Growth and Competition in a Light Gradient

Abstract

In this paper a general model for growth and competition in a light gradient is developed. The model is based on a few qualitative assumptions: (i) biomass is continuously distributed over depth; (ii) the light gradient is one-dimensional and uni-directional; (iii) photosynthesis is positively related to the local light intensity; and (iv) biomass growth is governed by the carbon balance. By introducing the concept of "quantum return", it is shown that growth can be quantified directly in terms of the light gradient. In monoculture, growth leads to a globally stable equilibrium, at which the light intensity at the bottom of the light gradient is reduced to a "critical light intensity" I*out, I*out is not affected by the background turbidity but negatively related to the light supply. When all species are similarly distributed over the light gradient, the outcome of competition can be inferred from this monoculture characteristic: the species with lowest I*out will competitively exclude all other species. In contrast, spatial differentiation of the competitors may lead to a completely different situation: several species may co-exist, and the species with lowest I*out may be competitively displaced by species with a better position in the light gradient.