Nearly two dozen marine scientists from around the world have issued a warning about an often-overlooked side effect of climate change and pollution.
"Combined effects of nutrient loading and climate change are greatly increasing the number and size of "dead zones" in the open ocean and coastal waters, where oxygen is too low to support most marine life", said Dr Vladimir Ryabinin, executive secretary of the International Oceanographic Commission, which formed GO2NE.
The study, which was funded by an global organization affiliated to UNESCO (the United Nations Educational, Scientific and Cultural Organization), reports that this problem has been worsening since the 1950s.
That includes the St. Lawrence Seaway and oceans off Canada's West Coast.
"This is a problem we can solve".
"There will always be some areas of the sea that have low oxygen - just as there are deserts on land - the problem is when these areas expand and replace more productive ecosystems", said Breitburg. "We're seeing the marine animals leaving those areas". Better data collection would also help, the researchers said, including real-time observations and more advanced forecasts that could potentially predict when and where low-oxygen events could occur years in advance.
Oxygen-deprived dead zones were first identified in estuaries in the mid-19th century and their oxygen depletion was linked to the presence of urban sewage in the water, said the study's lead author, Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Centre.
Analyses of measurements at sites around the world indicate that oxygen minimum zones in the open ocean have expanded by several million square kilometers and that hundreds of coastal sites now have oxygen concentrations low enough to limit the distribution and abundance of animals, and also alter the cycling of important nutrients.
"It's huge", said Gilbert.
They highlight efforts to provide better sewage treatment in Chesapeake Bay in the eastern United States, which have resulted in substantial increases in water oxygen levels.
Pollution and climate change both play significant roles in depleting the ocean's oxygen levels and the authors emphasise the role humans must play in addressing these issues.More news: 'The Shape of Water' leads race for BAFTAs
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LightRocket via Getty Images St. Lawrence River.
Industries and the agricultural sector release their nitrates into the ocean, creating algal blooms similar to those that can affect freshwater systems.
There are two kinds of low oxygen zones: the ones closer to the coast and directly affected by onshore activities are classified as coastal zones, and those indirectly impacted are called open ocean low-oxygen zones.
Climate change, Gilbert said, delivers a "triple whammy". On top of that, warmer water simply doesn't hold as much oxygen and less oxygen dissolves and gets into the water, she said.
Second, different layers in the oceans don't mix as much as upper waters warm. There is no proper way for ventilation of deeper layers.
"One of the reasons why (marine animals) cannot tolerate very hot water is that they have to breathe more".
One of the reasons why (marine animals) can not tolerate very warm waters is they need to breathe more. "The decline in oxygen can cause major changes in ocean productivity, biodiversity, and biogeochemical cycles as we have observed in the dead zones of the Baltic Sea".
The problem won't go away any time soon. He informed that the increasing rate of oxygen due to global warming would increase more by 2100 and it will keep increasing more.
But unfortunately, even "ambitious emission reductions", Breitburg said, might not prevent the predicted "further oxygen declines during the 21st century".
It takes time to recover from bleaching, and the increased frequency means coral doesn't get the chance to recover before the next outbreak, Eakin said.