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Climate-driven trends in contemporary ocean productivity

Abstract

Contributing roughly half of the biosphere’s net primary production (NPP)1,2, photosynthesis by oceanic phytoplankton is a vital link in the cycling of carbon between living and inorganic stocks. Each day, more than a hundred million tons of carbon in the form of CO2 are fixed into organic material by these ubiquitous, microscopic plants of the upper ocean, and each day a similar amount of organic carbon is transferred into marine ecosystems by sinking and grazing. The distribution of phytoplankton biomass and NPP is defined by the availability of light and nutrients (nitrogen, phosphate, iron). These growth-limiting factors are in turn regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, atmospheric dust deposition, and the solar cycle. Satellite measurements of ocean colour provide a means of quantifying ocean productivity on a global scale and linking its variability to environmental factors. Here we describe global ocean NPP changes detected from space over the past decade. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C yr-1), followed by a prolonged decrease averaging 190 Tg C yr-1. These trends are driven by changes occurring in the expansive stratified low-latitude oceans and are tightly coupled to coincident climate variability. This link between the physical environment and ocean biology functions through changes in upper-ocean temperature and stratification, which influence the availability of nutrients for phytoplankton growth. The observed reductions in ocean productivity during the recent post-1999 warming period provide insight on how future climate change can alter marine food webs.

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Acknowledgements

We thank the Ocean Biology Processing Group at the Goddard Space Flight Center, Greenbelt, Maryland for their diligence in providing the highest quality ocean colour products possible and the NASA Ocean Biology and Biogeochemistry Program for support.

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Correspondence to Michael J. Behrenfeld.

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Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

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Supplementary Notes

This file contains Supplementary Figures 1–4, Supplementary Methods and additional references. (PDF 1259 kb)

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Further reading

Figure 1: Distribution and trends in global ocean phytoplankton productivity (NPP) and chlorophyll standing stocks.
Figure 2: Ocean productivity is closely coupled to climate variability.
Figure 3: Climate controls on ocean productivity cause NPP to vary inversely with changes in SST.

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