Abstract During the spring of 1996, continuously monitoring fluorometers were deployed in the Humber plume during the spring phytoplankton bloom. The most striking feature of these records is the sharp increase and decrease in the chlorophyll-a concentration marking the onset and end of the spring bloom. A coupled physical–biogeochemical water column model has been used to successfully hindcast the observed spring bloom. Sensitivity analysis demonstrates that the onset of primary production occurs when the euphotic layer depth is >15% of the depth of the water column. The magnitude and duration of the simulated bloom is controlled by silicate limitation for diatoms and by grazing pressure for autotrophic flagellates. The implications of these results for the forecasting of phytoplankton blooms are discussed.