Breathing life into the Indian Ocean by predicting ‘dead zones’

By Kevin McElwee

In 2001, off India’s coastal state of Goa, the shrimp catch dropped by 80 percent in just a few years. The die-off was later traced to a dip in the ocean’s oxygen level.

“It was a massive event that almost collapsed the fisheries on the western Indian coast,” said Laure Resplandy, an assistant professor of geosciences and the Princeton Environmental Institute, who studies how our oceans, atmosphere and climate interact.

“One of the Indian fishing industry’s biggest sources of income is the export of shrimp to the United States.”

Resplandy believes that researchers, aided by advanced computing techniques, will soon be able to predict events such as these before they occur, allowing governments to warn communities or to prevent such occurrences altogether. She and her collaborators, including scientists at the Geophysical Fluid Dynamics Laboratory (GFDL), a division of the U.S. National Oceanic and Atmospheric Administration located about 3 miles from Princeton’s campus, are developing computer models aimed at making this goal a reality.

Tropical regions in the Indian Ocean are predisposed to low oxygen levels because exchanges with the atmosphere are limited and the water is warm. In higher latitudes, rough seas can take in and dissolve more oxygen from the atmosphere, and colder oceans can contain more oxygen.

“If you leave your Coke in the sun, it’ll  lose most of its fizz fast,” Resplandy explained. “Warmer liquids hold less gas than colder ones.”

Although low-oxygen regions occur naturally, human activity is the likely culprit when oxygen levels plummet so far that ocean life starts to die. In many coastal areas, agricultural fertilizers spur the growth of algae, which in turn become food for microbes that consume the oxygen, creating massive “dead zones” devoid of aquatic life. Because India’s agriculture is growing along with its booming population, the Indian Ocean is particularly susceptible.

To learn more about how and when these coastal dead zones can arise, Resplandy and her colleagues developed models with hundreds of factors, including algal production of oxygen, the mixing of fresh and salt water, and the ocean eddies that promote algae growth and oxygen exchanges between surface and deeper waters. She also includes variables controlling the monsoon, the massive system that dictates almost all weather patterns in South Asia.

Her models don’t have enough computing power to track every single wave and eddy in the ocean, but with informed estimations, she can create a model that approximates what to expect. She and her collaborators run these models on supercomputers in Princeton’s TIGER research computing cluster and at Oak Ridge National Laboratory. They then compare their models’ predictions to the measurements of oxygen levels, water temperatures and algae growth collected over the past century. If a model draws an accurate picture for recent events, its predictions for the future are more trustworthy.

John Dunne is a research oceanographer at GFDL who collaborates with Resplandy. He heads GFDL’s Biogeochemistry, Ecosystems, and Climate Group and has over 20 years of experience in collecting field observations, analyzing data and performing modeling.

Dunne was impressed with Resplandy’s breadth of research. “She now has many examples of research in which she has become engaged and advanced the understanding of the topic,” he said. “She’s a person that Princeton is lucky to have in their arsenal of scientific innovators.”

Resplandy was drawn to the Indian Ocean when writing her doctoral dissertation while at the Laboratoire d’Océanographie et du Climat in France, where she developed a model for addressing similar biogeochemical questions that is still used by the Indian government.

More recently, her models offered oceanographers a detailed picture of how oceans and rivers carry the Earth’s carbon and impact the storage of carbon on land, research that was published in June 2018 in Nature Geoscience. She has also received funding from NASA to collaborate with colleagues at the Jet Propulsion Laboratory and the Scripps Institution of Oceanography on a study of carbon fluxes during El Niño events.

Resplandy hopes that her new project on oxygen levels in the Indian Ocean, which is funded through the Princeton Environmental Institute’s Grand Challenges program, can also aid Indian officials. She doesn’t see herself as a policy advocate, but she expects her models to lead to preventative measures, such as safeguards to reduce fertilizer runoff.

She recognizes the magnitude of the questions she is trying to answer. “It’s challenging,” she said. “I’d do something else if it weren’t.”