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Small-scale turbulent mixing plays a pivotal role in shaping the circulation and a broad range of physical and biogeochemical ocean processes. Despite advances in understanding geophysical processes responsible for this mixing, the nature and importance of biomixing -turbulent mixing caused by marine biota -remains controversial. A major source of uncertainty pertains to the efficiency of biomixing -the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically -, which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here, we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in a coastal upwelling area. We show that turbulent dissipation is elevated 10 to 100-fold (reaching 10 −6 -10 −5 W kg −1 ) every night during due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently to dissipation, and occurs with an efficiency comparable

Bays within eastern boundary upwelling systems (EBUS) are ecological hot-spots featuring a diverse range of spatio-temporal dynamics. At the EBUSs' poleward limit, upwelling occurs in short-lived (<1 week) pulses modulated by synoptic wind variability. The circulations in long, narrow bays can respond to these fluctuations within few hours. The short-term biological response to these pulses was investigated in two of these bays (Rias Baixas, NW-Iberia) with a two-week quasi-synoptic spatiotemporal survey in the summer 2018. A four-day-long upwelling pulse caused deep, nutrient-rich isopycnals to rise into the euphotic zone inside the bays, triggering a rapid (˜1.7 days) nutrient uptake and formation of a subsurface chlorophyll maximum (˜3.8 days). The phytoplankton biomass was transported rapidly toward deep, offshore waters when the winds weakened. These results suggest that high productivity in narrow bays is controlled by the transient exposure of deep, nutrient-rich waters to light during upwelling pulses.

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Coastal upwelling bays are ecological hotspots with exceptionally high productivity and fish catch. One unresolved question in these regions is why bays are more productive than shelf upwelled waters. Our study in two bays within the upwelling region off NW-Iberia, shows that the rapid response of phytoplankton growth to wind pulses inside the bays could explain their overall elevated biological productivity. The described mechanism underpinning the fast response may have implications for shellfish farming and the early detection of harmful algal blooms. Our results reveal that predictions of the evolution of coastal upwelling ecosystems under global change scenarios should consider not only trends in mean wind intensity, but also changes in the patterns of short-term wind variability.