EcoWatch – Oceans
Ocean’s Largest Dead Zones Mapped by MIT Scientists
Olivia Rosane – January 26, 2022
Oxygen-deficient zones intensity across the eastern Pacific Ocean, where copper colors represent the locations of consistently lowest oxygen concentrations and deep teal indicates regions without sufficiently low dissolved oxygen. Jarek Kwiecinski and Andrew Babbin
When you think of the tropical Pacific, you might picture a rainbow of fish ribboning their way between pinnacles of coral, or large sea turtles swimming beneath diamonds of sunlight. But there are two mysterious zones in the Pacific Ocean where life like this cannot survive.
That is because they are the two largest oxygen-deficient zones (ODZ) in the world, which means they are a no-go zone for most aerobic (oxygen-dependent) organisms. Two Massachusetts Institute of Technology (MIT) scientists recently succeeded in making the most detailed atlas to date of these important oceanic regions, revealing crucial new facts about them in the process. The new high-resolution atlas was described last month in the journal Global Biogeochemical Cycles.
“We learned just how big these two zones in the Pacific are, reducing the uncertainty in the measurement, their horizontal extent, how much and where these zones are ventilated by oxygenated waters, and so much more,” Andrew Babbin told EcoWatch in an email. Babbin is one of the atlas’s two developers and Cecil and Ida Green Career Development Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “Being able to visualize in high resolution the low oxygen zones really is a necessary first step to fully understanding the processes and phenomena that lead to their emergence,” he said.
Natural Dead Zones
Oxygen-deficient zones can also be referred to as hypoxic zones or dead zones, as the National Oceanic and Atmospheric Administration explains. They can be caused by human activity, especially nutrient pollution. For example, the world’s second-largest dead zone is in the Gulf of Mexico, and is largely caused by the runoff of nitrogen and phosphorus from cities and factory farms.
The new atlas focuses on two naturally-occurring ODZs in the tropical Pacific, however. One is located off the coast of South America and measures about 600,000 cubic kilometers (approximately 143,948 cubic miles), or the equivalent of 240-billion Olympic swimming pools, MIT News reported. The second is around three times larger and located in the northern hemisphere, off the coast of Central America.
Both natural and anthropogenic ODZs have something in common: too many nutrients. In the case of the Pacific ODZs, Babbin said, those nutrients build up because of wind patterns that push water offshore.
“Deeper water then upwells to fill in this void, bringing higher nutrients to the surface,” Babbin told EcoWatch. “Those nutrients stimulate a massive amount of growth of phytoplankton, akin to how we fertilize crop lands and even our potted plants at home. When those phytoplankton then sink, heterotrophic bacteria act to decompose the organic material, consuming oxygen just like humans do to respire our food.”
However, because of where these zones are located, it takes a long time for oxygen-rich waters to reach the area and replenish what the bacteria gobble up.
“In essence, the biological demand of oxygen outpaces the physical resupply,” Babbin concluded.
While these specific zones aren’t caused by human pollution, understanding them is still important in the context of human activity. ODZs can emit the greenhouse gas nitrous oxide, and there is a concern that the climate crisis may cause them to expand.