Reduced mixing generates oscillations and chaos in the deep chlorophyll maximum

Deep chlorophyll maxima (DCMs) are widespread through large parts of the oceans1-5. 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 generally argued that DCMs are stable features. Here we show, however, that reduced vertical mixing can generate oscillations and even chaos in DCMs. These fluctuations in phytoplankton biomass and species composition in the DCM are caused by differences in time scale between two processes: rapid export of sinking phytoplankton withdrawing nutrients from the euphotic zone and the slow upward flux of nutrients fuelling new DCM maxima. Climate models predict that global warming will reduce vertical mixing in the oceans6-9. Our model analysis indicates that this will generate more variability in the phytoplankton dynamics of DCMs, thereby enhancing variability in oceanic primary production and carbon export.