Stream water chemistry integrates hydrological and biogeochemical processes occurring within its drainage area, and thus 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). For instance, spatial variation in nutrient concentration along the stream has been attributed to changes in soil nitrification rates (Bohlen et al., 2001), soil organic carbon availability (Johnson et al., 2000), and organic soil depth across altitudinal gradients (Lawrence et al., 2000). Moreover, nutrient cycling within the riparian zone can strongly influence stream nutrient concentrations along the stream because these ecosystems are hot spots of biogeochemical processing (McClain et al., 2003; Vidon et al., 2010). In addition, processes occurring at the riparian–stream interface have a larger influence on stream water chemistry than those occurring at catchment locations further from the stream (Ross et al., 2012). Finally, stream ecosystems have a strong capacity to transform and retain nutrients; thus, in-stream biogeochemical processes can further influence nutrient chemistry along the stream (Peterson et al., 2001; Dent et al., 2007). Therefore, consideration of these multiple sources of variation in stream water chemistry is important to understand drivers of stream nutrient dynamics.

Our understanding of nutrient biogeochemistry within riparian zones and streams is mainly based on field studies performed at the plot scale or in small stream reaches (a few hundred meters) (Lowrance et al., 1997; Peterson et al., 2001; Sabater et al., 2003; Mayer et al., 2007; von Schiller et al., 2015). These empirical studies have widely demonstrated the potential of riparian and stream ecosystems as either sinks or sources of nutrients, which ultimately influence the transport of nutrients to downstream ecosystems. Riparian and stream biota are capable of decreasing the concentration of essential nutrients, such as dissolved inorganic nitrogen (DIN) and phosphate, especially with increasing water storage and residence time (Valett et al., 1996; Hedin et al., 1998; Peterson et al., 2001; Vidon and Hill, 2004). Conversely, riparian forests can become sources rather than sinks of nutrients when N2-fixing species predominate (Helfield and Naiman, 2002; Compton et al., 2003), and in-stream nutrient release can be important during some periods (Bernhardt et al., 2002; von Schiller et al., 2015). Moreover, there is an intimate hydrological linkage between riparian and stream ecosystems that can result in strong biogeochemical feedbacks between these two compartments (e.g., Morrice et al., 1997; Martí et al., 2000; Bernal and Sabater, 2012). However, studies integrating biogeochemical processes of these two nearby ecosystems are rare (but see Dent et al., 2007), and the exchange of water and nutrients between stream and groundwater is unknown in most studies assessing in-stream gross and net nutrient uptake (Roberts and Mulholland, 2007; Covino et al., 2010; von Schiller et al., 2011).

There is a wide body of knowledge showing the potential of riparian and stream ecosystems to modify either groundwater or stream nutrient concentrations. However, a comprehensive view of the influence of riparian and in-stream processes on stream water chemistry at the catchment scale is still lacking (but see Meyer and Likens, 1979). This gap of knowledge mostly exists because hydrological and biogeochemical processes can vary substantially along the stream (Covino and McGlynn, 2007; Jencso et al., 2010), which limits our ability to extrapolate small plot- and reach-scale measurements to larger spatial scales. Some authors have proposed that nutrient concentrations should decline along the stream if in-stream net uptake is high enough and riparian groundwater inputs are relatively small (Brookshire et al., 2009). This declining pattern is not systematically observed in reach-scale studies, which could bring us to the conclusion that terrestrial inputs are the major driver of stream water chemistry because in-stream gross uptake and release counterbalance each other most of the time (Brookshire et al., 2009). However, synoptic studies have revealed that nutrient concentrations are patchy and highly variable along the stream as a result of spatial patterns in upwelling and in-stream nutrient processing (Dent and Grimm, 1999). Thus, in-stream nutrient cycling could be substantial, but it might not necessarily lead to longitudinal increases or declines in nutrient concentration, a question that probably needs to be addressed at spatial scales larger than a few hundred meters.

