Contrasting functional responses of benthic and hyporheic stream biofilms to light availability and macronutrient stoichiometry

Nutrient dynamics in headwater streams are governed by benthic and hyporheic biofilms, with carbon (C) : nitrogen (N) : phosphorus (P) ratios driving the heterotrophic microbial biofilm development through nutrient limitation. Furthermore, heterotrophic responses to changes in C : N : P ratios are probably modulated by autotrophic responses to light and C : N : P ratios, which modify the amount and composition of photosynthetic exudates and increase competition for nutrients. Effects on functional properties like the use of organic compounds by the heterotrophic biofilm community are largely unknown. We conducted a stream mesocosm experiment with a factorial design with different C : N : P ratios and light availability levels to test direct and indirect effects on heterotrophic biofilm functioning via community‐level physiological profiles in benthic and hyporheic biofilms. When inducing a resource C : N : P ratio closer to heterotrophic microbial biomass C : N : P ratios, we found an increased functional diversity of metabolized substrates, especially in hyporheic biofilms. Furthermore, this alteration shaped substrate preferences toward less P‐containing substrates and more N‐containing substrates in early‐stage biofilms. Despite the absence of a direct impact of light on hyporheic biofilms, we detected a propagation of a benthic autotrophic effect into hyporheic biofilms. Light availability induced effects on hyporheic bacterial density and the use of phenolic compounds, amino acids, and carbohydrates. In benthic biofilms, only the use of carbohydrates was affected by light. These results emphasize the significant indirect effects of benthic autotrophs on the functionality of hyporheic microbial heterotrophs and suggest consequences of human impacts, such as nutrient inputs and clear‐cutting, on stream nutrient cycling.