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

Abstract. Headwater streams are recipients of water sources draining through terrestrial ecosystems. At the same time, stream biota can transform and retain nutrients dissolved in stream water. Yet studies considering simultaneously these two sources of variation in stream nutrient chemistry are rare. To fill this gap of knowledge, we analyzed stream water and riparian groundwater concentrations and fluxes as well as in-stream net uptake rates for nitrate (NO₃⁻), ammonium (NH₄⁺), and soluble reactive phosphorus (SRP) along a 3.7 km reach on an annual basis. Chloride concentrations (used as conservative tracer) indicated a strong hydrological connection at the riparian–stream interface. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high in-stream biogeochemical processing. In-stream net nutrient uptake (F_sw) was highly variable across contiguous segments and over time, but its temporal variation was not related to the vegetative period of the riparian forest. For NH₄⁺, the occurrence of F_sw > 0 μg N m⁻¹ s⁻¹ (gross uptake > release) was high along the reach, while for NO₃⁻, the occurrence of F_sw < 0 μg N m⁻¹ s⁻¹ (gross uptake < release) increased along the reach. Within segments and dates, F_sw, 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-stream net uptake reduced stream NH₄⁺ flux up to 90%, while the stream acted mostly as a source of NO₃⁻ and SRP. During the dormant period, concentrations decreased along the reach for NO₃⁻, but increased for NH₄⁺ 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 hydrological mixing with terrestrial inputs and in-stream nutrient processing. Therefore, the assessment of these two sources of variation in stream water chemistry is crucial to understand the contribution of in-stream processes to stream nutrient dynamics at relevant ecological scales.