Light regime and consumer conrol of autotrophic biomass

1. Autotrophic biomass is often regulated by resource supply and consumer presence. Most studies on top-down vs. bottom-up control of plant biomass focus on nutrients as basal resources, whereas light has received considerably less attention. 2. Effects of light and nutrients may differ because light represents a vertical resource and plant adaptations to acquire light might counteract or facilitate consumer effects. I tested the interaction between light supply and consumer presence in a meta-analysis of experiments that manipulated both these factors factorially and measured the biomass of benthic algae (periphyton). 3. Both grazer removal and light enhancement had positive effects on algal biomass. These two factors showed strong interactions, which exceeded previously observed interaction terms between nutrient supply and grazer removal. Positive light effects on algal biomass were primarily observed in the absence of grazers. Grazer effects became stronger at high light supply, which indicated that high light favours algal growth types that are easily ingested. 4. Further analyses showed that light effects increased with trophic state of the habitat and with algal biomass, indicating enhanced importance of light limitation and self-shading at high nutrient supply. Grazer effects depended on grazer biomass and varied with type of experimental manipulation. 5. This study implies that light is of similar importance as nutrients in resource control of benthic algal biomass. High light levels simultaneously enhance consumer control of algal biomass, resulting in strong resource vs. consumer interactions. Light competition and consumer presence pose a trade-off on algal growth, where growth form may distinguish between adaptations to acquire light and to avoid consumption.