Annual cycles and spring blooms in phytoplankton: don't abandon Sverdrup completely

The critical-depth model for the onset of the spring phytoplankton bloom in the North Atlantic has recently been called into question by several researchers. The critical-depth model considers that the spring bloom starts when the mixed layer shoals to become shallower than a critical depth. Satellite and in situ measurements of chlorophyll are used here to show that the critical-depth model is indeed flawed. It is shown that the critical-depth model does not apply in the spring because the basic assumption of an upper layer that is well-mixed in plankton is not met. Instead, the spring bloom forms in shallow near-surface layers that deepen with the onset of thermal stratification. A stratification-onset model for the annual cycle in plankton is proposed that adheres to the conventional idea that the spring bloom represents a change from a deep-mixed regime to a shallow light-driven regime, but where the upper layers are not well mixed in plankton in spring and so the critical-depth model does not apply. Ironically, perhaps, the critical-depth model applies in the autumn and winter when plankton are well-mixed to the seasonal thermocline, so that the critical-depth model can be used to determine whether net winter production is positive or negative.