Abstract
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 (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 NH4+, the occurrence of Fsw > 0 μg N m−1 s−1 (gross uptake > release) was high along the reach, while for NO3−, the occurrence of Fsw < 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-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.
Highlights
Stream water chemistry integrates hydrological and biogeochemical processes occurring within its drainage area, and the temporal variation in stream solute concentrations at the catchment outlet is considered a good indicator of the response of terrestrial and aquatic ecosystems to environmental drivers (Bormann and Likens, 1967; Bernhardt et al, 2003; Houlton et al, 2003)
At the whole-reach scale, gross hydrological gains exceed gross losses in 8 out of 10 field dates (Fig. 2c and d). This was especially noticeable in at the beginning of riparian leaf-out (April) and December 2011, the two sampling dates most influenced by storm events
Our results revealed that in-stream processes were highly variable over time and space, though in most cases this variability could not be associated with either physical longitudinal gradients or shifts in environmental conditions between the dormant and vegetative period
Summary
Stream water chemistry integrates hydrological and biogeochemical processes occurring within its drainage area, and the temporal variation in stream solute concentrations at the catchment outlet is considered a good indicator of the response of terrestrial and aquatic ecosystems to environmental drivers (Bormann and Likens, 1967; Bernhardt et al, 2003; Houlton et al, 2003). Less attention has been paid to the spatial variation in water chemistry along the stream, though it can be considerably important because stream nutrient concentrations are influenced by changes in hydrological flow paths, vegetation cover, and soil characteristics (Dent and Grimm, 1999; Likens and Buso, 2006). Stream ecosystems have a strong capacity to transform and retain nutrients; in-stream biogeochemical processes can further influence nutrient chemistry along the stream (Peterson et al, 2001; Dent et al, 2007). Consideration of these multiple sources of variation in stream water chemistry is important to understand drivers of stream nutrient dynamics
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