The annual cycle of the North Atlantic phytoplankton

Phytoplankton populations in the North Atlantic rise at the end of winter and decay in the fall. This yearly modulation of the phytoplankton standing stock exerts a strong control on the entire marine food web and the sequestration of organic carbon in the deep ocean. Yet, it remains poorly understood what are the environmental and ecological factors that control these periodic fluctuations. In this presentation, we will use a combination of bio–Argo floats observations collected in the North Atlantic and numerical simulations to study in detail the full yearly cycle of phytoplankton. Our analysis suggest that the phytoplankton standing stock is controlled by the rate of grazing during most of the year, except at the end of winter and in late fall when atmospheric forcing suddenly promotes strong and weak division rates respectively. At these times the phytoplankton population rise and fall precipitously, before the grazing rate catches up and a new equilibrium is established.