  An extension of the Truscott-Brindley model (Bull. Math. Biol. {\bf 56}, 981
(1994)) is derived to account for the effect of demographic fluctuations. In
the presence of seasonal forcing, and sufficiently shallow water conditions,
the fluctuations induced by the discreteness of the zooplankton component
appear sufficient to cause switching between the bloom and no-bloom cycle
predicted at the mean-field level by the model.The destabilization persists in
the thermodynamic limit of a water basin infinitely extended in the horizontal
direction.
