Headwater streams have a strong capacity to transform and retain nutrients, and thus, a longitudinal decrease in stream nutrient concentrations would be expected from in-stream nutrient removal alone. Yet, a number of other factors within the catchment, including biogeochemical processing within the riparian zone and export to streams, can contribute to stream nutrient concentration, which may overcome the effect of in-stream biogeochemical processing. To explore this idea, we analyzed the longitudinal patterns of stream and riparian groundwater concentrations for chloride (Cl − ), nitrate (NO 3 − ), ammonium (NH 4 + ), and phosphate (PO 4 3− ) along a 3.7 km reach at an annual scale. The reach showed a gradual increase in stream and riparian width, riparian tree basal area, and abundance of riparian N 2 -fixing tree species. Concentrations of Cl − indicated a~strong hydrological connection at the riparian-stream edge. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high biogeochemical processing at the riparian-stream edge and within the stream. A mass balance approach along the reach indicated that, on average, in-stream net nutrient uptake prevailed over release for NH 4 + and PO 4 3− , but not for NO 3 − . On an annual basis, in-stream processes contributed to change stream input fluxes by 11%, 26%, and 29% for NO 3 − , NH 4 + , and PO 4 3− , respectively. Yet, longitudinal trends in concentration were not consistent with the prevailing in-stream biogeochem ical processes. During the riparian dormant period, stream concentration decreased along the reach for NO 3 − , but increased for NH 4 + and PO 4 3− . During the riparian vegetative period, NO 3 − and PO 4 3− increased along the reach while NH 4 + showed no clear pattern. These longitudinal trends were partially related to riparian forest features and groundwater inputs, especially for NO 3 − and PO 4 3− . Our study suggests that even though in-stream biogeochemical processing was substantial, the riparian zone can modulate the longitudinal variation in stream nutrient chemistry in this headwater stream.