New research offers clues on global pollution pathways
Polluted dust from Asia is cutting oxygen levels in the tropical Pacific Ocean, researchers said this week, releasing a new study that traces a chain reaction that starts with land-based industrial pollution in China and other Asian countries.
“There’s a growing awareness that oxygen levels in the ocean may be changing over time,” said Taka Ito, an associate professor at Georgia Tech’s School of Earth and Atmospheric Sciences. “One reason for that is the warming environment – warm water holds less gas. But in the tropical Pacific, the oxygen level has been falling at a much faster rate than the temperature change can explain,” Ito said.
The study found that the pollution raises levels of iron and nitrogen in the ocean off the coast of East Asia. Ocean currents then carried the nutrients to tropical regions, where it feeds photosynthesizing phytoplankton. As those organisms feed on the nutrients, it has a negative effect on the dissolved oxygen levels deeper in the ocean.
The study, which was published May 16 in Nature Geoscience, was sponsored by the National Science Foundation, a Georgia Power Faculty Scholar Chair and a Cullen-Peck Faculty Fellowship.
“If you have more active photosynthesis at the surface, it produces more organic matter, and some of that sinks down,” Ito said. “And as it sinks down, there’s bacteria that consume that organic matter. Like us breathing in oxygen and exhaling CO2, the bacteria consume oxygen in the subsurface ocean, and there is a tendency to deplete more oxygen.”
The effect is strongest in the tropics, where dissolved oxygen is already low.
Scientists who worked on the study said the research shows how extensive the impact of industrial pollution can be.
“The scientific community always thought that the impact of air pollution is felt in the vicinity of where it deposits ,” said Athanasios Nenes, a professor in the School of Earth and Atmospheric Sciences and the School of Chemical and Biomolecular Engineering at Georgia Tech. “This study shows that the iron can circulate across the ocean and affect ecosystems thousands of kilometers away.”Athanasios Nenes, a professor in the School of Earth and Atmospheric Sciences and the School of Chemical and Biomolecular Engineering at Georgia Tech.
The scientists set out to discover why oxygen levels in the tropics had been declining since the 1970s. They developed a complex model that includes atmospheric chemistry, biogeochemical cycles, and ocean circulation. It shows how the polluted, iron-rich dust that settles over the Northern Pacific gets carried by ocean currents east toward North America, down the coast and then back west along the equator.
In their model, the researchers accounted for other factors that can also impact oxygen levels, such as water temperature and ocean current variability — all of which can factor into declining oxygen levels that have serious implications for marine life.
“Many living organisms depend on oxygen that is dissolved in seawater,” Ito said. “So if it gets low enough, it can cause problems, and it might change habitats for marine organisms.”
Sometimes the dead zones grow toward coastal waters, where they kill of displace fish, crabs and other organisms that are economically, biologically and culturally important.
Such hypoxic events” may become more frequent as the oxygen-minimum zones grow, Ito said, adding that the increasing phytoplankton activity is a double-edged sword.
“Phytoplankton is an essential part of the living ocean,” he said. “It serves as the base of food chain and absorbs atmospheric carbon dioxide. But if the pollution continues to supply excess nutrients, the process of the decomposition depletes oxygen from the deeper waters, and this deep oxygen is not easily replaced.”
The study also expands on the understanding of dust as a transporter of pollution, Nenes said.
“Dust has always attracted of a lot of interest because of its impact on the health of people,” Nenes said. “This is really the first study showing that dust can have a huge impact on the health of the oceans in ways that we’ve never understood before. It just raises the need to understand what we’re doing to marine ecosystems that feed populations worldwide.”