How do sinking phytoplankton species manage to persist?

Phytoplankton requires light for photosynthesis, but most phytoplankton species are heavier than water and sink. How can these sinking species persist? Here we show, by means of an advection-diffusion-reaction equation of light-limited phytoplankton, that the answer lies in the turbulent motion of water that re-disperses phytoplankton over the vertical water column. More specifically, we show that there is a turbulence window sustaining sinking phytoplankton species. If turbulent diffusion is too high, phytoplankton is mixed to great depths and the depth-averaged light conditions are too low to allow net positive population growth. Conversely, if turbulent diffusion is too low, sinking phytoplankton populations end up at the ocean floor and succumb in the dark. At intermediate levels of turbulent diffusion, however, the model predicts that phytoplankton populations can outgrow both mixing rates and sinking rates. In this way, the reproducing population as a whole can maintain a position in the well-lit zone near the top of the water column, even if all individuals within the population have a tendency to sink. This theory unites earlier classic results by Sverdrup and Riley as well as our own recent findings, and provides a novel conceptual framework for the understanding of phytoplankton dynamics under influence of mixing processes.