Studies on factors controlling periphyton biomass are important for understanding food web structure and ecosystem functioning in tropical streams. Multiple factors potentially influence periphyton in streams, including floods, periods of low flow, nutrients, light availability, and grazing. Although periphyton are important components that promote secondary production, studies concerning their biomass in heterotrophic streams are scarce. Our goal is to assess the main biotic and abiotic controls that influence periphyton biomass production in an unimpacted tropical stream at the Luquillo Experimental Forest, Puerto Rico. We predict that periphyton biomass will be significantly influenced by canopy cover, discharge, and shrimp populations. Our study is part of the Stream Flow Reduction Experiment (StreamFRE), which addresses multiple ecosystem responses to prolonged low-flow conditions, including food web dynamics. The main drivers that will be examined are water discharge, nutrients (P and N), canopy cover, and shrimp populations, while benthic organic matter (i.e., AFDM) and chlorophyll will serve as biomass indicators. Preliminary analyses indicate that hydrology is a dominant driver of benthic organic matter and chlorophyll dynamics over the study period. In contrast, canopy cover showed no relation to periphyton biomass even following hurricane defoliation of riparian vegetation. Experimental manipulation of shrimp grazing highlighted their critical role in periphyton biomass in our study stream. Overall, our research shows that periphyton biomass is controlled by both biotic and abiotic factors, with top-down control by shrimp playing a key role in its dynamics. Even responses to major disturbances like droughts and hurricanes are mediated by shrimp activity.