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Unexpected winter phytoplankton blooms in the North Atlantic subpolar gyre

Abstract

In mid- and high-latitude oceans, winter surface cooling and strong winds drive turbulent mixing that carries phytoplankton to depths of several hundred metres, well below the sunlit layer. This downward mixing, in combination with low solar radiation, drastically limits phytoplankton growth during the winter, especially that of the diatoms and other species that are involved in seeding the spring bloom. Here we present observational evidence for widespread winter phytoplankton blooms in a large part of the North Atlantic subpolar gyre from autonomous profiling floats equipped with biogeochemical sensors. These blooms were triggered by intermittent restratification of the mixed layer when mixed-layer eddies led to a horizontal transport of lighter water over denser layers. Combining a bio-optical index with complementary chemotaxonomic and modelling approaches, we show that these restratification events increase phytoplankton residence time in the sunlight zone, resulting in greater light interception and the emergence of winter blooms. Restratification also caused a phytoplankton community shift from pico- and nanophytoplankton to phototrophic diatoms. We conclude that transient winter blooms can maintain active diatom populations throughout the winter months, directly seeding the spring bloom and potentially making a significant contribution to over-winter carbon export.

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Figure 1: Winter mixing intermittency in the North Atlantic subpolar gyre.
Figure 2: Winter phytoplankton blooms in the North Atlantic subpolar gyre.
Figure 3: The light environment impacts the phytoplankton community structure.

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Acknowledgements

We thank N. Briggs, M. J. Perry, E. D’Asaro, B. Gentili, E. Boss and F. Benedetti for fruitful discussions, C. Schmechtig for BGC-Argo float data management and J. Ras for proofreading the manuscript. We also thank S. Wright for sharing the CHEMTAX software v.1.95, and C. de Boyer Montégut for providing the MLD climatology. This work represents a contribution to the remOcean project (REMotely sensed biogeochemical cycles in the OCEAN, GA 246777) funded by the European Research Council, the ATLANTOS EU project (grant agreement 2014-633211) funded by H2020 program and the Italian Flagship Program RITMARE.

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D.I. and L.L. designed the study. L.L., M.A., K.F.S., H.C., A.P., M.R.D’A. and D.I. conducted the data analysis. L.L. and M.A. wrote the manuscript. All authors commented on the manuscript.

Corresponding author

Correspondence to L. Lacour.

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The authors declare no competing financial interests.

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Lacour, L., Ardyna, M., Stec, K. et al. Unexpected winter phytoplankton blooms in the North Atlantic subpolar gyre. Nature Geosci 10, 836–839 (2017). https://doi.org/10.1038/ngeo3035

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