A simple model that describes the dynamics of nutrientdriven phytoplankton blooms is presented. Apart from complicated simulation studies, very few models reported in the literature have taken this “bottom-up” approach. Yet, as discussed and justified from a theoretical standpoint, many blooms are strongly controlled by nutrients rather than by higher trophic levels. The analysis identifies an important threshold effect: a bloom will only be triggered when nutrients exceed a certain defined level. This threshold effect should be generic to both natural blooms and most simulation models. Furthermore, predictions are given as to how the peak of the bloom is determined by initial conditions. A number of counterintuitivePmax results are found. In particular, it is shown that increasing initial nutrient or phytoplankton levels can act to decrease . CorrectPmax predictions require an understanding of such factors as the timing of the bloom and the period of nutrient buildup before the bloom.