Marine Life

Loss of oxygen in ocean a major threat to marine life

written by Oceanographic Staff

A new report from the International Union for Conservation of Nature (IUCN) that was release at the UN Climate Change conference in Madrid, states that the loss of oxygen from the ocean, resulting in ‘dead zones’, is increasingly threatening fish species and disrupting ecosystems.

It’s the largest peer-reviewed study so far into the causes, impacts and possible solutions to ocean deoxygenation. Ocean regions with low oxygen concentrations are expanding. There are now approximately 700 sites worldwide that are affected by low oxygen conditions. In the 1960s there were only around 45. In the same time period, the volume of areas completely depleted of oxygen in the global ocean has quadrupled. The major drivers of ocean oxygen loss are climate change and nutrient pollution, with the latter affecting coastal areas.

ocean oxygen Deoxygenation

“With this report, the scale of damage climate change is wreaking upon the ocean comes into stark focus. As the warming ocean loses oxygen, the delicate balance of marine life is thrown into disarray,” said Dr Grethel Aguilar, IUCN Acting Director General. “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.”

Deoxygenation is starting to alter the balance of marine life, favouring low-oxygen tolerant species, such as jellyfish, at the expense of low-oxygen sensitive ones, which include most fish. Some of the ocean’s most productive biomes 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 the smallest of changes in ocean oxygen. Impacts here will ultimately ripple out and affect hundreds of millions of people.

“We are now seeing increasingly low levels of dissolved oxygen across large areas of the open ocean. This is perhaps the ultimate wake-up call from the uncontrolled experiment humanity is unleashing on the world’s ocean as carbon emissions continue to increase,” said Dan Laffoley, Senior Advisor Marine Science and Conservation in IUCN’s Global Marine and Polar Programme and a co-editor of the report. “Ocean oxygen depletion is menacing marine ecosystems already under stress from ocean warming and acidification. To stop the worrying expansion of oxygen-poor areas, we need to decisively curb greenhouse gas emissions as well as nutrient pollution from agriculture and other sources.”

Species groups such as tuna, marlin and sharks are particularly sensitive to low oxygen because of their energy demands and large size. These species are being driven into increasingly shallow surface layers of oxygen-rich water, making them more vulnerable to overfishing. Very low ocean oxygen can also affect basic processes like the cycling of elements crucial for life on Earth, such as nitrogen and phosphorous.

The ocean is expected to lose three to four percent of its oxygen globally by the year 2100 if everything continues ‘business-as-usual’. However, the global average masks local changes, for example, that they will be more severe in mid-high latitudes. Most of the losses are predicted to be concentrated in the upper 1000m of the water column, which is richest in marine biodiversity.

“Urgent global action to overcome and reverse the effects of ocean deoxygenation is needed,” said IUCN Global Marine and Polar Programme Director Minna Epps. “Decisions taken at the ongoing climate conference will determine whether our ocean continues to sustain a rich variety of life, or whether habitable, oxygen-rich marine areas are increasingly, progressively and irrevocably lost.”

World leaders will come together in June 2020 at the IUCN World Conservation Congress in Marseille to discuss and drive further action towards restoring ocean health.

To read the full report and summary for policy makers, click here.

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