The goal of this study was to gain a better understanding of the influence of riparian groundwater inputs and in-stream biogeochemical processing on stream nutrient chemistry and fluxes in a headwater forested catchment. To approach this question, we explored the longitudinal pattern of stream nutrient (nitrate, ammonium, and phosphate) concentration along a 3.7 km reach over 1.5 years. We chose a headwater catchment as a model system to investigate drivers of spatial patterns in stream water chemistry because they typically show pronounced changes in riparian and stream features across relatively short distances (Uehlinger, 2000). First, we evaluated riparian groundwater inputs and in-stream nutrient processing as sources of variation in stream nutrient concentration along the reach. We expected stream and riparian groundwater nutrient concentrations to be similar and strongly correlated if riparian groundwater is a major source of nutrients to the stream. In addition, we estimated the in-stream nutrient-processing capacity for 14 contiguous segments along the reach with a mass balance approach. Second, we evaluated the relative contribution of riparian groundwater inputs and in-stream biogeochemical processing to stream nutrient fluxes at the whole-reach scale by applying a mass balance approach that included all hydrological input and output fluxes along the reach.

The research was conducted in the Font del Regàs catchment (14.2 km2) (Fig. 1), located in the Montseny Natural Park, NE Spain (41∘50′ N, 2∘30′ E; 300–1200 m a.s.l.) during the period 2010–2011. Total inorganic N deposition in this area oscillates between 15 and 30 kg N ha-1 yr-1 (Àvila and Rodà, 2012). The climate at the Montseny Mountains is subhumid Mediterranean. The long-term mean annual precipitation is 925 ± 151 mm and the long-term mean annual air temperature is 12.1 ± 2.5 ∘C (mean ± SD, period: 1940–2000; Catalan Meteorological Service: http://www.meteo.cat/observacions/xema/). During the study period, mean annual precipitation (975 mm) and temperature (12.9 ∘C) fell within the long-term average (data from a meteorological station within the study catchment). In this period, summer 2010 was the driest season (140 mm), while most of the precipitation occurred in winter 2010 (370 mm) and autumn 2011 (555 mm) (Fig. 2a).

The catchment is dominated by biotitic granite (ICC, 2010) and it has steep slopes (28 %). Evergreen oak (Quercus ilex) and beech (Fagus sylvatica) forests cover 54 and 38 % of the catchment area, respectively (Fig. 1). The upper part of the catchment (2 %) is covered by heathlands and grasslands (ICC, 2010). The catchment has a low population density (< 1 person km-2) which is concentrated in the valley bottom. Hillslope soils (pH ∼ 6) are sandy, with a high content of rocks (33–36 %). Soils at the hillslopes have a 3 cm depth O horizon and a 5 to 15 cm depth A horizon (averaged from 10 soil profiles).

The riparian zone is relatively flat (slope < 10 %), and it covers 6 % of the catchment area. Riparian soils (pH ∼ 7) are sandy loam with low rock content (13 %) and a 5 cm depth organic layer followed by a 30 cm depth A horizon (averaged from five soil profiles). Along the 3.7 km reach, the width of the riparian zone increases from 6 to 32 m, whereas the total basal area of riparian trees increases 12-fold (based on forest inventories of 30 m plots every ca. 150 m) (Fig. S1 in the Supplement). Alnus glutinosa, Robinia pseudoacacia, Platanus hybrida, and Fraxinus excelsior are the most abundant riparian tree species followed by Corylus avellana, Populus tremula, Populus nigra, and Sambucus nigra. The abundance of N2-fixing species (A. glutinosa and R. pseudoacacia) increases from 0 to > 60 % along the longitudinal profile (Fig. S1). During base flow conditions, riparian groundwater (< 1.5 m from the stream channel) flows well below the soil surface (0.5 ± 0.1 m), and thus the interaction with the riparian organic soil is minimal (averaged from 15 piezometers, n=165) (Fig. S1). During the period of study, riparian groundwater temperature ranged from 5 to 19.5 ∘C.

