Deep chlorophyll maxima (DCMs) are widespread in large parts of the world's oceans1,2,3,4,5,6,7. These deep layers of high chlorophyll concentration reflect a compromise of phytoplankton growth exposed to two opposing resource gradients: light supplied from above and nutrients supplied from below. It is often argued that DCMs are stable features. Here we show, however, that reduced vertical mixing can generate oscillations and chaos in phytoplankton biomass and species composition of DCMs. These fluctuations are caused by a difference in the timescales of two processes: (1) rapid export of sinking plankton, withdrawing nutrients from the euphotic zone and (2) a slow upward flux of nutrients fuelling new phytoplankton production. Climate models predict that global warming will reduce vertical mixing in the oceans8,9,10,11. Our model indicates that reduced mixing will generate more variability in DCMs, thereby enhancing variability in oceanic primary production and in carbon export into the ocean interior.
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We thank R. R. Bidigare for HPLC pigment analyses, and M. Stomp, J.G. Verwer and J. Williams for discussions. J.H. was supported by the Earth and Life Sciences Foundation (ALW), which is subsidized by the Netherlands Organization for Scientific Research (NWO). N.N.P.T. was supported by the Computational Science program of NWO. D.M.K. acknowledges support from the US National Science Foundation and the Gordon and Betty Moore Foundation. B.S. acknowledges support from the Dutch BSIK/BRICKS project. Author Contributions J.H. and N.N.P.T. contributed equally to this work. J.H., N.N.P.T. and B.S. developed the model structure. N.N.P.T. and B.S. wrote the numerical code. D.M.K. provided data from the Hawaii Ocean Time-series program. J.H. wrote the paper. All authors discussed the results and commented on the manuscript.
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Huisman, J., Pham Thi, N., Karl, D. et al. Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum. Nature 439, 322–325 (2006). https://doi.org/10.1038/nature04245
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