Diatom flotation at the onset of the spring phytoplankton bloom: an in situ experiment

We used a new type of sediment trap to conduct an in situ test on the buoyancy properties of diatoms before and during the growth phase of the spring phytoplankton bloom (SPB). Diatoms shifted from a sinking pattern before the bloom, while their populations were not growing, to a neutrally buoyant pattern during bloom development, when calm conditions prevailed, light was abundant and phytoplankton were actively growing. This shift was mainly due to the upward motion of centric diatoms during the growth phase. Our field experiment confirms laboratory experiments and field observations showing that diatoms, the paradigm of sinking phytoplankton, approach neutral buoyancy when conditions are adequate for growth, a fact that is not taken into account in most SPB and phytoplankton dynamics models.