Dust, Pollution, and Decreasing Oxygen in the Tropical Pacific
For the past several decades, researchers have been tracking the decline in dissolved oxygen in the tropical Pacific Ocean. Less dissolved oxygen means that fewer aquatic creatures can survive there, which is bad news for that region’s ecosystem.
But while they could agree on the decline, researchers had trouble deciphering the cause. Now a new theory, put forth by scientists at Georgia Tech, NASA, NC State and the University of Washington and based on computer modeling, may provide part of the answer – air pollution.
Air pollution doesn’t just sink into the ocean and disrupt the environment. Instead, there’s a chain of events that begins with dust and ends with decomposition and oxygen use.
Dust is always in the air around us, taken up by wind and transported all over the globe until it settles out someplace else, like the ocean. The dust contains the mineral form of iron, which would normally just oxidize, or rust, and sink. But when dust interacts with an acidic environment, like that produced by atmospheric pollution, it releases soluble iron. Increasing quantities of this soluble atmospheric anthropogenic iron and fixed nitrogen entering the open ocean increase the amount of phytoplankton – those tiny plants that many aquatic creatures love to eat.
More food for the fishes is a good thing, right? Not necessarily. The increased amount of plankton also increases the decomposition of organic material as it sinks through undersea currents and falls to the bottom of the sea. The process of decomposition takes oxygen out of the water.
The researchers, led by Georgia Tech’s Taka Ito, decided to test their theory by constructing a computer model that reproduced the pattern of oxygen changes from 1970 to 1990. They found that while temperature change was small and didn’t affect the oxygen levels, iron and nitrogen did.
“There are multiple sources of iron that find their way into the ocean, from the rivers, air, sediments and hydrothermal vents,” says Nicholas Meskhidze, associate professor of marine, earth and atmospheric sciences and a co-author of a paper describing the research, “but the farther you get from the coasts, the source with the most impact comes from the atmosphere, due to its conversion of the iron into a soluble form. And very small amounts of soluble iron can have a large effect.
“This model allows us to trace the impact of aerosol pollution on oxygen levels in the ocean and gives us a window into what may happen if these levels increase.”
The research appears in Nature Geoscience.