Low-oxygen zones in the open ocean have quadrupled in number since the 1950s, spreading across millions of square kilometers. Closer to shore, in estuaries and seas, they have grown more than tenfold. That was the finding of a review published in the journal Science in January 2018, the broadest global look at the problem so far.
“Oxygen is fundamental to life in the oceans,” said Denise Breitburg, the lead author and a marine ecologist at the Smithsonian Environmental Research Center. “The decline in ocean oxygen ranks among the most serious effects of human activities on Earth’s environment.”
Two causes, two places
Out at sea, warming is the main driver. Warm water holds less oxygen, and a warmer surface makes it harder for oxygen to reach the depths. Melting polar ice adds a second effect: a buoyant layer forms on top of colder, saltier water and acts as a lid, so less surface water mixes downward. All of the ocean’s oxygen enters at the surface, from the air or from phytoplankton, which means less mixing leaves the deep short.
Near the coast, the culprit is mostly nutrient pollution. Phosphorus and nitrogen from farm fertilizer wash into rivers and estuaries and feed algal blooms. When the algae die and decompose, they use up the oxygen in the water.
The measured decline
Direct measurements show the global ocean has lost about 2% of its oxygen over the past 50 years. The loss is uneven. Some tropical regions have lost 40% over the same period, according to recent studies. A separate report projects a further 3 to 6% drop during the 21st century as surface waters warm.
Phytoplankton, which produce much of the oxygen we breathe, have also declined. Some scientists have suggested a global drop of 40% since 1950, about 1% a year, and tied it to rising sea surface temperatures.
There are more than 400 dead zones worldwide. The Baltic Sea holds the largest. The second largest sits in the northern Gulf of Mexico, an area the size of 5 million soccer fields, where it has been killing fish, crustaceans and the fisheries that depend on them.
What it does to marine life
In a dead zone, most animals suffocate. A few species survive, but biodiversity collapses. Fish can swim away, though that shrinks their habitat and exposes them to more predators. Even where oxygen stays above lethal levels, low oxygen can stunt growth, hinder reproduction or trigger disease, the researchers said. It can also cause the ocean to release nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide.
Warmer water makes the problem worse, because marine animals need more oxygen as temperatures rise. Vladimir Ryabinin of the Intergovernmental Oceanographic Commission said the 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.”
The review’s authors belong to GO2NE, the Global Ocean Oxygen Network, founded in 2016. They proposed a three-part plan: tackle climate change and nutrient pollution, protect the most vulnerable marine life, and track low-oxygen areas better worldwide.
Breitburg pointed to Chesapeake Bay as proof that local action works. It once held a huge dead zone, and after improvements in sewage treatment and farming practices it has almost no zero-oxygen water left. “This is a problem we can solve,” she said.
Sources
- Oxygen & Ocean — The Conscious Challenge
- Hidden 'Dead Zones' in The Ocean Have Quadrupled Since The '50s, And That's Really Bad
