World's oceans losing oxygen, threatening marine life, study finds

The oxygen content in the world's oceans has decreased by more than two per cent in the past 50 years and could decrease by seven per cent by 2100, a new study shows.

The study, conducted at the GEOMAR Helmholtz Centre for Ocean Research in Keil, Germany, is being called the the first in-depth study of global ocean oxygen content, examining how global warming is impacting oceanic oxygen and why it is a concern.

After studying 50 years of data, researchers found the greatest volume of oxygen losses had occurred in the North Pacific, while the largest percentage loss was in the Arctic Ocean.

The main cause for the drop in oceanic oxygen is climate change, they say, specifically rising water temperatures.

Warmer water temperatures account for 15 per cent of oxygen loss, the researchers found, the majority from reduced stratification when surface water doesn't sink to the ocean floor caused by changing temperatures in the Arctic and the melting of sea ice.

Consequences

Warmwater holds less dissolved oxygen and is less dense than coldwater (4 degrees Celsius), so it is less efficient at circulating to the bottom of the ocean.

"Since large fishes in particular avoid or do not survive in areas with low oxygen content, these changes can have far-reaching biological consequences," lead author Sunke Schmidtko said.

• Climate change causing more dead zones in oceans
• Scientists propose refreezing Arctic to fight climate change

An increase in melted sea ice led to more plankton growth and decomposition, the study states. The decomposed plankton decreases oxygen levels and can lead to dead zones in the ocean water.

The dead zones are often in shallower waters where fish can't thrive, the research states. These dead zones also pump out nitrous oxide, which is a greenhouse gas and further contributes to climate change.

"While the slight decrease of oxygen in the atmosphere is currently considered non-critical, the oxygen losses in the ocean can have far-reaching consequences because of the uneven distribution. For fisheries and coastal economies this process may have detrimental consequences," co-author Lothar Stramma said.

The study is published in the journal Nature.