A simple system of parametrically forced ordinary differential equations is used to model autumn phytoplankton blooms in temperate oceans by a mechanism involving deepening of the upper mixed layer. Blooms are triggered provided the increase in nutrients in the mixed layer is rapid within the first few days of deepening and provided light-limited phytoplankton growth rate is relatively high. Blooms exist as transient trajectories between quasi-equilibrium states, rather than as bifurcations of

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... teady states; therefore very gradual deepening cannot trigger blooms. Very rapid deepening also prevents blooms due to the deleterious effect on phytoplankton growth rate. The mechanisms identified by this simple model are vindicated by considering alternative grazing and deepening regimes and by comparison with a more ecologically complex model (Fasham, 1993, in The Global Carbon Cycle, Springer-Verlag). Modelled estimates of primary productivity from both the simple model and the complex model parameterized for Ocean Weather Station 'India' are around 0.5 g C m À2 day À1 during the autumn bloom, therefore comprising a significant component of annual production in temperate areas.