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The balance of physical and biological processes governing phytoplankton growth rates and the accumulation of biomass is widely debated in the literature, notably during the winter–spring transition. Here we show, in a temperate shelf sea that variability in the depth of the actively mixing surface layer is the leading order control. During a 2-week period preceding the peak of the spring bloom we observe two distinct regimes; first, growth within the euphotic zone during the day and re-distribution of new biomass to the seasonal pycnocline at night by convective mixing; then, more rapid biomass accumulation trapped within a shallower, wind-driven actively mixing layer that was decoupled from the pycnocline below. Our observations of the bloom in the Celtic Sea, Northwest European Shelf, were made using ocean gliders and include measurements of the dissipation of turbulent kinetic energy. A 1-D phytoplankton growth model driven by our measurements of dissipation and incident irradiance replicates the observed bloom and reinforces the conclusion that physical processes that mediate light availability were key. Day-to-day variability in cloud cover and the ability of phytoplankton to acclimate to their light environment were also important factors in determining growth rates, and the timing of the biomass peak. Our results emphasize the need for accurate turbulent mixing parameterizations in coupled hydrodynamic-ecosystem models. Our findings are applicable to any region where wind-driven mixing can modify nutrient and light availability, especially across subpolar shelves in the northern hemisphere where light rather than nutrients is typically the limiting factor on phytoplankton growth. Marine phytoplankton are responsible for 50% of primary production on Earth and form the base of the global ocean food web (Field et al. 1998). Seasonal phytoplankton blooms make a significant contribution to oceanic primary production, air-sea CO2 fluxes and carbon sequestration, as well as being key to the life cycles and trophic interactions of marine organisms (Lutz et al. 2007; Koeller et al. 2009; Signorini et al. 2012). Shelf seas contribute between 10% and 30% of global marine primary production, a disproportionately large contribution relative to their size (Mackenzie et al. 2005), they are a net sink for atmospheric carbon dioxide (Laruelle et al. 2018) and are responsible for up to 50% of the organic carbon supplied to the deep ocean (Jahnke 2010). A bloom occurs when phytoplankton growth rates exceed losses such that a sustained period of growth leads to a (net) accumulation of biomass. Controls on growth rates include light availability, nutrient supply, and temperature, while losses may occur through mortality (e.g., grazing, viral lysis), advection, sinking or detrainment out of the euphotic zone. Hence, a range of biological and physical controls govern the balance between phytoplankton production and loss (see Lindemann and John 2014 for a review). Several hypotheses concerning the mechanisms for bloom initiation have emerged, with three main themes: critical depth, critical turbulence, and dilution re-coupling. While we do not implicitly test these hypotheses, we briefly review them here in order to provide the reader with an overview of the relevant physical and biological drivers behind increases in phytoplankton growth rates and the accumulation of biomass. Many of these drivers are relevant throughout the year, not just for spring bloom initiation. Sverdrup (1953) introduced the concept of a critical depth, based upon the assumption that phytoplankton growth was proportional to light, such that it decayed exponentially with *Correspondence: [email protected] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Additional Supporting Information may be found in the online version of this article.