LIGHT-LIMITED GROWTH AND COMPETITION FOR LIGHT IN WELL-MIXED AQUATIC ENVIRONMENTS - AN ELEMENTARY MODEL

Light is never distributed homogeneously since it forms a gradient ove r biomass. As a consequence, the common theories on nutrient competiti on are not applicable to competition for light. In this paper, we inve stigate a model for light-limited growth and competition among phytopl ankton species in a mixed water column. The model is based on standard assumptions such as Lambert-Beer's law of light absorption, a Monod e quation for carbon uptake, and constant specific carbon losses. By int roducing the concept of quantum return, we show that the dynamics of g rowth and competition can be quantified not only in terms of depth but also directly in terms of light availability. We argue that the cruci al measure for phytoplankton growth is not a ''critical depth'' but a ''critical light intensity,'' I-out. For each species, I-out* corresp onds to the equilibrium light intensity at the bottom of a water colum n when the species is grown in monoculture. I-out plays a role simila r to the ''critical nutrient concentration'' R used in models of nutr ient-limited growth. For a constant light supply, the species with the lowest I-out will competitively exclude all other species. There are , however, some important differences between R and I-out*. Whereas R reflects both the local and the total balance between nutrient uptak e and nutrient losses, I-out only reflects the total carbon balance. Moreover, I-out decreases with increasing light supply, whereas R* is independent of the nutrient supply. As a consequence, (1) the outcome of competition for light may depend on the light supply, (2) the comp ensation point is not a good predictor for the outcome of competition, (3) the resource ratio hypothesis does not apply when species compete for both nutrients and light. The outcome of competition for nutrient s and light may depend on the nutrient and light supply, on the mixing depth, and on the background turbidity due to inanimate substances.