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Heterotrophic organisms in streams use dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) from the water column to meet their growth and energy requirements. However, the role of DOC availability in driving DIN uptake in headwater streams is still poorly understood. In this study, we focus on how DOC:DIN stoichiometry and DOC bioreactivity control ammonium (NH4+) uptake and heterotrophic aerobic respiration, and how this influence varies among seasons in a forested Mediterranean headwater stream.We estimated in‐stream NH4+ uptake rates seasonally by conducting whole‐reach constant‐rate additions of NH4+ with and without amendments of either lignin (recalcitrant DOC) or acetate (labile DOC). During each addition, we characterised microbial community composition by molecular analyses, stream metabolism with the single‐station method, and heterotrophic aerobic respiration by adding a metabolic tracer (resazurin).The stream was heterotrophic (net ecosystem production <0) regardless of the season, with a microbial community mostly composed of heterotrophic bacteria. In‐stream NH4+ uptake rates were not related to either background NH4+ or DOC concentrations. Instead, these rates increased with increasing the molar ratio of NH4+ to nitrate (NO3−) (NH4+:NO3−) and DOC to DIN (DOC:DIN).Whole‐reach heterotrophic aerobic respiration rates showed the same relationship against stoichiometric ratios as NH4+ uptake rates. Furthermore, in‐stream NH4+ uptake rates were from 5% to >800% higher during the co‐additions of acetate than when adding NH4+ either alone or with lignin.Our results indicate that in‐stream NH4+ uptake was largely controlled by heterotrophic bacteria, and that the stoichiometric balance between organic resources and nutrients was key to explaining the variability of in‐stream NH4+ uptake and heterotrophic aerobic respiration. Moreover, the observed increase in NH4+ uptake during acetate additions suggests that heterotrophic activity was limited by labile DOC availability.Our study highlights that both DOC:DIN stoichiometry and DOC bioreactivity are relevant factors driving the seasonal pattern of in‐stream N processing in this forested Mediterranean headwater stream. [ABSTRACT FROM AUTHOR]

A longstanding challenge in stream ecology is to understand how landscape configuration organizes spatial patterns of ecosystem function via lateral groundwater connections. We combined laboratory bioassays and field additions of a metabolic tracer (resazurin) to test how groundwater‐stream confluences, or "discrete riparian inflow points" (DRIPs), regulate heterotrophic microbial activity along a boreal stream. We hypothesized that DRIPs shape spatial patterns and rates of aquatic heterotrophic microbial activity by supplying labile dissolved organic matter (DOM) to streams. Laboratory bioassays showed that the potential influence of DRIPs on heterotrophic activity varied spatially and temporally, and was related to their DOM content and composition. At the reach scale, DRIP‐stream confluences elevated the spatial heterogeneity and whole‐reach rates of heterotrophic activity, especially during periods of high land–water hydrological connectivity. Collectively, our results show how the arrangement of lateral groundwater connections influence heterotrophic activity in streams with implications for watershed biogeochemical cycles. [ABSTRACT FROM AUTHOR]

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:NO3- 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:NO3- stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO 3- concentrations and DOC:NO3- 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:NO3- stoichiometry for stream heterotrophic microorganisms (corresponding to DOC:NO3- 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:NO3- 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. [ABSTRACT FROM AUTHOR]

Headwater streams are control points for carbon dioxide (CO2) emissions to the atmosphere, with relative contributions to CO2 emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in‐stream metabolic processes. We analyzed continuous measurements of stream dissolved CO2 and oxygen (O2) concentrations during spring and early summer in two Mediterranean headwater streams from which we evaluated the contribution of in‐stream net ecosystem production (NEP) to CO2 emission. The two streams exhibited contrasting hydrological regimes: one was non‐perennial with relatively small groundwater inflows, while the other was perennial and received significant lateral groundwater inputs. The non‐perennial stream exhibited strong inverse coupling between instantaneous and daily CO2 and O2 concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO2–O2 relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. Mean NEP contribution to CO2 emission was 51% and 57% at the non‐perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO2 concentration, in‐stream CO2 production consistently and substantially contributed to total atmospheric CO2 flux in both streams. We conclude that in‐stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed. [ABSTRACT FROM AUTHOR]

