20 January 2021 : Crossing a Trichodesmium bloom!

Trichodesmium is a fascinating phytoplankton for oceanographers. Discovered in the mid-1960s, it has the particularity of capturing atmospheric nitrogen, a process called nitrogen fixation. As the atmosphere is an almost unlimited reservoir of nitrogen (80%), this capacity is a real advantage in subtropical waters, which are very poor in alternative nitrogen sources. A few days ago, while passing off the southern tip of Madagascar, we were lucky enough to come across a Trichodesmium bloom. The biologists went into a frenzy! Hugo with the bucket, Marion with the net and Christophe with the photos!

Indeed, when the weather is calm, the Trichodesmium cells rise to the surface and form accumulations of several hundred metres that drift with the currents. Although these accumulations are made up of microscopic cells, they can be seen from space. There is nothing more tempting than to fish for them from the shore!

By throwing a plankton net into the water we were able to recover and observe these Trichodesmium under the microscope. Continuous measurements of nitrogen fixation fluxes are carried out on board, which will allow us to better understand the importance of this process in the South Indian Ocean and the Southern Ocean, which are largely under-sampled.

Accumulation de Trichodesmium en surface. © Christophe Cassou
Throwing a plankton net. © Christophe Cassou
Trichodesmium seen under the microscope. © Hugo Berthelot

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