A mysterious change in the food web of the Pacific Ocean started in the mid-19th century, and the skeletons of deep-sea coral tell the tale.

Hundreds, even thousands, of feet beneath the ocean surface, deep sea corals live for centuries. As the grow, the tiny creatures collect a chemical record of what they eat. Marine scientists recently constructed a 1,000 year-long history of North Pacific corals’ cuisine by analyzing the nitrogen trapped in the coral skeletons.

The changing levels of different types of nitrogen, called isotopes, revealed information about the conditions in the ecosystem of the North Pacific subtropical gyre, a 20 million square kilometer counterclockwise circulation of the ocean’s waters.

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For most of the past millennium, the nitrogen in the Pacific Ocean food chain came from dissolved nitrate rising from deeper in the sea. However, abruptly 150 years ago, the coral recorded a dramatic change in the source of nitrogen entering the marine food chain. Since approximately 1850, more of the chemical has been coming from microorganisms that transform nitrogen, similarly to how beans and other legumes fix nitrogen on land. Since approximately 1850, the increase in nitrogen from microorganisms increased by 17 to 27 percent.

“In comparison to other transitions in the paleoceanographic record, it’s gigantic,” said lead author Owen Sherwood of the University of Colorado, Boulder, in a press release. The study was published Dec. 15 in the journal Nature.

The cause of the food chain change may have to do with an expansion and warming of the North Pacific subtropical gyre itself. Marine scientists have also observed the gyre changing again over the past few decades.

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“Our new records from deep-sea corals now show that the decadal-scale changes are really only small oscillations superimposed on a dramatic long-term shift at the base of the Pacific ecosystem,” study co-author Matthew McCarthy of the University of California at Santa Cruz said in a press release. “This long-term perspective may help us better predict the effects of global warming on open ocean regions.”

IMAGE: A rockfish hides in a deep sea coral, Primnoa pacifica, in Juan Perez Sound in Haida Gwaii, British Columbia (Geofflos, Wikimedia Commons)