Based on a model of light lindted growth, Huisman and Weissing found that in a well mixed water column with constant light supply (energy reaching the water surface), equilibrium growth and competition of phytoplankton for light can be characterised by a critical light intensity at the base of the colunm (I*out). The present study attempts to give a further insight into this model. We first analyse the dependence of the critical light intensity on four parameters: initial slope of the photosynthesis-inntensity (p-I) curve, maxinaal photosynthetic rate, the light-saturated parameter Ik and specific carbon loss rate. Increases in the first two parameters tend to reduce the critical light intensity and increases in the last two tend to increase the critical light intensity. Then we analyse the performance of the model under variable light supply with a time-scale of 1 day (24hr). Within this time-scale, the critical light intensity changes with time. However, the equilibrium growth and the outcome of competition for light can be adequately characterised by critical light extinction defined as the upper lindt of total light extinction due to both biomass and non-living matter in the water column. Under constant light supply, a critical light intensity uniquely corresponds to a critical light extinction. Therefore, critical light extinction can be utilised to predict the equilibrimn growth and the outcome of competition under both constant and variable light supply. By changing the maximal light supply at noon, seasonal succession of species composition of conmmnities is investigated. The possible effect of two typical photoresponses, photoadaptation and photoinhibition, on growth and competition are discussed.