Kelly McSweeney

May 16th 2018

Gasping for Air in the Ocean’s Oxygen Minimum Zone


An oxygen minimum zone in the ocean is often called a “dead zone” because the dissolved oxygen levels are so low that most sea creatures can’t survive. There has been an increase in the number of low-oxygen zones, which poses big threats to oceans. This is urgent, since — just like humans on land — marine animals need oxygen to survive.

A Growing Problem

A team of experts from a United Nations group, called the Global Ocean Oxygen Network, released a study in the January 2018 issue of Science about the growing number of low-oxygen sites in the ocean.

According to the scientists, oxygen concentrations in ocean water have been declining since at least the middle of the 20th century. They found that oxygen minimum zones in the open ocean have expanded by several million square kilometers and the problem is even worse in coastal water; some sites now have oxygen concentrations low enough that animals are fleeing.

What Causes Deoxygenation in the Ocean?

A phenomenon called nutrient loading is the main reason we’re seeing an increase in low-oxygen zones, according to the Global Ocean Oxygen Network and the National Oceanic and Atmospheric Administration (NOAA). This is a process where polluted water from urban centers and land-based agriculture trickles into streams and rivers that flow into the ocean. This water is full of fertilizers and other nutrients that create problems when they enter natural environments.

NOAA scientist Alan Lewitus explains that excessive nutrients in the water can stimulate explosive blooms of algae. When the blooms die, they sink to the bottom of the sea where bacteria decompose them and consume oxygen from the water, ultimately making the environment uninhabitable.

According to the United Nations Development Programme, dead zones are costing fisheries, the tourism industry and other coastal livelihood tens of billions of dollars annually and will only continue to increase unless something changes.

Marine life is already vulnerable because of plastic pollution, acidification and other threats to oceans. The oxygen problem is also connected to rising ocean temperatures caused by climate change. Lisa Levin, one of the authors of the Global Ocean Oxygen Network study, explained to Newsweek that warm water is inhospitable to oxygen.

As the ocean gets warmer, the water’s overall oxygen concentration decreases. Rising temperatures also slow down ocean circulation and cause layers of water to stratify so the high-oxygen surface waters don’t mix as well with deep water in an oxygen minimum zone, according to Scientific American.

Three Steps to Help the Ocean Breathe Again

“‘This is a problem we can solve,'” says Denise Breitburg, the lead author of the Global Ocean Oxygen Network study. She continues in a statement, “‘Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline.'”

The scientists highlighted three main ways to fix the ocean’s oxygen problem. First, we can stop adding to the problem by addressing two of the causes: nutrient pollution and climate change. The authors suggest that cutting fossil fuel emissions would help fight climate change, and new policies can help reduce the amount of fertilizers leaching into groundwater. The Chesapeake Bay is a success story where better sewage treatment, farming practices and laws like the Clean Air Act have helped improve the ocean’s health.

Secondly, there are steps we can take to protect fish near low-oxygen zones, such as creating marine-protected areas or no-catch zones animals could use to flee low oxygen. Lastly, we can work on improving tracking this problem around the world. Scientists have been working on creating models to help predict where low-oxygen zones will be in the future, but they need more data about current oxygen levels in the ocean worldwide.

Scientists around the world are grappling with many complicated threats to oceans, but eliminating low-oxygen zones is extremely important to the health of creatures and humans alike.