Research brief: How phytoplankton will be affected by climate change

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 ARCCSS researchers 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.

The 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.

The study proposes a new interpretation on how ongoing changes in the Southern Ocean impact phytoplankton activity and highlights the presence of threshold depths for the winter mixed-layer above which phytoplankton may struggle to survive.


  • Paper: Llort, J., Lévy, M., Sallée, J. B., & Tagliabue, A. ( 2019). Non‐monotonic response of primary production and export to changes in mixed‐layer depth in the Southern Ocean. Geophysical Research Letters, 46.
  • Picture (top left): Plankton by Jesse Claggett Flickr
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