The compensation irradiance for phytoplankton is calculated from the time course of fluorescence-derived chlorophyll-a measured on moored sensors in the North Atlantic in spring. Following the initiation of the spring bloom, the time at which chlorophyll-a begins to decrease at a particular depth in a stratified water column is used to compute the compensation irradiance from the critical depth criterion. Over the depth range of 50--110 m, the compensation irradiance is estimated to be in the range from 0.1--0.3 mol photons m-2 d-1.