In a new study co-led by Patrick Rafter of the University of South Florida, researchers show that warming in the tropical Pacific - home to some of the world's most productive fisheries - may not trigger the severe declines predicted by earlier models. Instead, the region's fisheries could remain productive even as ocean temperatures rise.
Rafter, a chemical oceanographer at USF's College of Marine Science, said the findings are welcome news.
"Our measurements suggest that, on a warmer planet, the availability of marine nutrients to fuel plant growth and fisheries may not necessarily decline," Rafter said.
The paper highlights a cutting-edge approach to predicting future ocean conditions by examining the distant past. Further study could reveal more reason for optimism about global ocean productivity.
The team turned to the Pliocene Epoch, 5.3 to 2.6 million years ago, when ocean warming trends were similar to today's. By analyzing nitrogen isotopes preserved in the shells of tiny plankton called foraminifera (forams), researchers reconstructed nutrient characteristics in the tropical Pacific.
Today, nutrient upwelling in the region supports vast blooms of plankton - the base of the marine food chain. During warming events like El Nino, this process weakens, reducing nutrients and harming fisheries. Previous studies suggested such conditions could become permanent in a hotter world.
But Rafter and his colleagues found no evidence of reduced nitrate concentrations - a key nutrient for plankton - in the eastern tropical Pacific over the last five million years. The results suggest that nutrient upwelling and biological productivity remained stable despite higher global temperatures.
"We've used this nitrogen isotope like a geochemical fingerprint," Rafter said. "We don't have a time machine, but we can use our detective toolkit to reconstruct what happened in the ocean the last time Earth was as warm as today."
Extracting the isotopes required painstaking work. Researchers from USF, the University of Massachusetts Boston, the University of California Irvine and Princeton University hand-sorted foram shells from deep-sea cores, dissolved them and analyzed the nitrogen isotopes with the help of bacteria.
"Analyzing nitrogen isotopes derived from forams has allowed us to reconstruct the past with precision," Rafter said. "We can compare these past conditions to today and make better predictions about the future. The methods we've used represent a big step forward in improving our predictive capabilities."
For Jesse Farmer, co-lead author and assistant professor at UMass Boston, the findings provide cautious optimism.
"Our current warming is happening so quickly that the ocean may behave differently than it does when it's been warm for a long time, as was the case in the Pliocene," Farmer said, also noting modern threats such as ocean acidification and overfishing. Still, he added: "It's good news that the nutrient supply to the eastern Pacific food web will be maintained in a warmer ocean."
Looking ahead, the team plans to apply their "detective toolkit" to other parts of the ocean.
"We're looking at a changing system," Rafter said. "What's clear from this study is that the system is more complicated than we previously thought."
Much of the research for the study was conducted while Rafter and Farmer were postdocs at Princeton in the lab of Daniel Sigman, the paper's senior author.
Research Report:Persistent eastern equatorial Pacific Ocean upwelling since the warm Pliocene
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