The 3.7 km study reach is a second-order stream along the first 1.5 km and a third-order stream for the remaining 63 % of its length. The geomorphology of the stream bed changes substantially with stream order. The stream bed along the second-order section is mainly composed of rocks and cobbles (70 %) with a small contribution of sand (∼ 10 %). At the valley bottom, sands and gravels represent 44 % of the stream substrate and the presence of rocks is minor (14 %). Mean wetted width and water velocity increase between the second- and third-order section (from 1.6 to 2.7 m and from 0.24 to 0.35 m s-1, respectively) (Fig. S1). During the study period, stream water temperature ranged from 5 to 18 ∘C. Stream discharge was low in summer (0.33 mm) and peaked in spring (0.79 mm).

In terms of hydrology, the study headwater stream was a net gaining reach, though the hydrological interaction between the riparian zone and the stream was complex as indicated by the longitudinal variation in net riparian groundwater inputs. Moreover, the longitudinal decrease in area-specific discharge suggests that hydrological retention increased at the valley bottom compared to upstream segments as reported in previous studies (Covino et al., 2010). Despite the complex hydrological processes along the reach, the strong positive correlation between stream and riparian groundwater Cl- concentration suggests high hydrological connectivity at the riparian–stream interface (Butturini et al., 2003). In addition, we found that the permanent tributaries, which comprised ∼ 50 % of the catchment area, contributed 56 % of stream discharge, and thus were an essential component for understanding stream nutrient chemistry and loads. Hydrological mixing of stream water with water from tributaries could partially explain the longitudinal increase in Cl- because its concentration was higher at the tributaries than at the main stream, especially during the vegetative period. In addition, riparian groundwater inputs to the stream could further contribute to the longitudinal increase in stream Cl- concentration because they contributed 26 % of stream discharge and also exhibited higher Cl- concentration than stream water.

Based on the strong hydrological connectivity between the stream and the riparian groundwater and the large contribution of tributaries to stream discharge, one would expect a strong influence of these water sources on the longitudinal variation in stream nutrient chemistry. However, the relationship between stream and riparian groundwater nutrient concentration was from moderate to weak for NO3- and SRP, and zero for NH4+. Further, the contribution of tributaries to stream nutrient fluxes was relatively small (from 21 to 34 %) compared to their contribution to stream Cl- and water fluxes (>50 %). Together these data suggest that longitudinal patterns of stream nutrient concentration could not be explained by hydrological mixing alone, thus pointing to in-stream biogeochemical processing as a likely mechanism to modify nutrient concentrations along the study reach. In fact, the estimates of in-stream net nutrient uptake (Fsw) at the different stream segments supported this idea and agreed with previous studies showing that in-stream processes can mediate stream nutrient chemistry and downstream nutrient export (McClain et al., 2003; Harms and Grimm, 2008).

Our results revealed an extremely high variability in Fsw, which could range by up to one order of magnitude, across individual segments and over time, which agrees with findings from other headwater streams (von Schiller et al., 2011). However, some general trends appeared when comparing patterns for the different studied nutrients. For instance, the frequency of dates for which in-stream gross uptake and release were imbalanced (Fsw≠0) was higher for NH4+ (80 %) and SRP (68 %) than for NO3- (37 %). Further, the potential of in-stream processes to modify stream fluxes within stream segments (|Fsw×x/Fin|) was 3-fold higher for NH4+ and SRP than for NO3-. Our findings are concordant with studies performed at short stream reaches (< 300 m) worldwide, which show that in-stream gross uptake velocity (as a proxy of nutrient demand) is typically higher for NH4+ and SRP than for NO3- (Ensign and Doyle, 2006). This difference among nutrients is commonly attributed to the higher biological demand for NH4+ and SRP than for NO3-. However, we found that Fsw was similar among nutrients; thus, differences in |Fsw×x/Fin| were mainly associated with differences in the concentration of the inputs, which tend to be 20-fold lower for NH4+ and SRP than for NO3-. Divergences between Fsw and |Fsw×x/Fin| were even more remarkable when nutrient budgets were considered at the whole-reach scale, especially for DIN forms. NO3- and NH4+ showed no differences in Fsw between the two scales of observation; however, they showed a substantial increase in |Fsw×x/Fin| at the whole-reach scale (length of kilometers) compared to the segment scale (length of hundreds of meters). Similarly, previous nutrient spiraling studies have reported an increase in the proportion of nutrient removal with stream order despite no changes in gross uptake rates among stream reaches (Ensign and Doyle, 2006; Wollheim et al., 2006). This pattern has been attributed to variation in intrinsic stream characteristics, such as stream nutrient concentration, discharge, stream width, and the size of the hyporheic zone (Wollheim et al., 2006; Alexander et al., 2009), which may also hold for our study since these characteristics varied along the 3.7 km reach. However, our results also indicate that the assessment of riparian groundwater inputs is crucial to understand the contribution of in-stream processes to stream nutrient fluxes. Overall, our findings add to the growing evidence that streams are hot spots of nutrient processing (Peterson et al., 2001; Dent et al., 2007), and that in-stream processes can substantially modify stream nutrient fluxes at the catchment scale (Ensign and Doyle, 2006; Bernal et al., 2012).

