The North Atlantic Spring Bloom and the Critical Depth Hypothesis: A reassessment using Bio-ARGO profiles and long time series of satellite data

The Critical Depth Hypothesis (CDH), one of the fundamental tenets of plankton ecology, posits that the spring bloom in the North Atlantic Ocean is initiated when the mixed layer depth (MLD) becomes shallower than the critical depth (Z_cr), a condition that ensures that vertically integrated net rates of phytoplankton productivity exceed community loss rates, and allows phytoplankton to grow and form large blooms. Recent studies have challenged the concept of the CDH in the North Atlantic and other locations which experience spring blooms, and instead, have proposed alternate hypotheses and physical and biological mechanisms that deviate from those on which the CDH was based upon. In this study, we have used a new optical model for estimating the diffuse attenuation coefficients of visible light in the water column, and spatially and temporally enhanced bio-optical datasets from Suomi-VIIRS and Bio-ARGO floats to show that current misgivings about the CDH stem from lack of both adequate and accurate measurements of compensation irradiance (I_c), i.e. the light intensity in the water column where photosynthetic and ecosystem community loss processes are in balance, and a key input for calculating Z_cr. Variations in Z_cr, MLD and changes in chlorophyll a concentration, over seasonal, annual and interannual time scales (2002-2018), across different latitudinal bands in the North Atlantic, lend strong support for the CDH.