Predictors of light-limited growth and competition of phytoplankton in a well-mixed water column

Based on a model of light limited growth, Huisman and Weissing found that i n a well mixed water column with constant light supply (energy reaching the water surface), equilibrium growth and competition of phytoplankton for li ght can be characterised by a critical light intensity at the base of the c olumn (I-out*). The present study attempts to give a further insight into t his model. We first analyse the dependence of the critical light intensity on four parameters: initial slope of the photosynthesis-intensity (p-I) cur ve, maximal photosynthetic rate, the light-saturated parameter Ik and speci fic 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 (24 hr). Within thi s time-scale, the critical light intensity changes with time. However, the equilibrium growth and the outcome of competition for light can be adequate ly characterised by critical light extinction defined as the upper limit of total light extinction due to both biomass and non-living matter in the wa ter column. Under constant light supply, a critical light intensity uniquel y corresponds to a critical light extinction. Therefore, critical light ext inction can be utilised to predict the equilibrium growth and the outcome o f competition under both constant and variable light supply. By changing th e maximal light supply at noon, seasonal succession of species composition of communities is investigated. The possible effect of two typical photores ponses, photoadaptation and photoinhibition, on growth and competition are discussed. (C) 1999 Academic Press.