The chemistry of small watershed streams has played an important role in the development of ecosystem ecology.  The controls of stream nutrient chemistry however are debated and often split along terrestrial and aquatic disciplines.  Here, we develop and test a simple equilibrium model of nutrient cycling in streams that captures the essential physical and biotic properties of streams and explore the use of stream chemistry to understand watershed biogeochemistry.  Without major directional change in groundwater nutrient concentrations, we assume that divergence from constant longitudinal stream nutrient concentrations reflects biotic imbalance between internal uptake and regeneration.  We test this idea using 142 longitudinal profiles of ambient dissolved nutrient concentrations in 14 small watershed streams across 4 major biomes.  We show strong evidence for steady state: overall, ambient profiles were flat for all nutrient forms.  The few cases of downstream declines were in open-canopied systems undergoing biomass accumulation.  In contrast to the ideas that small streams regulate watershed losses or are biogeochemically inert pipes, our analysis supports the idea that while stream ecosystems display high rates of nutrient cycling, they also tend strongly toward system steady state, allowing use of their chemistry as an integrated measure of terrestrial nutrient losses.