In situ evaluation of the initiation of the North Atlantic phytoplankton bloom

Abstract

Two years of continuous physical and optical measurements from a profiling float in the western subarctic North Atlantic are used to analyze seasonal phytoplankton dynamics. The observed annual cycle challenges the traditional view that initiation of spring accumulations of phytoplankton in the upper water column requires a critical stratification threshold (known as the ‘Gran effect’ or the ‘Sverdrup Hypothesis’). Instead, we find that biomass accumulation begins in mid‐winter when light levels are minimal and near‐surface mixing is deepest. These observations are consistent with the recently proposed dilution–recoupling hypothesis which states that deep winter mixing in the North Atlantic is essential for bloom formation as it decouples phytoplankton growth from grazing losses, thereby allowing net biomass accumulation despite low‐light conditions.