Revisiting Sverdrup's critical depth hypothesis

Published more than 60 years ago in this Journal, the article in which Sverdrup proposed the concept of critical depth to explain the initiation of the spring bloom in the North Atlantic has accrued an exceptionally large number of citations and continues to be cited more than 50 times per year. The framework provided by Sverdrup has now been applied, adapted, and tested across many aquatic systems worldwide. Datasets have been collected; models have been built on the framework: these studies have generated new insights into phytoplankton dynamics and interesting debates on the relative importance of the various factors responsible for phytoplankton blooms. This article theme set presents some of the most recent efforts to discuss and test Sverdrup's critical depth hypothesis using a diverse set of approaches, ranging from controlled experiments to field observations as well as numerical and analytical models. The set of papers celebrates an elegant and powerful hypothesis that has had long-lasting influence. It is to be expected that it will also stimulate future research, adding even more to our understanding of one of the most fundamental processes in biological oceanography.