MEPS 443:39-50 (2011)  -  doi:10.3354/meps09453

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

Stephen M. Chiswell*

National Institute of Water & Atmospheric Research (NIWA) Ltd, Private Bag 14-901, Kilbirnie, Wellington, New Zealand

ABSTRACT: 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.

KEY WORDS: Phytoplankton · Spring bloom · Annual cycle · Remote sensing · Critical-depth model

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Cite this article as: Chiswell SM (2011) Annual cycles and spring blooms in phytoplankton: don’t abandon Sverdrup completely. Mar Ecol Prog Ser 443:39-50

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