alarming part: the fossil record shows that lanternfish populations crashed during past periods of extreme oxygen loss. They only returned when oxygen levels rose again, about 6,000 years ago.

“If a group with such massive biomass disappears, other marine species are also likely to be at risk,” warns Sven Pallacks, Ph.D in Environmental Science and Technology and lead author of the study.

The research team, which included scientists from Scripps Institution of Oceanography, Woods Hole, and other global institutions, used tiny fish ear bones (called otoliths) to reconstruct past populations. The Eastern Mediterranean, which has swung between oxygen-poor conditions over millennia, offered a perfect natural archive for this work.

Why does this matter?

The twilight zone isn’t just home to strange, glowing fish, it’s a critical area that plays a paramount role in Earth’s climate system and carbon cycle. If these ecosystems collapse, it could throw ocean food webs out of balance, reduce the ocean’s ability to store carbon, and even threaten global food security.

In other words, ocean deoxygenation isn’t just a deep-sea problem, it’s a planet-wide problem with long-term consequences.

What happens when oceans lose oxygen?

Great question! Several things happen:

Spread of dead zones and shrinking habitats

Oxygen-poor regions, also known as dead zones, already exist in coastal areas and are spreading in open oceans. These hypoxic zones lead to reduced biodiversity, mass die-offs in benthic communities, and the proliferation of stress-tolerant species such as jellyfish.

Fish losing their homes

Many fish can’t survive in deeper oxygen-starved layers, causing habitat compression. This forces them into shallower zones, where competition increases and ecosystems become unbalanced. Overall marine productivity and fisheries can decline, especially in tropical upwelling systems.

Weaker carbon sink: Warming feedback loop

Warmer waters holds less oxygen and slows ocean mixing, meaning carbon absorbed at the surface doesn’t sink as efficiently. That reduces the ocean’s capacity as a carbon sink and more CO2 remains in the atmosphere, amplifying warming.

Shift in microbial and chemical cycles

As oxygen drops, microbial metabolism shifts. Oxygen-depleted zones may release more greenhouse gases like nitrous oxide and methane. Oxygen-deficient areas also shift from carbon sinks to sources in some cases.

Long-term planetary consequences

Model predictions estimate than even if CO2 emissions stopped today, deep ocean oxygen would continue falling for centuries, losing over 10% of its pre-industrial content by the year 2650. That would reduce biologically viable habitats by up to 25% in many deep regions. The result: fragile deep-sea ecosystems, diminished biodiversity, and disrupted food webs.

Broader implications of declining oxygen in the oceans

Ecosystem collapse and food security risks: If lanternfish vanish, as seen over past deoxygenation events, they take with them a foundational link in the ocean’s carbon and food web. Similar effects could cascade to fisheries and human food systems.

Climate tipping points: Combined threads like warming, acidification, and low oxygen create a triple whammy on marine life. In tropical waters, these factors are pushing ecosystems towards critical tipping points, like during the Permian extinction era.

Human costs: Coastal economies, tourism, prawn farming (shrimp farming, for the American), fisheries, and aquaculture are all vulnerable to oxygen loss. This could disrupt livelihoods for hundred of millions of people, especially in poorer countries dependent on the ocean for food.

Carbon cycle feedbacks: Reduced oxygen sets off feedbacks: less carbon uptake, more greenhouse gas release, and slower recovery even if emissions are curbed.

So, where are we headed?

Ocean oxygen levels are still decreasing, predicted to fall by several per cent globally by the end of this century, and even more in the deep sea, this decline will carry on for centuries!

That could lead to vast habitat loss, biodiversity collapses, reduced fish yields, and weakened carbon storage.

These changes raise the risk of climate feedback loops, food insecurity, and major shifts in how marine ecosystems function globally.

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