1. Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands
2. Center for Mathematics and Computer Science (CWI), PO Box 94079, 1090 GB Amsterdam, The Netherlands
3. School of Ocean and Earth Science and Technology, University of Hawaii, 1000 Pope Road, Honolulu, Hawaii 96822, USA
4. *These authors contributed equally to this work

Correspondence to: Jef Huisman^1,4 Correspondence and requests for materials should be addressed to J.H. (Email: jef.huisman@science.uva.nl). The time-series data from the Hawaii Ocean Time-series program are deposited at http://hahana.soest.hawaii.edu/hot/hot-dogs.

Abstract

Deep chlorophyll maxima (DCMs) are widespread in large parts of the world's oceans^{1, 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 oceans^{8, 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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