Plastics, especially microplastics (<5 mm in length), are anthropogenic polymer particles that have been detected in almost all environments. Microplastics are extremely persistent pollutants and act as long-lasting reactive surfaces for additives, organic matter, and toxic substances. Biofilms are microbial assemblages that act as a sink for particulate matter, including microplastics. They are ubiquitous in freshwater ecosystems and provide key services that promote biodiversity and help sustain ecosystem function. Here, we provide a conceptual framework to describe the transient storage of microplastics in fluvial biofilm and develop hypotheses to help explain how microplastics and biofilms interact in fluvial ecosystems. We identify lines of future research that need to be addressed to better manage microplastics and biofilms, including how the sorption and desorption of environmental contaminants in microplastics affect biofilms and how microbial exchange between microplastics and the biofilm matrix affects biofilm characteristics like antibiotic resistance, speciation, biodiversity, species composition, and function. We also address the uptake mechanisms of microplastics by consumers and their propagation through the food web. [ABSTRACT FROM AUTHOR]

Wastewater treatment plant (WWTP) effluents alter water chemistry and in-stream nutrient uptake rates of receiving freshwaters, thus changing the magnitude and fate of the nutrients exported. In Mediterranean regions, the dilution capacity of receiving streams can vary strongly over time due to the seasonal occurrence of floods and droughts, causing temporal variability of nutrient uptake. We assessed the temporal patterns and the controlling factors of net nutrient uptake in an intermittent Mediterranean stream receiving WWTP effluent inputs. We compiled the longitudinal concentration profiles of ambient dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) along a 800 m reach on 47 sampling dates between 2001 and 2017, encompassing a wide range of hydrological conditions. We estimated net nutrient uptake in the receiving stream. In 72% of the dates, high rates of net ammonium uptake co-occurred with net releases of either nitrate or nitrite. This pattern suggests that the receiving stream has a high nitrification capacity. Conversely, 75% of the dates did not show any longitudinal pattern in SRP concentration, suggesting that uptake and release processes for this element were either counterbalanced or both occurred at very low rates. Finally, net ammonium uptake was low when the stream had a low dilution capacity (< 40%) and ammonium concentration was high. Overall, we demonstrate that consideration of the receiving stream's dilution capacity is imperative to the management of freshwaters to guarantee an adequate dilution of WWTP effluent inputs and avoid saturation of in-stream nutrient uptake capacity under low flow conditions in urban landscapes. [ABSTRACT FROM AUTHOR]

Cessation of cattle grazing has resulted in the reestablishment of wetlands in some streams of the U.S. Southwest. Decades of cattle grazing prevented vascular plant growth in Sycamore Creek (Arizona, U.S.A.), resulting in stream reaches dominated by diatoms and filamentous green algae. Establishment of vascular plants can profoundly modify ecosystem processes; yet, the effects on nitrogen (N) cycling remain unexplored. We examined the consequences of this ecosystem state shift on N cycling in this N‐limited desert stream. We compared results from whole‐reach ammonium‐N stable isotope (15NH4+) tracer additions conducted before (pre‐wetland state) and 13 yr after (wetland state) free‐range cattle removal from the watershed. Water column estimations showed that in‐stream N uptake and storage were higher in the pre‐wetland than in the wetland state. N turnover was also higher in the pre‐wetland state, indicating that assimilated N was retained for shorter time in stream biomass. In addition, N uptake was mostly driven by assimilatory uptake regardless of the ecosystem state considered. Water column trends were mechanistically explained by the fact that the dominant primary uptake compartments in the pre‐wetland state (i.e., algae and diatoms) had higher assimilatory uptake and turnover rates than those in the wetland state (i.e., vascular plants). Overall, results show that the shift in the composition and dominance of primary producers induced by the cessation of cattle grazing within the stream‐riparian corridor changes in‐stream N processing from a dominance of intense and fast N recycling to a prevalence of N retention in biomass of primary producers. [ABSTRACT FROM AUTHOR]

