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Headwater streams are recipients of water sources draining throughterrestrial ecosystems. At the same time, stream biota can transform andretain nutrients dissolved in stream water. Yet studies consideringsimultaneously these two sources of variation in stream nutrient chemistryare rare. To fill this gap of knowledge, we analyzed stream water andriparian groundwater concentrations and fluxes as well as in-stream netuptake rates for nitrate (NO3−), ammonium (NH4+), and solublereactive phosphorus (SRP) along a 3.7 km reach on an annual basis. Chlorideconcentrations (used as conservative tracer) indicated a strong hydrologicalconnection at the riparian–stream interface. However, stream and ripariangroundwater nutrient concentrations showed a moderate to null correlation,suggesting high in-stream biogeochemical processing. In-stream net nutrientuptake (Fsw) was highly variable across contiguous segments andover time, but its temporal variation was not related to the vegetativeperiod of the riparian forest. For NH4+, the occurrence ofFsw > 0 μg N m−1 s−1 (gross uptake >release) was high along the reach, while for NO3−, the occurrence ofFsw < 0 μg N m−1 s−1 (gross uptake< release) increased along the reach. Within segments and dates,Fsw, whether negative or positive, accounted for a median of 6,18, and 20% of the inputs of NO3−, NH4+, and SRP,respectively. Whole-reach mass balance calculations indicated that in-streamnet uptake reduced stream NH4+ flux up to 90%, while the streamacted mostly as a source of NO3− and SRP. During the dormant period,concentrations decreased along the reach for NO3−, but increased forNH4+ and SRP. During the vegetative period, NH4+ decreased,SRP increased, and NO3− showed a U-shaped pattern along the reach.These longitudinal trends resulted from the combination of hydrologicalmixing with terrestrial inputs and in-stream nutrient processing. Therefore,the assessment of these two sources of variation in stream water chemistry iscrucial to understand the contribution of in-stream processes to streamnutrient dynamics at relevant ecological scales

Riparian and in-stream controls on nutrient concentrations and fluxes in a headwater forested stream