We deployed sensors for physical and biogeochemical measurements on one Eulerian mooring and two Lagrangian biogeochemical Argo‐floats on the Kerguelen Plateau. High temporal and vertical resolution measurements revealed an abrupt shoaling of both the mixed‐layer depth and mixing‐layer depth. The sudden stratification was concomitant with the start of significant biological activity detected by chlorophyll‐a accumulation, oxygen oversaturation and dissolved inorganic carbon drawdown. The net community production computed in the mixing‐layer during the onset period of 9 days was 119±7 mmol m‐2 d‐1. While it is generally admitted that bloom initiation is mostly driven by the onset of positive heat fluxes, our results suggest this is not a sufficient condition. Here we report that the decrease in the depth over which wind mixes the upper layer drives the initiation of the bloom. These results suggest that future atmospheric changes in Southern Ocean could impact the phenology of the blooms.

Plain Language Summary

The region of the Kerguelen Plateau is well known as a naturally iron fertilized region and it supports a productive marine ecosystem. In the present study, we investigate the evolution of the biogeochemical and physical conditions during the 2016 phytoplankton bloom event near the Kerguelen Plateau. We use a unique combination of moored instruments and profiling floats in order to follow the phytoplankton evolution at vertical, spatial and temporal scales and to understand the main physical drivers supporting such an abrupt bloom initiation that occurs only over a 9‐days period. The large phytoplankton bloom develops during a major shallowing event bringing in few days the mixed‐layer depth from its typical winter value to its typical summer value. This abrupt stratification of the water column is driven by a decline of the wind stress. These results have important implications considering that the wind regimes are predicted to intensify in the future in the Southern Ocean.

Key Points

Phytoplankton onset is detected from in‐situ high resolution multisensors data gathered by one mooring and two Biogeochemical Argo floats

The bloom starts approximately two months after the net heat flux becomes positive, during an abrupt shoaling event of the mixing‐layer

The bloom starts only after a decrease in the depth over which winds actively mix the upper ocean leading to a decrease of the turbulence