Climate change predictions for Southern Europe indicate an increase in the frequency and duration of summer low flows and water scarcity in streams. Droughts can cause substantial changes in aquatic communities and biogeochemical processes because they modify stream environmental conditions. Among the many physical, chemical, and biological variables that influence stream metabolism, nutrients may affect algae and heterotrophic microorganisms, and nutrient concentrations may vary with reduction in water discharge. We experimentally manipulated stream discharge and used a before–after control–impact paired design to assess how reduction of stream discharge affected metabolism (i.e., ecosystem respiration [ER] and gross primary production [GPP]). The study was done in 2 lowland forested streams in northwestern Spain with contrasting nutrient concentrations (i.e., mesotrophic: Caselas stream; eutrophic: Pego stream). In the 2 streams, metabolism was estimated before and after discharge manipulation. Prior to discharge reduction, the 2 streams were heterotrophic (i.e., GPP∶ER < 1), and GPP and ER were related in the eutrophic Pego. Discharge reduction increased GPP at the impacted reaches of the mesotrophic Caselas and decoupled GPP and ER at the Pego. An information-theoretic approach was used to assess which combinations of physical, chemical, and biological variables were most important to explain the variation in ER and GPP under the different hydrologic conditions. The observed differences between the 2 streams suggest that the metabolic response to stream drought can be modulated by the interplay between nutrient availability and the density of consumers feeding on resources, among other environmental variables. We show that longer summer periods and reduced stream discharge have the potential to increase daily hypoxia in nutrient-rich lowland stream ecosystems. Increased hypoxia can threaten biodiversity of stream ecosystems and reduce consumer pressure on basal instream resources, such as algae, bacteria, and fungi, which may, in turn, favor instream GPP. In addition, drying conditions seemed to favor autotrophic over heterotrophic activity under moderate nutrient availability (i.e., increase in the GPP∶ER ratio in the impacted Caselas reaches). Therefore, drought conditions not only affect overall daily rates of metabolic activity, but also the relative importance of different energy sources and organic matter for instream function. Our results show that discharge reduction caused by increased water scarcity, due to both climate change and water demand by human activities, can influence energy flow and organic matter dynamics through effects on the metabolism of forested lowland streams. [ABSTRACT FROM AUTHOR]

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface‐subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio‐temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local‐to‐regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies. Key Points: The complexity of HZ rivers and controls exceeds the range of conditions considered in previous conceptualizations and model formulationsUnderstanding organizational principles of hyporheic exchange flow (HEF) and biogeochemical cycling in landscapes is key for generalizing process knowledgeLocal HEF, metabolism, and biogeochemical cycling are embedded within the larger context of fluvial and terrestrial ecosystem structure [ABSTRACT FROM AUTHOR]

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 determining dissolved organic carbon (DOC) and nitrate (NO3-) concentrations and DOC:NO3- molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO3- supply and DOC:NO3- stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO3- concentrations, and DOC:NO3- 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 NO3- from riparian soils to the stream during large storms. Stream DOC and NO3- concentrations were generally low (overall average ± SD was 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:NO3- stoichiometry for stream heterotrophic microorganisms (corresponding to DOC:NO3- 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 large storm events highlighted different patterns of DOC and NO3- mobilization depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of riparian groundwater tables, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO3- concentrations compared to events preceded by wet conditions. These results suggest that (i) increased supply of limited resources during storms can promote in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were gradually overcome downstream, likely due to higher C inputs from riparian forests present at lower elevations. The contrasting spatiotemporal patterns in DOC and NO3-- availability and DOC:NO3- stoichiometry observed at the study stream suggests that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments. [ABSTRACT FROM AUTHOR]

