Seasonal shifts in the importance of bottom-up and top-down factors on stream periphyton community structure

We examined the importance of temporal variability in top-down and bottom-up effects on the accumulation of stream periphyton, which are complex associations of autotrophic and heterotrophic microorganisms. Periphyton contributes to primary production and nutrient cycling and serves as a food resource for herbivores (grazers). Periphyton growth is often limited by the availability of nitrogen and phosphorus, and biomass can be controlled by grazers. In this study we experimentally manipulated nutrients and grazers simultaneously to determine the relative contribution of bottom-up and top-down controls on periphyton over time. We used nutrient diffusing substrates to regulate nutrient concentrations and an underwater electric field to exclude grazing insects in three sequential 16-17 day experiments from August to October in montane Colorado, USA. We measured algal biomass, periphyton organic mass, and algal community composition in each experiment and determined densities of streambed i...