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

Abstract

14 páginas, 7 figuras, 4 tablasHeadwater 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 (NO 3 ), ammonium (NHC 4 ), 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 (Fsw) 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 NHC 4 , the occurrence of Fsw >0 μg Nm1 s1 (gross uptake > release) was high along the reach, while for NO 3 , the occurrence of Fsw <0 μg Nm1 s1 (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 NO 3 , NHC 4 , and SRP, respectively. Whole-reach mass balance calculations indicated that instream net uptake reduced stream NHC 4 flux up to 90 %, while the stream acted mostly as a source of NO 3 and SRP. During the dormant period, concentrations decreased along the reach for NO 3 , but increased for NHC 4 and SRP. During the vegetative period, NHC 4 decreased, SRP increased, and NO 3 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.S. Bernal and A. Lupon were funded by the Spanish Ministry of Economy and Competitiveness (MINECO) with a Juan de la Cierva contract (JCI-2010-06397) and an FPU grant (AP-2009-3711). S. Bernal received additional funds from the Spanish Research Council (CSIC) (JAEDOC027) and the MICECO-funded project MED_FORESTREAM (CGL2011- 30590). M. Ribot was funded through a technical training contract from the MINECO-funded project ISONEF (CGL2008-05504- C02-02/BOS) and MED_FORESTREAM. Additional financial support was provided by the European Union-funded project REFRESH (FP7-ENV-2009-1-244121) and the MINECO-funded project MONTES-Consolider (CSD 2008-00040).Peer reviewe