Oxygen-deficient ‘dead zones’ in the Pacific Ocean are continuing to expand, according to a study which included researchers at The University of Western Australia.

The study, published in the Proceedings of the National Academy of Sciences, found oceanic ‘dead zones’ had been expanding for eight million years and human activity was contributing to the problem.

The researchers said oxygen was critical to the health of marine ecosystems and predicting future changes in ocean deoxygenation was important for societies that rely on fisheries.

Adjunct Professor Birger Rasmussen, from UWA’s School of Earth Sciences, said the research showed oceans had been losing oxygen for the past 50 years in the open ocean and coastal areas partly because of climate change and coastal nutrient discharge.

“Our study indicates that the modern oxygen deficient zones are underpinned by a historically high oceanic concentration of phosphate,” Professor Rasmussen said. 

A rise in nutrients fuels increased biological activity in the surface ocean. When the organic particles sink they undergo bacterial degradation which consumes oxygen, leading to oxygen-poor zones which may be deadly to marine life. 

The study noted there were three major oxygen depleted zones, at depth of 100 to 150m, with two in the Pacific Ocean.

“Coastal ocean ‘dead zones’ are mainly caused by the input of products such as fertiliser,” Professor Rasmussen said.

He said the oxygen depleted zones occurred naturally in the open ocean but it was unclear how they would change with global warming and an increase in nutrients.

“This study might help better predict the future behaviour of open ocean zones with depleted oxygen,” he said.