The onset of the spring phytoplankton bloom in the coastal North Sea supports the Disturbance Recovery Hypothesis

The spring phytoplankton bloom is a key event in temperate and polar seas, yet the mechanisms that trigger it remain under debate. Some hypotheses claim that the spring bloom onset occurs when light is no longer limiting, allowing phytoplankton division rates to surpass a critical threshold. In contrast, the Disturbance Recovery Hypothesis (DRH) proposes that the onset responds to an imbalance between phytoplankton growth and loss processes, allowing phytoplankton biomass to start accumulating, and this can occur even when light is still limiting. Although several studies have shown that the DRH can explain the spring bloom onset in oceanic waters, it is less certain whether and how it also applies to coastal areas. To address this question at a coastal location in the Scottish North Sea, we combined 21 years (1997-2017) of weekly in situ chlorophyll and environmental data with meteorological information. Additionally, we also analyzed phytoplankton cell counts estimated using microscopy (2000-2017) and flow cytometry (2015-2017). The onset of phytoplankton biomass accumulation occurred around the same date each year, 16 +/- 11 d (mean +/- SD) after the winter solstice, when light limitation for growth was strongest. Also, negative and positive biomass accumulation rates (r) occurred respectively before and after the winter solstice at similar light levels. The seasonal change from negative to positive r was mainly driven by the rate of change in light availability rather than light itself. Our results support the validity of the DRH for the studied coastal region and suggest its applicability to other coastal areas.