The swirling currents of the North Pacific Subtropical Gyre have long been considered to be a vast marine desert home to sparsely scattered microscopic life-forms. But decades of observations from a Hawaiian research station indicate cyclical changes in the availability of phosphorus and iron, both key components for life. Now a new study has identified the source of these mysterious nutrient oscillations: Asian dust storms.
“Historically, these regions of the Pacific were thought to be barren, but it turns out, they’re actually quite dynamic,” says Ricardo Letelier, a microbial ecologist at Oregon State University who, in collaboration with David Karl at the University of Hawai‘i, led the new study, published in the Proceedings of the National Academy of Sciences of the United States of America.
Letelier and colleagues drew upon 3 decades of data collected at Station ALOHA (A Long-term Oligotrophic Habitat Assessment), an oceanographic research station operated by the Hawaii Ocean Time-series program and funded by the National Science Foundation. They found that the levels of phosphorus and iron varied significantly both seasonally and over longer timescales.
Modeling helped solidify the link between nutrient cycles and dust storms, Letelier says. “Dust deposition has a very strong seasonal cycle. Most comes from dust storms in Asia that take place in the late winter and early spring.”
A major contribution of this study was the detection of changes at interannual timescales, corresponding to changes in the Pacific Decadal Oscillation (PDO), a recurring pattern of ocean-atmosphere climate variability similar to El Niño. In years when the PDO is weak, leading to low-pressure zones in the northwest Pacific, winds from Asia become stronger, bringing more dust. In strong PDO years, the high-pressure zone results in weaker winds from Asia and less dust transport.
The study offers a “pretty convincing link between the cycles of the PDO and the supply of nutrients like iron from dust,” says Dave Hutchins, a marine biogeochemist at the University of Southern California who was not involved in the new study. “They did a really nice job of pulling together different lines of evidence and modeling them and seeing how they fit into the big picture story of this region of the ocean.”
Climate change is expected to impact the PDO and the large-scale wind patterns that sweep across the Pacific Ocean, Letelier says. As the Arctic warms, the temperature gradient between the Arctic and subtropical regions will decrease, influencing the PDO, El Niño, and other large-scale climate oscillations. How these changes will affect the transport of dust and nutrients into the Pacific and influence biological activity in the North Pacific is an area that needs more research, he says.
It’s not just a matter of wind: Increasing desertification due to climate change may also create more dust. In addition, anthropogenic pollution could also change the character of the dust being transported, Letelier says. “I don’t know if we can say if there will be more or less dust transport in the future, but what is likely is that the system will become more unpredictable.”
—Mary Caperton Morton (@theblondecoyote), Science Writer