Interactive effects of resources and food web configutation on periphytic biomass and nutrient stoichiometry

The importance of resource- or predator-based forces to the structure and functioning of ecological communities has been a conceptual dichotomy for ecologists although both theoretical and empirical evidences have highlighted the importance of the interacting effects between nutrients and predation. Here we determined the individual and combined effects of fish, nutrients and light on periphyton biomass and nutrient stoichiometry in a field mesocosms experiment in a tropical coastal lagoon. Treatments consisted  of a factorial combination of nutrient (nitrogen and phosphorus) and fish addition. Light was manipulated placing periphyton artificial substrates at two different depths.  Periphyton nutrient stoichiometry (C:N:P ratios) was profoundly affected by nutrients and light but not affected by fish presence. Nutrient addition and fish presence had contrasting effects on periphytic C:Chla ratio, although the magnitude of the effects were different depending on light availability. Fish, nutrients and light had significant individual and interactive effects on periphytic total and algal biomass. Nutrients increased total periphytic biomass, but fish presence reduced periphytic algal biomass in high light conditions. Light availability was an important factor determining both total and algal biomass accrual throughout the experiment. Our results pointed out that both top-down and bottom-up effects as well as their interactions are crucial to the periphyton dynamics. We suggest that the strength of the trophic cascade will depend on such interactions and fish foraging behavior may have a central role influencing the direction and magnitude of the trophic cascades.