Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum

Green-belt development In many parts of the oceans a layer known as a ‘deep chlorophyll maximum’ develops about 50 to 100 metres below the surface as various opposing forces achieve a balance. Phytoplankton sinking from surface waters and taking nutrients with them meet an upward flux of nutrients fuelling new growth at a depth where there is still sufficient light. These chlorophyll-rich layers play an important role in ocean productivity. A new study shows that reduced vertical mixing can induce oscillations and chaos in phytoplankton biomass and species composition in deep chlorophyll maxima via a mismatch in the time scales of the processes that create them. This runs counter to the widely held belief that deep chlorophyll maxima are stable features that track seasonal changes in light and nutrient conditions. Climate change scenarios predict that global warming will suppress vertical mixing in the oceans: this could destabilize the phytoplankton dynamics in the deep chlorophyll maximum, with implications for oceanic primary production, phytoplankton species composition, and carbon export. 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.