Worldwide, about 60.4 million cubic kilometers of ocean water is hypoxic, with dissolved oxygen near 1.6 milligrams per liter. A scientist writing in The Conversation in November 2016 put that in terms of the Empire State Building: it would take close to 58 billion of them to hold that much water.
The author belongs to the Global Ocean Oxygen Network, or GO2NE, a working group of the Intergovernmental Oceanographic Commission and UNESCO formed in 2015 to raise awareness of the problem and push for research and policy.
How oxygen disappears
Oceans, coastal seas, estuaries, and many rivers and lakes are all losing dissolved oxygen. The problem has long been tied to sewage and fertilizer runoff. Climate change now adds to it, often independent of nutrient loads, and the scale is global.
The chain starts with nutrients. Runoff from farmland and cities feeds aquatic plants, and an excess can set off massive algal blooms. When the algae die, microbes break them down and use up the dissolved oxygen.
Warming adds a second push. Warmer water is less dense and holds less oxygen than cold water, and salt lowers the capacity further. As seas heat up, layers separate more sharply, a process called stratification, and deeper water loses even more oxygen.
Water with less than two milligrams of oxygen per liter is called hypoxic, a term borrowed from medicine. That level is generally harmful to fish and shellfish. Water with none at all is anoxic, and few higher organisms can live in it.
Where it is happening
Dead zones are growing off the Oregon coast and the upwelling zones off Peru, and in the Baltic and Black seas, the Bay of Bengal, the South China Sea and the Gulf of Mexico. Low-oxygen areas in the deep ocean, called oxygen minimum zones, are also expanding, mainly because of warming.
Long records show the pattern. In the Adriatic Sea, summer oxygen was stable until the middle of the 20th century, then hypoxia began in 1960 and worsened. One study using data back to 1899 found growing hypoxic and anoxic bottom water in the Baltic, where the affected zone is now about three times the size of the Gulf of Mexico dead zone.
What it means for fish
Water holds much less oxygen than air, so fish have to work harder to breathe. Experiments show that low oxygen limits their metabolic scope and slows their metabolism, cutting feeding and growth.
One study that combined rising temperatures with falling oxygen predicted that maximum fish size will shrink by 10 percent on average across all seas, with wide variation by species. In regions that rely on fish for protein, that loss lands on populations already hit by overfishing and pollution.
Baltic cod have been in serious decline in condition and size over two decades. Hypoxia was the strongest explanatory variable, followed by a shortage of prey and a 30 percent loss of living space the author calls “habitat squeeze.” In the tropics, oxygen-rich water has grown shallower, which appears to crowd some tuna and billfish into a thinner layer and may make them easier to catch. In the Gulf of Mexico, hypoxia stress on the endocrine system has cut reproduction in Atlantic croaker.
Projections
Climate models run under assumptions ranging from business as usual to deep cuts in carbon dioxide emissions all project that ocean oxygen will fall by 1.8 to 3.45 percent over the next century. The effects will not be even. The north Atlantic, north Pacific and Antarctic waters are expected to be hit hardest.
The oceans have lost oxygen before, but the author says clear evidence ties the present changes largely to human actions, chiefly higher water temperatures and continued nutrient inputs from farms and cities. Cutting runoff and halting greenhouse gas emissions would require a total overhaul of how societies do business, the author wrote, and policy action is needed now.
