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Dissolved organic phosphorus concentrations in the surface ocean controlled by both phosphate and iron stress

Abstract

Dissolved organic phosphorus (DOP) has a dual role in the surface ocean as both a product of primary production and as an organic nutrient that fuels primary production and nitrogen fixation, especially in oligotrophic gyres. Although poorly constrained, the geographic distribution and environmental controls of surface ocean DOP concentrations influence the distributions and rates of primary production and nitrogen fixation in the global ocean. Here we pair DOP concentration measurements with a metric of phosphate stress, satellite-based chlorophyll a concentrations and a satellite-based iron stress proxy to explore their relationship with upper 50 m DOP stocks. Our results suggest that phosphate and iron stress work together to control surface ocean DOP concentrations at basin scales. Specifically, upper 50 m DOP stocks decrease with increasing phosphate stress, while alleviated iron stress leads to either surface DOP accumulation or loss depending on phosphate availability. Our work extends the relationship between DOP concentrations and phosphate availability to the global ocean, suggests a linkage between marine phosphorus cycling and iron availability and establishes a predictive framework for DOP distributions and their use as an organic nutrient source that supports global ocean fertility.

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Fig. 1: Distribution of DOP concentrations and the relationship between the upper 50 m DOP stocks and P* over the global ocean.
Fig. 2: Relationships between upper 50 m DOP stocks, surface chlorophyll a concentration and NPQ-corrected φsat.
Fig. 3: Conceptual model of factors influencing surface ocean DOP distributions with representative ocean regions.

Data availability

Original DOP data used in this study can be found and freely accessed in the DOPv2021 database archived on the BCO DMO website (https://www.bco-dmo.org/dataset/855139) or in the Woods Hole Open Access Server (https://doi.org/10.26008/1912/bco-dmo.855139.2)61. The Level-3 satellite product can be downloaded from the NASA OceanColor website (https://oceancolor.gsfc.nasa.gov/l3/).

Code availability

Code and data used to reproduce Figs. 1 and 2 are archived at https://github.com/zliangocean/DOP.

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Acknowledgements

The work was supported by NSF OCE-1829797 (A.N.K.) and NSF OCE-1829916 (R.T.L.). We also gratefully acknowledge T.K. Westberry who provided useful feedback on NPQ-corrected φsat.

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Contributions

Z.L. performed the analysis. Z.L. and A.N.K. designed the study. Z.L., A.N.K. and R.T.L. wrote the paper. A.N.K. and R.T.L. led the project.

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Correspondence to Zhou Liang.

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Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: James Super and Kyle Frischkorn, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Table 1 R2 of observed DOP concentration vs. predicted surface ocean DOP concentration based on different machine learning models (SVM, boosted tree and Gaussian process regression) with different predictors

Extended Data Fig. 1 Boxplots of mean DOP concentrations in the surface ocean (0–50 m) in different ocean basins.

Asterisks denote confidence levels when testing for unique mean concentrations between basins with ****: P< 0.0001, ***: P< 0.001, *: P< 0.05, using Dunn test (pairwise Kruskal–Wallis test) with Bonferroni correction. Black dots above the Eastern North Pacific and Gulf of Mexico are outliers. Center line is median and box limits are upper and lower quartiles. Whiskers show 1.5x interquartile range.

Extended Data Fig. 2 Correlation between observed upper 50 m DOP stock (mmol m2) and mean upper 50 m P* (µM) computed from the same samples.

Black solid line is the best fit line using a Type II linear regression model and dashed blue lines are the 95% confidence level. Three stations from the BIOSOPE cruise had only phosphate but no nitrate concentration measurements and they are not included in this figure.

Extended Data Fig. 3 Correlation between observed upper 50 m DOP stock (mmol m−2) and climatological mean P* (µM) between 100 m and 250 m computed from the World Ocean Atlas 2013 (ref. 24).

Black solid line is the best fit line using a Type II linear regression model and dashed blue lines are the 95% confidence level. There are 12 stations containing DOP data with a bottom depth <100 m which have not been included in this figure.

Extended Data Fig. 4 Annual mean surface geostrophic currents (0.25° × 0.25°) and identified surface current convergence zones (SCZ).

Annual mean geostrophic currents (0.25° × 0.25°) are obtained from the Copernicus Marine Environmental Monitoring Service (marine.copernicus.eu). Three surface current convergence zones (SCZ) are identified in the North Pacific, South Pacific and South Atlantic by red circle.

Extended Data Fig. 5

Predicted surface ocean DOP concentrations (µM) based on the linear relationship between DOP concentrations and P* (Fig. 1; see main text).

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Supplementary Figs. 1 and 2.

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Liang, Z., Letscher, R.T. & Knapp, A.N. Dissolved organic phosphorus concentrations in the surface ocean controlled by both phosphate and iron stress. Nat. Geosci. 15, 651–657 (2022). https://doi.org/10.1038/s41561-022-00988-1

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