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Ongoing and future changes in wind and temperature are predicted to alter upper ocean vertical mixing across the Southern Ocean. How these changes will affect primary production (PP) remains unclear as mixing influences the two controlling factors: light and iron. We used a large ensemble of 1-D-biogeochemical model simulations to explore the impacts of changes in mixed-layer depths on PP in the Southern Ocean. In summer, shoaling mixed-layer depth always reduced depth-integrated PP, despite increasing production rates. In winter, shoaling mixed layers had a two-staged impact: for moderate shoaling PP increased as light conditions improved, but more pronounced shoaling decreased iron supply, which reduced PP. The fraction of PP exported below 100 m also presented a nonmonotonic behavior. This suggests a potential future shift from a situation where reduced winter mixing increases PP and export, to a situation where PP and export may collapse if the ML shoals above a threshold depth.

Plain Language Summary In the Southern Ocean, atmospheric warming associated to climate change is altering the depth at which surface waters are stirred, the so-called mixed-layer depth. A change in the mixed-layer depth impacts the phytoplankton cells that inhabit it by altering their two main limiting factors: iron and light. However, the sign and magnitude of this impact are still not clear. In this work we used mathematical simulations to explain how changes in the seasonal mixed-layer depth modify the supply of iron and the amount of light, and how these changes impact phytoplankton activity. Our results show that mixed-layer depth changes in summer and in winter have different impacts. Reducing summer mixed-layer depth did not change the iron supply, but it reduced the volume of water where phytoplankton thrived. In winter, shallower mixed-layer depth altered iron and light but in opposed ways. At first, phytoplankton increased its activity as more light became available. However, a continued shallowing of the mixed-layer depth eventually reduced the iron supply and the phytoplankton activity. Our study proposes a new interpretation on how ongoing changes in the Southern Ocean impact phytoplankton activity and alerts of the presence of threshold depths for the winter mixed layer above which phytoplankton may struggle to survive.