If spring is the season of lilies of the valley, it is also the season of phytoplankton blooms in the North Atlantic Ocean. This phenomenon is one of the most important factors in the efficiency of the biological carbon pump in the ocean. Like the cherry blossom in Japan, the phytoplankton bloom period extends over several months because of its geographical extent. It starts at the beginning of March off the Azores, and you have to wait until the end of June to see it appear off Ireland. Oceanographers have known about this phenomenon for a long time, and Riley’s pioneering work in 1942 explained its primary cause. The conditions favourable to flowering are linked to the stability of the surface waters where the phytoplankton will develop. In winter, highly unstable waters induce a deep mixing, which deprives these microscopic plants of the light they need to photosynthesise. In spring, the first rays of sunshine warm the surface of the ocean, putting an end to this vertical mixing and allowing flowering to begin. Satellites, which measure the “colour of the water”, now make it possible to observe the northward spread of this vast green belt between March and June, year after year.

But on closer inspection, the start of the phytoplankton bloom is far from uniform, rather like that hypothetical wandering sprig of lily of the valley. This is what a team of researchers from the IPSL has shown for the first time. Using satellite images of water colour, they have detected differences in the day on which flowering begins, over a distance of less than 10 km. But the problem is that these differences do not occur between two fixed regions, and cannot be pinpointed geographically. Phytoplankton is transported by turbulent currents, and everything moves very quickly. Using complementary satellite data and surface temperature data, they showed that phytoplankton first began to bloom in moving fronts linked to current turbulence. These fronts are well known theoretically, particularly to meteorologists, as they are the oceanic cousins of atmospheric fronts. They are also well reproduced by ocean models. It is therefore thanks to the advent of high spatial resolution models capable of reproducing these fronts that we have begun to suspect the possible existence of such discrepancies in the start of flowering. Fronts have the specific feature of opposing vertical mixing, which theoretically allows flowering to start a little before spring. But this phenomenon had not been verified until now, and more importantly, its extent was unknown, its fluctuating nature making it difficult, if not impossible, to detect. What this spatial data has enabled us to do is to compartmentalise this turbulent environment into frontal and non-frontal zones on a daily basis, and to compare phytoplankton growth statistics in these two ephemeral environments.

These statistics revealed that phytoplankton flowering started one to two weeks earlier in the fronts. Another result was that the bloom was also two to three times more intense in the fronts, which act as fertilisers, providing nutrients as they are consumed. These mechanisms could have a cascading effect on ocean ecosystems, their biodiversity and their capacity to fix carbon, since phytoplankton is the first rung in the ocean food web.

More