Nutrient uptake in a stream affected by hydropower plants: comparison between stream channels and diversion canals

12 páginas, 3 figuras, 4 tablas.Small hydropower plants divert part of the water from wide and physically complex stream channels with active hyporheic areas to narrow and hydraulically simple concrete canals, and thus, might affect nutrient dynamics. We compared nutrient uptake in diversion canals and in stream channels in the Leitzaran Stream (Basque Country, northern Spain). We predicted that simple morphology in diversion canals will result in lower nutrient uptake in canals than in stream channels. Periphytic chlorophyll and biomass did not differ significantly between reach types. Water was significantly deeper and faster in canals than in stream channels, but the transient storage zone did not differ significantly between reach types. There were no significant differences between uptake length for neither phosphate nor ammonium between reach types. Uptake length in both stream channels and diversion canals decreased with discharge, in a pattern similar to that previously described for pristine rivers across the world. Uptake velocity and uptake rate for phosphate did not differ signifi- cantly between reach types, but in the case of ammonium both retention metrics were significantly larger in the diversion canals. Results suggest that although hydropower schemes have minor effects on nutrient retention, these depend on the proportion of flow diverted.This research has been funded by the Spanish Department of Science and Technology, the University of the Basque Country, and the European Regional Development Fund, through projects 9/UPV00118.310-14476/ 2002 and BOS2003-04466.Peer reviewe

Green light: gross primary production influences seasonal stream N export by controlling fine-scale N dynamics

Monitoring nutrient concentrations at fine-scale temporal resolution contributes to a better understanding of nutrient cycling in stream ecosystems. However, the mechanisms underlying fine-scale nutrient dynamics and its implications for budget catchment fluxes are still poorly understood. To gain understanding on patterns and controls of fine-scale stream nitrogen (N) 18 dynamics and to assess how they affect hydrological N fluxes, we explored diel variation in stream nitrate (NO3-) concentration along a headwater stream with increasing riparian area and channel width. At the down-stream site, the highest day-night variations occurred in early-spring when stream NO3- concentrations were 13% higher at night than during day time. Such day-night variations were strongly related to daily light inputs (R2=0.74) and gross primary production (GPP) (R2=0.74), and they showed an excellent fit with day-night NO3- variations predicted from GPP (R2=0.85). These results suggest that diel fluctuations in stream NO3- concentration were mainly driven by photoautotrophic N uptake. Terrestrial influences were discarded because no simultaneous diel variations in stream discharge, riparian groundwater level, or riparian solute concentration were observed. In contrast to the down-stream site, no diel variations in NO3- concentration occurred at the up-stream site likely because water temperature was colder (10 vs. 12 ºC) and light availability was lower (4 vs. 9 mol m-2 d-1). Although daily GPP was between 10-100 folds lower than daily respiration, photoautotrophic N uptake contributed to a 10% reduction in spring NO3- loads at the down-stream site. Our study clearly shows that the activity of photoautotrophs can substantially change over time and along the stream continuum in response to key environmental drivers such as light and temperature, and further that its capacity to regulate diel and seasonal N fluxes can be important even in low productivity streams.Financial supported was provided by the Spanish Government through the projects MONTES-Consolider (CSD2008- 00040-MONTES) and MEDFORESTREAM (CGL2011-30590).Peer reviewe

The influence of riparian evapotranspiration on stream hydrology and nitrogen retention in a subhumid Mediterranean catchment

Contiene 6 figuras, 3 tablasRiparian evapotranspiration (ET) can influence stream hydrology at catchment scale by promoting the net loss of water from the stream towards the riparian zone (i.e., stream hydrological retention). However, the consequences of stream hydrological retention on nitrogen dynamics are not well understood. To fill this gap of knowledge, we investigated changes in riparian ET, stream discharge, and nutrient chemistry in two contiguous reaches (headwater and valley) with contrasted riparian forest size in a small forested Mediterranean catchment. Additionally, riparian groundwater level (hgw) was measured at the valley reach. The temporal pattern of riparian ET was similar between reaches, was positively correlated with hgw (ρ = 0.60), and negatively correlated with net riparian groundwater inputs (ρ < -0.55). During the vegetative period, stream hydrological retention occurred only at the valley reach (59% of the time), and was accompanied by in-stream nitrate release and ammonium uptake. During the dormant period, when the stream gained water from riparian groundwater, results showed small influences of riparian ET on stream hydrology and nitrogen concentrations. Despite being a small component of annual water budgets (4.5%), our results highlight that riparian ET drives stream and groundwater hydrology in this Mediterranean catchment and, furthermore, question the potential of the riparian zone as a natural filter of nitrogen loads.Financial supported was provided by the Spanish Government through the projects MONTES-Consolider (CSD2008-00040-MONTES), MEDFORESTREAM (CGL2011-30590), and MEDSOUL (CGL2014-59977-C3-2). AL was supported by a FPU PhD fellowship from the Spanish Ministry of Education and Science (AP-2009-3711) and the MEDSOUL project. SB work was funded by the Spanish Research Council (JAE-DOC027), the Spanish CICT (Juan de la Cierva contract JCI-2008-177), European Social Funds (FSE), and the MEDFORESTREAM and NICUS (CGL-2014-55234-JIN) projects. SP was supported by a FPI PhD fellowship from the Spanish Ministry of Economy and Competitiveness (BES-2012-054572).Peer reviewe

Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO3_{3} –^{–} stoichiometry along a headwater Mediterranean stream

In forest headwater streams, metabolic processes are predominately heterotrophic and depend on both the availability of carbon (C) and nitrogen (N) and a favourable C:N stoichiometry. In this context, hydrological conditions and the presence of riparian forests adjacent to streams can play an important, yet understudied role in determining dissolved organic carbon (DOC) and nitrate (NO$_{3}$ $^{–}$) concentrations and DOC : NO$_{3}$ $^{–}$ molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO$_{3}$ $^{–}$ supply and DOC : NO$_{3}$ $^{–}$ stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO$_{3}$ $^{–}$ concentrations and DOC : NO$_{3}$ $^{–}$ molar ratios during both base flow and storm flow conditions at three stream locations along a longitudinal gradient of increased riparian forest coverage. Further, we performed an event analysis to examine the hydroclimatic conditions that favour the transfer of DOC and NO$_{3}$ $^{–}$ from riparian soils to the stream during storms. Stream DOC and NO$_{3}$ $^{–}$ concentrations were generally low (overall averages ± SD were 1.0 ± 0.6 mg C L $^{–1}$ and 0.20 ± 0.09 mg N L $^{–1}$), although significantly higher during storm flow compared to base flow conditions in all three stream sites. Optimal DOC : NO$_{3}$ $^{–}$ stoichiometry for stream heterotrophic microorganisms (corresponding to DOC : NO$_{3}$ $^{–}$ molar ratios between 4.8 and 11.7) was prevalent at the midstream and downstream sites under both flow conditions, whereas C-limited conditions were prevalent at the upstream site, which had no surrounding riparian forest. The hydroclimatic analysis of storms suggested that large and medium storm events display a distinct mechanism of DOC and NO$_{3}$ $^{–}$ mobilization. In comparison to large storms, medium storm events showed limited hydrological responses that led to significantly lower increases in stream DOC and NO$_{3}$ $^{–}$ concentrations. During large storm events, different patterns of DOC and NO$_{3}$ $^{–}$ mobilization arise, depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of the riparian groundwater table, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO$_{3}$ $^{–}$ concentrations compared to large storm events preceded by wet conditions. Our results suggest that (i) increased supply of limited resources during storms can potentially sustain in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were overcome downstream, likely due to higher C inputs from riparian forests present at lower elevations. The contrasting spatiotemporal patterns in DOC and NO$_{3}$ $^{–}$ availability and DOC : NO$_{3}$ $^{–}$ stoichiometry observed at the studied stream suggest that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments

Differences in ammonium oxidizer abundance and N uptake capacity between epilithic and epipsammic biofilms in an urban stream

The capacity of stream biofilms to transform and assimilate N in highly N-loaded streams is essential to guarantee the water quality of freshwater resources in urbanized areas. However, the degree of N saturation experienced by urban streams and their response to acute increases in N concentration are largely unknown. We measured changes in the rates of NH4+ uptake (UNH4) and oxidation (UAO) resulting from experimental increases in NH4+-N concentration in mature biofilms growing downstream of a wastewater treatment plant (WWTP) and, thus, naturally exposed to high N concentration. We investigated the responses of UNH4 and UAO to NH4+-N increases and the abundance of NH4+ oxidizing bacteria and archaea (AOB and AOA) in epilithic and epipsammic biofilms. UNH4 and UAO increased with increasing NH4+-N concentration for the 2 biofilm types, suggesting no N saturation under ambient levels of NH4+-N. Thus, these biofilms can contribute to mitigating N excesses and the variability of NH4+-N concentrations from WWTP effluent inputs. The 2 biofilm types exhibited different Michaelis-Menten kinetics, indicating different capacity to respond to acute increases in NH4+-N concentration. Mean UNH4 and UAO were 5× higher in epilithic than epipsammic biofilms, coinciding with a higher abundance of AOA+AOB in the former than in the later (76 × 104 vs 14 × 104 copies/cm2). AOB derived from active sludge dominated in epilithic biofilms, so our results suggest that WWTP effluents can strongly influence in-stream NH4+ processing rates by increasing N inputs and by supplying AOA+AOB that are able to colonize some stream habitat

Effects of riparian vegetation removal on nutrient retention in a Mediterranean stream

