Oceanic nitrogen reservoir regulated by plankton diversity and ocean circulation

Abstract

The average nitrogen-to-phosphorus ratio of marine phytoplankton (16N:1P) is closely matched to the nutrient content of mean ocean waters (14.3N:1P). This condition is thought to arise from biological control over the ocean’s nitrogen budget, in which removal of bioavailable nitrogen by denitrifying bacteria ensures widespread selection for diazotrophic phytoplankton that replenish this essential nutrient when it limits the growth of other species1,2,3. Here we show that in the context of a realistic ocean circulation model, and a uniform N:P ratio of plankton biomass, this feedback mechanism yields an oceanic nitrate deficit more than double its observed value. The critical missing phenomenon is diversity in the metabolic N:P requirement of phytoplankton, which has recently been shown to exhibit large-scale patterns associated with species composition4. When we model these variations, such that diazotrophs compete with high N:P communities in subtropical regions, the ocean nitrogen inventory rises and may even exceed the average N:P ratio of plankton. The latter condition, previously considered impossible, is prevented in the modern ocean by shallow circulations that communicate stoichiometric signals from remote biomes dominated by diatoms with low N:P ratios. Large-scale patterns of plankton diversity and the circulation pathways connecting them are thus key factors determining the availability of fixed nitrogen in the ocean.

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Figure 1: Model scenarios and solutions.
Figure 2: Response of ΣN/ΣP to the degree of stoichiometric diversity, RO,ST.
Figure 3: Role of ocean circulation illustrated in a three-box model.
Figure 4: Influence of individual surface regions on ΣN/ΣP.

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Acknowledgements

We thank T. DeVries for providing the ocean circulation model. This work was funded by a NASA Earth Systems Science Fellowship (T.W.) and a grant from the Gordon and Betty Moore Foundation (C.D.).

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T.W. developed the model and performed simulations and analyses. Both authors designed the study and wrote the paper.

Correspondence to Thomas Weber.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Notes that discuss sensitivity testing and additional considerations to support the conclusions, Supplementary Figures 1-10, which illustrate the methods and present additional sensitivity testing results and Supplementary Tables 1-2. Supplementary Table 1 gives parameter values used in the ecosystem/biogeochemistry model and Supplementary Table 2 presents the results of the dissolved organic matter sensitivity tests. (PDF 2415 kb)

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Weber, T., Deutsch, C. Oceanic nitrogen reservoir regulated by plankton diversity and ocean circulation. Nature 489, 419–422 (2012). https://doi.org/10.1038/nature11357

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