The potential of in-stream processes to regulate stream nutrient fluxes was especially remarkable for NH4+. There was no relationship between stream and riparian groundwater NH4+ concentrations; further, whole-reach budgets indicated that in-stream net uptake could reduce the flux of NH4+ up to 90 % along the reach. This high in-stream bioreactive capacity could be favored by the sharp increase in redox conditions from riparian groundwater to stream water (Hill et al., 1998; Dent et al., 2007). Concordantly, NH4+ concentrations were higher in riparian groundwater than in the stream, while the opposite occurred for NO3- (although only during the vegetative period). These results suggest fast nitrification of groundwater inputs within the stream as environmental conditions become well oxygenated (Jones et al., 1995). However, the marked increase in stream NO3- concentration observed along the last 700 m of the reach during the vegetative period could not be explained entirely by nitrification of riparian groundwater NH4+ because this flux (Fgw,NH4 ∼ 2 µg N m-1 s-1) was not large enough to sustain in-stream NO3- release |Fsw,NO3 < 0| (∼ 10 µg N m-1 s-1). This finding suggests an additional source of N at the valley bottom. Previous studies have shown that leaf litter from riparian trees, and especially from N2-fixing species, can enhance in-stream nutrient cycling because of its high quality and degradability (Starry et al., 2005; Mineau et al., 2011). Thus, the increase in NO3- and SRP concentrations and in-stream NO3- release observed at the lowest part of the catchment during the vegetative period could result from the combination of warmer temperatures and the mineralization of large stocks of alder and black locust leaf litter stored in the stream bed (Strauss and Lamberti, 2000; Bernhardt et al., 2002; Starry et al., 2005). Alternatively, increases in stream NO3- and SRP concentration could result from human activities, which were concentrated at the lowest part of the catchment. However, regarding NO3-, anthropogenic sources seem unlikely because DIN concentrations at the tributary draining through the inhabited area were low. In contrast, this tributary showed high SRP concentrations (from 2- to 6-fold higher than in the main stream), though its discharge would have had to be ca. 4 times higher than expected for its drainage area (< 0.4 km2) to explain the observed changes in concentration. Another possible explanation for the increase in stream N concentration at the valley bottom could be increased N fixation by stream algae (Finlay et al., 2011). However, in-stream DIN release (NO3- and NH4+) peaked in late spring and summer (May and August 2011), when light penetration was limited by riparian canopy and in-stream photoautotrophic activity was low (Lupon et al., 2015). Altogether, these data suggest that the sharp increase in nutrient availability along the last 700 m of the reach was likely related to the massive presence of the invasive black locust at the valley bottom. Black locust is becoming widespread throughout riparian floodplains in the Iberian Peninsula (Castro-Díez et al., 2014), and its potential to subsidize N to stream ecosystems via root exudates and leaf litter could dramatically alter in-stream nutrient processing and downstream nutrient export (e.g., Stock et al., 1995; Mineau et al., 2011). However, further research is needed to test the hypothesis that this invasive species can alter stream nutrient dynamics in riparian floodplains.

