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Research: Oxygen Levels Continue Dropping In World's Waters

Jan 16, 2018

Scientists say that climate change is having an effect on the levels of the world’s oceans.

But it’s also apparently affecting the oxygen levels throughout the oceans, as well as our coastal waters including the Gulf of Mexico.

That’s according to a study published in the Jan. 4 issue of Science by a team of scientists from the Global Ocean Oxygen Network (GO2NE), a working group created by the United Nations’ Intergovernmental Oceanographic Commission.

One of the group’s members is Brad Seibel, a professor of biological oceanography at the University of South Florida College of Marine Science. He talked to University Beat about what they’ve discovered so far:

There’s two processes at work here – and mankind may be to blame for both.
“One (process) that’s occurring in the Gulf of Mexico is predominantly a coastal issue that’s caused by nutrient runoff, which causes phytoplankton blooms, that, when those phytoplankton die and sink, they’re consumed by microbes, and the process of consumption is aerobic, and so it consumes oxygen as well,” Seibel said.

“(This is caused by) things like fertilizers that are used in agriculture are washed into rivers, such as the Mississippi River and that ends up in the Gulf of Mexico and other coastal waters, and causes eutrophication, where the nutrients actually fuel phytoplankton blooms, so it’s effectively fertilizing the ocean.”

“The other process is deoxygenation, more broadly caused by the warming of the oceans, which reduces the solubility of oxygen, so there’s less oxygen in surface waters and it also causes what’s called ‘stratification,’ where you have warmer waters floating on top of colder, deeper waters and that prevents surface waters mixing with deeper waters, so the interior of the ocean is receiving less oxygen from the surface waters.”

A map showing where low-oxygen zones are in the world's oceans and coastal waters. (Data from World Ocean Atlas 2013 and provided by R. J. Diaz.)
Credit GO2NE working group

The size and range of these zones continues to grow.
The study found that the amount of water with zero oxygen in the open ocean is up more than fourfold in the last fifty years. And since 1950, low-oxygen sites in coastal waters, including estuaries and seas, have jumped more than tenfold.

“In the Gulf of Mexico, the low-oxygen zone forms seasonally and the size varies from year to year, but can be as large as the size of New Jersey, and the oxygen content is anywhere from 50 percent of normal saturation down to complete anoxia – the absence of oxygen,” Siebel said. “And throughout the world, these sorts of low-oxygen zones are forming more frequently, they do occur naturally, but they’re appearing in more areas and are increasing in size and occurring more frequently.” 

This lack of oxygen can have serious repercussions for sea life -- and, as a result, humans.
"One of the biggest concerns for the coastal hypoxia, in terms of the effects on humans, are fisheries resources." 

"As oxygen declines, the more mobile organisms can leave the area and seek waters with higher oxygen, which ... can concentrate organisms in specific areas that are then targeted by fisheries, so it can make them more vulnerable to fisheries. Organisms that can't move, such as clams and more sedentary organisms are either killed or their populations are severely reduced in some areas."

What can be done?
"Certainly, the ultimate cause for the more oceanic (deoxygenation) is the carbon dioxide emissions and any reduction in CO2 emissions will prevent or postpone additional warming. Coastally, alternative practices for fertilization and agriculture can reduce emission runoffs and that is happening in some parts of the world, and it's had positive local effects."