The major nutrients nitrate and phosphate have one of the strongest correlations in the sea, with a slope similar to the average nitrogen (N) to phosphorus (P) content of plankton biomass (N/P = 16:1). The processes through which this global relationship emerges despite the wide range of N/P ratios at the organism level are not known. Here we use an ocean circulation model and observed nutrient distributions to show that the N/P ratio of biological nutrient removal varies across latitude in Southern Ocean surface waters, from 12:1 in the polar ocean to 20:1 in the sub-Antarctic zone. These variations are governed by regional differences in the species composition of the plankton community. The covariation of dissolved nitrate and phosphate is maintained by ocean circulation, which mixes the shallow subsurface nutrients between distinct biogeographic provinces. Climate-driven shifts in these marine biomes may alter the mean N/P ratio and the associated carbon export by Southern Ocean ecosystems.
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We thank S. Khatiwala for providing the transport matrix. This work was funded by grants from the National Science Foundation and the Gordon and Betty Moore Foundation. Additional support for T.S.W. was provided by a Pauley Fellowship from UCLA.
The authors declare no competing financial interests.
This file contains Supplementary Methods, Supplementary Tables 1-3, Supplementary Notes on sensitivity testing, Supplementary Figures 1-6 with legends and additional references. (PDF 4248 kb)
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Weber, T., Deutsch, C. Ocean nutrient ratios governed by plankton biogeography. Nature 467, 550–554 (2010). https://doi.org/10.1038/nature09403
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