By Anders Lorenzen

Loss of oxygen from the world`s oceans driven by climate change is increasingly threatening the ability of oceans to support life. So says the conservation group, the International Union for Conservation of Nature (IUCN), in a report released earlier this month at the COP25 summit in Madrid.

They found that the loss of oxygen is threatening fish species and disrupting ecosystems. It is driven by climate change and nutrient pollution. and is especially threatening species such as tuna, marlin and sharks. Many tuna species are already threatened by overfishing and fishing methods.

The report: “Ocean deoxygenation: Everyone’s problem“ is the largest peer-reviewed study so far looking at the causes, impacts and possible solutions to ocean deoxygenation. It finds that ocean regions with low oxygen concentrations are expanding and that around 700 sites worldwide are now affected by low oxygen conditions. This is a rise from around 45 in the 1960s. Additionally, during the same period, the volume anoxic waters which are areas completely depleted of oxygen has quadrupled.

Climate change and nutrient pollution the main culprits

As the ocean warms, its waters hold less oxygen and become more buoyant. This leads to a reduced mixing of the oxygen-rich water near the surface with the ocean depths, which naturally contain less oxygen. Nutrient pollution causes oxygen loss in coastal waters as fertiliser, sewage, animal and aquaculture waste cause excessive growth of algae, which in turn deplete oxygen as they decompose.

Dr Grethel Aguilar, IUCN Acting Director-General, explained that with this report it is now clear for everyone to see what scale of damage climate change is inflicting upon the oceans. She stated: “As the warming ocean loses oxygen, the delicate balance of marine life is thrown into disarray. The potentially dire effects on fisheries and vulnerable coastal communities mean that the decisions made at the ongoing UN Climate Change Conference are even more crucial. To curb ocean oxygen loss alongside the other disastrous impacts of climate change, world leaders must commit to immediate and substantial emission cuts.”

The Swedish government is a major funder of the report. Minister for Environment and Energy and Deputy Prime Minister, Isabella Lövin, said “Whilst we have known about dead zones in the ocean for many decades, ocean warming is now expected to further amplify deoxygenation across great swathes of the ocean. We need to work together to get the ocean oxygen budget back in balance. With this report, it is time to put ocean deoxygenation among our top priorities in order to restore ocean health.”

Altering marine life

The report describes how deoxygenation is so severe that it is actually starting to alter the balance of marine life. Meaning that in areas of reduced oxygen levels low-oxygen tolerant species such as microbes, jellyfish and some squid are more likely to survive at the expense of species more sensitive to low-oxygen which includes many marine species and most fish.

Tuna, marlin and sharks are particularly sensitive to low oxygen because of their large size and energy demands. These species are being driven into increasingly shallow surface layers of oxygen-rich water, making them more vulnerable to overfishing.

The report outlines just how devastating these changes can be for hundreds of millions of people depending on the oceans for their livelihood. It explains some of the ocean’s most productive biomes – which support one-fifth of the world’s wild marine fish harvest – are formed by ocean currents carrying nutrient-rich but oxygen-poor water to coasts that line the eastern edges of the world’s ocean basins. As naturally oxygen-poor systems, these areas are particularly vulnerable to even small changes in ocean oxygen.

Crucially, the report warns, low ocean oxygen can impact much more than the ocean species. It can also affect the basic processing of nitrogen and phosphorous crucial to sustaining life on Earth. By 2100 the ocean is expected to lose 3-4% of its oxygen inventory if we carry on with business as usual.

UCN Global Marine and Polar Programme Director, Minna Epps, expressed hope that world leaders would make progress on this issue in Madrid. She also highlighted this will be a top issue in June next year at the IUCN World Conservation Congress in Marseille, France.