The Tracer Additions for Spiraling Curve Characterization (TASCC) model has been rapidly adopted to interpret in‐stream nutrient spiraling metrics over a range of concentrations from breakthrough curves (BTCs) obtained during pulse solute injection experiments. TASCC analyses often identify hysteresis in the relationship between spiraling metrics and concentration as nutrient concentration in BTCs rises and falls. The mechanisms behind these hysteresis patterns have yet to be determined. We hypothesized that differences in the time a solute is exposed to bioactive environments (i.e., biophysical opportunity) between the rising and falling limbs of BTCs causes hysteresis in TASCCs. We tested this hypothesis using nitrate data from Elkhorn Creek (CO, USA) combined with a process‐based particle‐tracking model representing travel times and transformations along each flow path in the water column and hyporheic zone, from which the bioactive zone comprised only a thin superficial layer. In‐stream nitrate uptake was controlled by hyporheic exchange and the cumulative time nitrate spend in the bioactive layer. This bioactive residence time generally increased from the rising to the falling limb of the BTC, systematically generating hysteresis in the TASCC curves. Hysteresis decreased when nutrient uptake primarily occurred in the water column compared to the hyporheic zone, and with increasing the distance between the injection and sampling points. Hysteresis increased with the depth of the hyporheic bioactive layer. Our results emphasize that good characterization of spatial heterogeneity of surface‐subsurface flow paths and bioactive hot spots within streams is essential to understanding the mechanisms of in‐stream nutrient uptake. Plain Language Summary: Human activities negatively impact water quality by supplying excessive nutrients to streams. To investigate the capacity of streams to take up nutrients from the water column, scientists usually add nutrients to stream reaches, calculate the fraction of added nutrients that is taken up, and identify the environmental conditions controlling nutrient uptake. A common idea is that nutrient uptake increases with increasing water residence time because of increased contact time between solutes and organisms. Yet, water residence time only partially explains the temporal and spatial variability of nutrient uptake, and the reasons behind this variability are still not well understood. We use a mathematical model to test a simple idea: the organisms responsible for the uptake of nitrate, a major form of dissolved nitrogen, are confined within a thin layer in the stream sediments. We define the time spent within this bioactive layer as the "bioactive residence time." Greater bioactive residence time for nitrate corresponds with higher nitrate uptake along the whole reach. Our model successfully reproduces nitrate uptake patterns observed during nitrate addition experiments, suggesting that bioactive residence time is important for nutrient uptake. Our study sheds new light on how streams can improve water quality by decreasing nutrient loads. Key Points: Residence time in the hyporheic bioactive zone is a key control on nitrate uptake in streamsSpatial variability of hyporheic flow and biological activity creates different biophysical opportunities for nitrate removal during downstream transportHysteresis in TASCC curves emerges when biophysical opportunity increases from the rising to the falling limb of the breakthrough curve [ABSTRACT FROM AUTHOR]

Leaf litter can be retained in floodplains for several months before it enters rivers as lateral inputs. During this period, the environmental conditions on the floodplain can alter leaf litter chemistry and, consequently, affect its subsequent processing in the river. We analysed the effect of contrasting floodplain conditions on the chemical composition of leaf litter and its leachates, and how this affected their biodegradability and processing in rivers. To do so, we placed reed leaf litter (Phragmites australis) in open- and closed-canopy habitats of three floodplain sites with contrasting climates (semiarid Mediterranean, humid Mediterranean and continental) for 105 days. We then used litterbags in a river to examine the decomposition of preconditioned leaf litter in comparison with a control (non-preconditioned litter), and laboratory assays to examine the biodegradation of their leachates. Contrasting conditions on the floodplain prompted differences in the nutrient content of leaf litter among floodplain sites. Preconditioning caused a generalized decline in the C content and an increase in the lignin content of leaf litter. Even so, preconditioning did not affect litter decomposition rates in the river, although it did reduce decomposition efficiency and biodegradability of leachates. Shredder colonization of litter was variable and generally higher on preconditioned litter, but not significantly so. Different floodplain conditions had no influence on the aquatic processing of preconditioned litter. Our results demonstrate that the retention of leaf litter in terrestrial environments can affect C budgets of fluvial ecosystems and the recipient food web by reducing the input and the biodegradability of C and nutrients. [ABSTRACT FROM AUTHOR]