We examined the effects of riparian vegetation removal on algal dynamics and stream nutrient retention efficiency by comparing NH4-N and PO4-P uptake lengths from a logged and an unlogged reach in Riera Major, a forested Mediterranean stream in northeastern Spain. From June to September 1995, we executed 6 short-term additions of N (as NH4Cl) and P (as Na2HPO4) in a 200-m section to measure nutrient uptake lengths. The study site included 2 clearly differentiated reaches in terms of canopy cover by riparian trees: the first 100 m were completely logged (i.e., the logged reach) and the remaining 100 m were left intact (i.e., the shaded reach). Trees were removed from the banks of the logged reach in the winter previous to our sampling. In the shaded reach, riparian vegetation was dominated by alders (Alnus glutinosa). The study was conducted during summer and fall months when differences in light availability between the 2 reaches were greatest because of forest canopy conditions. Algal biomass and % of stream surface covered by algae were higher in the logged than in the shaded reach, indicating that logging had a stimulatory effect on algae in the stream. Overall, nutrient retention efficiency was higher (i.e., shorter uptake lengths) in the logged than in the shaded reach, especially for PO4-P. Despite a greater increase in PO4-P retention efficiency relative to that of NH4-N following logging, retention efficiency for NH4-N was higher than for PO4-P in both study reaches. The PO4-P mass-transfer coefficient was correlated with primary production in both study reaches, indicating that algal activity plays an important role in controlling PO4-P dynamics in this stream. In contrast, the NH4-N mass-transfer coefficient showed a positive relation-ship only with % of algal coverage in the logged reach, and was not correlated with any algal-related parameter in the shaded reach. The lack of correlation with algal production suggests that mechanisms other than algal activity (i.e., microbial heterotrophic processes or abiotic mechanisms) may also influence NH4-N retention in this stream. Overall, this study shows that logging disturbances in small shaded streams may alter in-stream ecological features that lead to changes in stream nutrient retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical processes with implications for stream ecosystem functioning

Biofilm growth and nitrogen uptake responses to increases in nitrate and ammonium availability

Nitrate (NO3 −) and ammonium (NH4 +) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO3 −:NH4 + ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO3 − or NH4 +. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO3 − and +NH4 +) in five streams. Biomass-specific uptake rates (U spec ) of NO3 − and NH4 + were then measured using in situ additions of 15N-labeled NO3 − and NH4 +. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U spec -NO3 − of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. U spec -NO3 − was one order of magnitude greater and more variable than U spec -NH4 +, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO3 − and +NH4 +) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and U spec -NO3 − and for the +NH4 + than for the +NO3 − treatments. In particular, enrichment with NH4 + reduced biofilm U spec -NO3 − uptake, which has important implications for N cycling in high NH4 + streams

A round-trip ticket: the importance of release processes for in-stream nutrient spiraling

Most nutrient-spiraling studies have focused on estimates of gross uptake (Ugross), which show that streams take up dissolved inorganic nutrients very efficiently. However, studies based on estimates of net uptake (Unet) emphasize that streams tend to be at biogeochemical steady state (i.e., Unet ≈ 0), at least on a time scale of hours. These findings suggest that streams can be highly reactive ecosystems but remain at short-term biogeochemical steady state if Ugross is counterbalanced by release (R), a process that remains widely unexplored. Here, we propose a novel approach to infer R by comparing Unet and Ugross estimated from ambient and plateau concentrations obtained from standard short-term nutrient additions along a reach. We used this approach to examine the temporal variation of R and its balance with Ugross in 2 streams with contrasting hydrological regime (i.e., perennial vs intermittent) during 2 years. We focused on the spiraling metrics of NH4+ and soluble reactive P (SRP), essential sources of N and P in stream ecosystems. R differed substantially between the 2 streams. The perennial stream had a higher proportion of dates with R > 0 and a 2× higher mean R than the intermittent stream for both nutrients. Despite these differences, the magnitude of R and Ugross tended to be similar for both nutrients within each stream, which lead to Unet ≈ 0 in most cases. A notable exception occurred for SRP in the intermittent stream, where R tended to be higher than Ugross during most of the winter period, probably because of desorption of P from stream sediments. Together, our findings shed light on the contribution of release processes to the dynamics of nutrient spiraling and support the idea that streams can be active ecosystems with high spiraling fluxes while simultaneously approaching short-term biogeochemical steady-state

In-stream net uptake regulates inorganic nitrogen export from catchments under base flow conditions

We aimed to investigate the temporal variation of in‐stream net dissolved inorganic nitrogen (DIN) areal uptake rates (UDIN, in μg N m−2 min−1) and its implications on regulating catchment N export, under base flow conditions. To do so, we estimated UDINfrom longitudinal profiles of ambient DIN concentration (nitrate + ammonium) in two streams on a monthly basis during two hydrological years (n = 45). We found that in‐stream DIN uptake and release did not offset each other (UDIN ≠ 0) in half of the dates, and that UDIN> 0 occurred mostly in autumn. Based on these reach‐scale uptake rates, we performed empirical calculations and model simulations to assess the potential of stream network DIN retention to regulate DIN export from catchments on an annual scale. The empirical approach consisted in up‐scalingUDIN by means of a dynamic stream network analysis that considered temporal and spatial variation of UDIN. The modeling approach consisted in applying different scenarios with the INCA model based on the natural range of empirical UDIN values. Our results showed that the contribution of stream network DIN retention to catchment DIN export increased when calculations accounted for the temporal variation of UDIN. Both approaches suggested that stream network DIN retention can significantly reduce DIN export from headwater catchments under base flow conditions (from 4% to 38%)