Use of pharmaceuticals and personal-care products (PPCPs) has been pervasive for decades. Caffeine is the most widely used drug in the US and caffeine has been detected in surface waters. We predicted that chronic exposure to caffeine concentrations measured in rivers would have consequences for stream ecosystems. We conducted an experiment using 24 recirculating indoor streams, partitioned among 4 caffeine concentrations: 1) control; 2) 0.1 μg L-1 (the median concentration observed in the US); 3) 10 μg L-1, (the highest concentration measured in the US) and 4) 1 mg L-1, (among the highest reported caffeine concentrations worldwide). Repeated measures ANOVA showed that caffeine significantly decreased (p<0.05) periphyton chlorophyll a and ash free dry mass (AFDM), ammonium uptake rates and substrate-specific metabolism at the lower dose, but all endpoints were less affected by high caffeine concentrations. Algal community composition was also influenced by caffeine exposure. Caffeine had non-linear effects, with low concentrations usually having stronger effects than high concentrations. The widespread presence of caffeine in surface waters, even in low concentrations, may have effects on stream ecosystems.