Rapid phytoplankton response to wind forcing influences productivity in upwelling bays

Bays are often ecological hotspots within highly-productive eastern boundary upwelling systems. Though the physics of such bays are well understood, there is no consensus about the factors underlying their high productivity. Three weeks of high-temporal-resolution observations in two long, narrow bays (Rias Baixas, NW-Iberia), showed that during an upwelling pulse, deep, nutrient-rich isopycnals rose into the euphotic zone inside the rias in a few hours. The response of the isopycnals to changes in wind forcing is approximately three times faster inside the rias than the Ekman spin-up time, triggering rapid nutrient uptake and subsequent formation of a subsurface chlorophyll and production maximum. The tight coupling and rapid response of phytoplankton growth to wind forcing could explain the higher productivity of the rias, and also be at play in other upwelling bays with similar morphologies and orientations. Resolving short-term variability of physical-biological coupling is crucial to discern the future evolution of upwelling bays.