It is worth noting that longitudinal trends in stream nutrient concentrations showed no simple relationship to in-stream processes. This finding evidenced that other sources of variation in stream water chemistry were counterbalancing the influence of in-stream processes on stream nutrient fluxes. In this sense, results from NH4+ were paradigmatic. The mass balance approach clearly showed that in-stream gross uptake of NH4+ exceeded release; concordantly, NH4+ concentration was consistently lower in the stream than in riparian groundwater. However, stream NH4+ concentration showed small longitudinal variation likely because in-stream net uptake balanced the elevated inputs from riparian groundwater. Therefore, our results challenge the idea that stream nutrient concentration should decrease in the downstream direction when in-stream processes are efficient in taking up nutrients from receiving waters (Brookshire et al., 2009). Conversely, our findings convincingly show that in-stream processes can strongly affect stream nutrient chemistry and downstream nutrient export even in the absence of consistent longitudinal gradients in nutrient concentration. For NO3-, our data suggest that the marked increase in concentration along the last 700 m could be a consequence of in-stream mineralization of N-rich leaf-litter stocks. However, the observed decrease in NO3- concentration along the first 1.5 km of the reach could barely be explained by in-stream processing alone because its contribution to reduce stream NO3- fluxes was too low, even when the whole-reach budget was recalculated excluding the last 700 m of the reach (Fsw=0.61 µg N m-1 s-1 and Fsw>0/Fin= 10 %). Therefore, the declining pattern was likely a combination of both in-stream nutrient processing and hydrological mixing with riparian groundwater and tributary inputs. For SRP, the longitudinal increase in concentration could neither be fully explained by in-stream release because Fsw,SRP < 0 was not widespread along the reach and the stream only contributed to input fluxes by 19 % (6 % when excluding the last 700 m). Again, stream nutrient chemistry along the reach was the combination of both in-stream nutrient processing and hydrological mixing as indicated by our whole-reach mass balance. Recent studies have concluded that riparian groundwater is a major driver of longitudinal patterns in stream nutrient concentration in headwater streams (Bernhardt et al., 2002; Asano et al., 2009; Scanlon et al., 2010). Our study adds to our knowledge of catchment biogeochemistry by showing that stream nutrient chemistry results from the combination of both hydrological mixing from the riparian zone and in-stream nutrient processing, which can play a pivotal role in shaping stream nutrient concentrations and fluxes at the catchment scale.

The synoptic approach adopted in this study highlighted that the Font del Regàs stream had a strong potential to transform nutrients. The longitudinal pattern in stream nutrient concentrations could not be explained solely by hydrological mixing with riparian groundwater and tributary sources because dissolved nutrients underwent biogeochemical transformation while traveling along the stream channel. 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. Nevertheless, results from a mass balance approach showed that in-stream processes contributed substantially to modify stream nutrient fluxes and that the stream could act either as a net nutrient sink (for NH4+) or as a net nutrient source (for SRP and NO3-) at the catchment scale. These results add to the growing evidence that in-stream biogeochemical processes need to be taken into consideration in either empirical or modeling approaches if we are to understand drivers of stream nutrient chemistry within catchments.

Recent studies have proposed that riparian groundwater is a major control of longitudinal patterns of nutrient concentration because in-stream gross nutrient uptake and release tend to counterbalance each other most of the time (Brookshire et al., 2009; Scanlon et al., 2010). Conversely, our study showed that in-stream processes can influence stream nutrient chemistry and downstream exports without generating longitudinal gradients in concentration and flux because changes in stream nutrient chemistry are the combination of both in-stream processing and nutrient inputs from terrestrial sources. Our results imply that the assessment of these two sources of variation in stream nutrient chemistry is crucial to understand the contribution of in-stream processes to stream nutrient dynamics at relevant ecological scales.

Reliable measurements of riparian groundwater inputs are difficult to obtain because spatial variability can be high (Lewis et al., 2006) and determination of the chemical signature of the groundwater that really enters the stream is still a great challenge (Brookshire et al., 2009). In this study, we installed 15 piezometers along the reach (one per sampling site), which may not be representative enough of the variation in riparian groundwater chemistry. However, and despite its limitations, riparian groundwater sampling near the stream can help to constrain the uncertainty associated with this water source and provide more reliable estimations of in-stream net nutrient uptake for both nutrient mass balance and spiraling empirical approaches (von Schiller et al., 2011).