Does winter physical intermittency support a biological preconditioning of the spring phytoplankton bloom?

Most of the recent studies on spring bloom dynamics have focused on the physical mechanisms driving its initiation. With few exceptions, the biological and ecological dimensions have been surprisingly less considered. Here, we investigate the transient winter blooms as a possible mechanism to impact the timing, magnitude and composition of the subsequent spring bloom. Using highly resolved Bio-Argo float data acquired in the North Atlantic subpolar gyre, we show that the winter mixing regime is characterized by intermittent periods of stratification or shallow mixing. Nearly 30% of the winter Bio-Argo profiles indeed show a mixed layer depth (MLD) below 100 m. This winter mixing intermittency creates transient high-nutrient and high-light environments, which promote surface phytoplankton accumulation. However, these short winter blooms seem to have only a minor impact on the overall phytoplankton stock (Chla and Bbp column integrated content). Rather, community composition might be significantly impacted, through competition for light. Complementary winter HPLC data acquired in the sub-polar gyre show that the relative abundance of the microphytoplankton (mainly diatoms) increases with the shallowing MLD, while the relative abundance of the nanophytoplankton (small flagellates) decreases. We hypothesize that transient winter blooms maintain an inoculum of opportunistic species that are adapted to high light environments similar to those involved during spring bloom initiation. The physical winter intermittency thus supports a biological preconditioning which could shape the spring bloom’s timing and magnitude.