Ocean nutrient ratios governed by plankton biogeography

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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.

At a glance


  1. Observed N* distribution in the Southern Ocean.
    Figure 1: Observed N* distribution in the Southern Ocean.

    a, Zonal mean section of N* ([NO3]16[PO43−]), with potential density anomaly contours (ρθ1,000kgm−3; thin lines) and schematic large-scale meridional overturning circulation patterns (arrows). Upwelling circumpolar deep water (CDW) is driven across the Antarctic polar front (APF) and the sub-Antarctic front (SAF) by Ekman transport, and subsides in the polar frontal zone (PFZ) to form Antarctic intermediate water (AAIW) and sub-Antarctic mode water (SAMW). b, Zonal mean vertical difference in N* between the surface and the thermocline (ΔN* = N*0–75mN*200–800m), globally and in individual sectors of the Southern Ocean.

  2. Redfield N* prediction.
    Figure 2: Redfield N* prediction.

    Southern Ocean surface N* gradient (dashed line) and ΔN* (solid line) predicted in model 1, assuming nutrient uptake and remineralization at the Redfield ratio. The northward increase in surface N* and low ΔN* values are inconsistent with observations.

  3. Diagnosed nutrient export ratios.
    Figure 3: Diagnosed nutrient export ratios.

    a, Spatial pattern of N/Pexp, derived in model 2 by diagnosing the export of NO3 and PO43− independently. b, Zonal mean export ratio (solid line) and contribution of diatoms to N export (dashed line) diagnosed in model 2. The grey envelope is an estimate of errors associated with the choice of model compensation depth (50–100m), the biological damping timescale (14d to 1yr) and the parameterization of dissolved organic matter (see Supplementary Information).

  4. Circulation averaging of remineralized nutrients.
    Figure 4: Circulation averaging of remineralized nutrients.

    Depth profiles of [NO3]rem/[PO43−]rem in the Antarctic (solid line), Polar frontal (dotted line) and sub-Antarctic (dashed line) zones predicted in model 3. The reduction in variability with depth results from the physical mixing of remineralized nutrients.


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Author information


  1. Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, California 90095, USA

    • Thomas S. Weber &
    • Curtis Deutsch


T.S.W. conducted the simulations and analysed the results. C.D. designed the study. Both authors wrote the paper.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (4.1M)

    This file contains Supplementary Methods, Supplementary Tables 1-3, Supplementary Notes on sensitivity testing, Supplementary Figures 1-6 with legends and additional references.

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