Phytoplankton & Climate Change: Model Shows Long-Held Constant In Ocean Nutrient Ratio May Vary

The future of these plants, called phytoplankton, is important because they exist at the base of the marine food web and represent a large source of food for fish. Also, they affect global climate by using atmospheric carbon dioxide, a greenhouse gas.

Phytoplankton depend upon nitrogen and phosphorus to grow and, ultimately, replenish the supply of these nutrients in the ocean. Since the 1930s, scientists have known that the average nitrogen-to-phosphorus (N:P) nutrient ratio of phytoplankton closely mirrors the N:P ratio in the ocean - 15:1 for the plants and 16:1 for the water. Scientists accepted this as a constant called the Redfield ratio, named after the late Harvard University scientist Alfred Redfield.

But researchers at the Georgia Institute of Technology and Princeton University designed a mathematical model based on phytoplankton physiology. It shows a broad range of N:P ratios are possible depending on the conditions under which species grow and compete. This research - part of a larger biocomplexity research project led by Professor Simon A. Levin at Princeton -- is published in the May 13 edition of the journal Nature.

"The take-home message is that this finding reinforces what some researchers have been saying lately - that N:P is not so fixed," said lead author Christopher Klausmeier, a Georgia Tech assistant professor of biology and former postdoctoral fellow at Princeton. Other authors are Elena Litchman, also of Georgia Tech, and Tanguy Daufresne and Levin of Princeton.

"This shows the range of ratios within which we could expect the ocean to change in the future," Klausmeier said. "Right now we have 16:1, but 500 years from now, if we have a different mix of growth conditions, then it might change the overall N:P needs of the phytoplankton community and the ocean."

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