‘Ocean deoxygenation’ ranks among the most important changes occurring in marine ecosystems
An international team of scientists warns that the ocean may run out of breath unless action is taken to rein in climate change and nutrient pollution. In the first sweeping look at the causes, consequences and solutions to low oxygen worldwide, published in Science, researchers reveal that the amount of oxygen in the world’s oceans and coastal waters is steadily decreasing.
The oxygen content of the open ocean and coastal waters has been declining for at least the past half century as a result of human activities that have increased global temperatures and nutrients discharged to coastal waters.
“Oxygen is fundamental to life in the oceans,” said Denise Breitburg, lead author and marine ecologist with the Smithsonian Environmental Research Center. “The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth’s environment.”
In the past 50 years, the amount of water in the open ocean with zero oxygen has gone up more than fourfold. In coastal water bodies, including estuaries and seas, low-oxygen sites have increased more than 10-fold since 1950. For the upper ocean, oxygen and heat content are highly correlated for the period of 1958-2015 with sharp increases in both deoxygenation and ocean heat content beginning in the mid 1980s.
Many areas around the globe are looking at how we used sound science to make wise environmental management decisions to improve the water quality of Chesapeake Bay.
The study came from a team of scientists from GO2NE (Global Ocean Oxygen Network), a new working group created in 2016 by the United Nation’s Intergovernmental Oceanographic Commission that includes Mike Roman and Kenny Rose from the University of Maryland Center for Environmental Science’s Horn Point Laboratory.
The review paper is the first to take such a sweeping look at the causes, consequences and solutions to low oxygen worldwide, in both the open ocean and coastal waters. The article highlights the biggest dangers to the ocean and society, and what it will take to keep Earth’s waters healthy and productive.
In areas traditionally called “dead zones,” like those in Chesapeake Bay and the Gulf of Mexico, oxygen plummets to levels so low many animals suffocate and die. As fish avoid these zones, their habitats shrink and they become more vulnerable to predators or fishing. But the problem goes far beyond “dead zones,” the authors point out.
Even smaller oxygen declines can stunt growth in animals, hinder reproduction and lead to disease or even death. It also can trigger the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide, and toxic hydrogen sulfide. While some animals can thrive in dead zones, overall biodiversity falls.
The ongoing recovery of Chesapeake Bay, where nitrogen pollution has dropped 24 percent since its peak thanks to better sewage treatment, better farming practices and successful laws like the Clean Air Act, is an example of what’s possible to reverse this trend. While some low-oxygen zones persist, the area of the Chesapeake with zero oxygen has almost disappeared.
“Many areas around the globe are looking at how we used sound science to make wise environmental management decisions to improve the water quality of Chesapeake Bay,” said Roman, co-author of the report and director of the UMCES' Horn Point Laboratory in Cambridge, Maryland.
Climate change is the key culprit in the open ocean. Warming surface waters make it harder for oxygen to reach the ocean interior. Furthermore, as the ocean as a whole gets warmer, it holds less oxygen. In coastal waters, excess nutrient pollution from land creates algal blooms, which drain oxygen as they die and decompose. In an unfortunate twist, animals also need more oxygen in warmer waters, even as it is disappearing.
People’s livelihoods are also on the line, the scientists reported, especially in developing nations. Smaller, artisanal fisheries may be unable to relocate when low oxygen destroys their harvests or forces fish to move elsewhere. In the Philippines, fish kills in a single town’s aquaculture pens cost more than $10 million. Coral reefs, a key tourism attraction in many countries, also can waste away without enough oxygen.
Some popular fisheries could benefit, at least in the short term. Nutrient pollution can stimulate production of food for fish. In addition, when fish are forced to crowd to escape low oxygen, they can become easier to catch. But in the long run, this could result in overfishing and damage to the economy.
“Getting the effects of low oxygen on fish populations and supporting food webs correct, especially as it worsens, will enable more effective analyses and decisions on how to sustainably manage many fisheries, from artisanal that support local communities to large-scale industrial fisheries,” said Rose, a co-author of the report and a professor at Horn Point Laboratory.
Winning the War: A Three-Pronged Approach
To keep low oxygen in check, the scientists said the world needs to take on the issue from three angles:
Address the causes: nutrient pollution and climate change. While neither issue is simple or easy, the steps needed to win can benefit people as well as the environment. Better septic systems and sanitation can protect human health and keep pollution out of the water. Cutting fossil fuel emissions not only cuts greenhouse gases and fights climate change, but also slashes dangerous air pollutants like mercury.
Protect vulnerable marine life. With some low oxygen unavoidable, it is crucial to protect at-risk fisheries from further stress. According to the GO2NE team, this could mean creating marine protected areas or no-catch zones in areas animals use to escape low oxygen, or switching to fish that are not as threatened by falling oxygen levels.
Improve low-oxygen tracking worldwide. Scientists have a decent grasp of how much oxygen the ocean could lose in the future, but they do not know exactly where those low-oxygen zones will be. Enhanced monitoring, especially in developing countries, and numerical models will help pinpoint which places are most at risk and determine the most effective solutions.
The Global Ocean Oxygen Network (GO2NE) is a scientific working group organized by the Intergovernmental Oceanographic Commission, part of the United Nations Educational, Scientific and Cultural Organization (UNESCO). Established in 2016, its scientists from around the world are committed to providing a global and multidisciplinary view of deoxygenation, advising policymakers on countering low oxygen and preserving marine resources.
Read the paper: "Declining oxygen in the global ocean and coastal waters"