In the oceans, ubiquitous microscopic phototrophs (phytoplankton) account for approximately half the production of organic matter on Earth. Analyses of satellite-derived phytoplankton concentration (available since 1979) have suggested decadal-scale fluctuations linked to climate forcing, but the length of this record is insufficient to resolve longer-term trends. Here we combine available ocean transparency measurements and in situ chlorophyll observations to estimate the time dependence of phytoplankton biomass at local, regional and global scales since 1899. We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ∼1% of the global median per year. Our analyses further reveal interannual to decadal phytoplankton fluctuations superimposed on long-term trends. These fluctuations are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures. We conclude that global phytoplankton concentration has declined over the past century; this decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries.
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We are grateful to all data providers, to J. Mills-Flemming, W. Blanchard, M. Dowd, C. Field and C. Minto for statistical advice, to T. Boyer, J. Smart, D. Ricard and D. Tittensor for help with data extraction, and to J. Mills-Flemming, W. Blanchard, W. Li, H. Lotze, C. Muir and M. Dowd for critical review. Funding was provided by the Natural Sciences and Engineering Research Council of Canada, the US Office of Naval Research, the Canada Foundation for Climate and Atmospheric Sciences, the National Aeronautics and Space Administration, the Sloan Foundation (Census of Marine Life FMAP Program), and the Lenfest Ocean Program.
The authors declare no competing financial interests.
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Boyce, D., Lewis, M. & Worm, B. Global phytoplankton decline over the past century. Nature 466, 591–596 (2010). https://doi.org/10.1038/nature09268
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