With the aim of describing annually recurring phytoplankton blooms we discuss an extension of the phytoplankton-zooplankton model introduced by [Truscott, J.E., Brindley, J., 1994. Ocean plankton populations as excitable media. Bull. Math. Biol. 56, 981-998]. The extension is a seasonal forcing of the phytoplankton growth rate driven by an oscillating temperature via a Q10 law. We observe bistable long-term behaviour of the ecological system, i.e. a bloom and non-bloom mode, the importance of timing, and noise-induced switchings between the bloom and non-bloom mode. We link the model results to existing Helgoland Roads long-term data series by analysing the latter using the novel method of bloom-triggered averaging, a tool borrowed from signal analysis of neurophysiological recordings. We find that on an average blooms are correlated with rapid upward temperature fluctuations and speculate on their possible role as trigger mechanisms. © 2006 Elsevier B.V. All rights reserved.