The release of soluble compounds (i.e., leachates) from allochthonous coarse particulate organic matter (CPOM) into stream water can be an important source of dissolved organic matter (DOM) and nutrients for instream microbial activity. Here, we assessed variability in the chemistry and the composition of DOM of leachates from different CPOM sources and examined how these characteristics influenced the activity of microbial assemblages in streams. Specifically, we considered leaf litter from 6 tree species that are broadly distributed in riparian zones of the Mediterranean region, a mixture of fruits from some of these species, and a mixture of twigs. We analyzed the leachates from each CPOM source for the concentration of dissolved forms of carbon, nitrogen, and phosphorus, as well as for the composition of DOM based on optical indices associated with the degree of aromaticity and molecular weight. Under laboratory conditions, we used the Resazurin–Resorufin system to estimate the microbial metabolic activity associated with each leachate type. Microbial metabolic activity varied among leachates from different CPOM sources and was positively related to the degree of aromaticity and the NO3– concentration of leachates. Hence, certain types of riparian CPOM inputs can constitute sources of high-quality DOM and dissolved nutrients for instream microbial assemblages. Thus, management of riparian vegetation should consider variation in leachate properties among plant species, as they can influence dissolved organic carbon and nutrient dynamics and heterotrophic activity in stream ecosystems. [ABSTRACT FROM AUTHOR]

Addition of isotopically labeled dissolved forms of nitrogen (N) and carbon (C) in streams has revealed that microbes associated with leaf litter can assimilate solutes from the water column. However, whether this assimilation varies among leaf litter species and at different decomposition stages is still largely unknown. Here, we explore differences in the uptake of NH4‐N and acetate from water column among leaf litter of five riparian tree species at five different times of decomposition. To do so, we conducted a 24 h addition of 15N‐NH4 and 13C‐acetate at constant rate to estimate the microbial uptake of these solutes from the water column. We measured microbial cellobiohydrolase activity and decomposition rates (k) associated with each leaf litter type to examine their influence on NH4‐N and acetate uptake. We found that mass specific N‐uptake (μg N mg N−1 d−1) was relatively constant during leaf litter decomposition whereas mass specific C‐uptake (mg C g C−1 d−1) increased over time. Among leaf litter species, cumulative values of N‐uptake during the study period were negatively related to leaf litter k whereas cumulative values of C‐uptake were positively related to k. Our study indicates that leaf litter species and their decomposition stage are important for understanding the reliance of leaf‐associated microbes on dissolved inorganic nitrogen and dissolved organic carbon from the water column. [ABSTRACT FROM AUTHOR]

In-stream nutrient retention is an important ecosystem function because it can regulate nutrient fate and export to downstream ecosystems. Temporal variation in nutrient retention in streams has been studied extensively at the annual and seasonal scale but less thoroughly at the diel scale. However, understanding temporal variability in nutrient uptake at the diel scale can increase understanding of the role of photoautotrophic primary production on nutrient uptake in streams, especially open-canopy streams. We hypothesized that nutrient retention mostly depends on autotrophic demand in open-canopy streams and that it varies following the diel pattern of gross primary production (GPP). We therefore evaluated the temporal variation in phosphate (PO43−) and ammonium (NH4+) uptake at a daily scale in a highly-productive Pampean stream that is dominated by a dense assemblage of macrophytes and filamentous algae. We conducted 6 slug additions of PO43− and NH4+ over a 24-h period and quantified reach-scale nutrient uptake concurrently with measurements of whole-stream metabolism and chemical variables during additions (including nitrates and nitrites). The study stream had extremely high uptake of PO43− and NH4+ (>90 and >75% retention of the P and N mass added, respectively). Uptake of PO43− did not vary throughout the day. Estimated PO43− uptake from GPP accounted for only a small fraction of observed PO43− uptake. Thus, another mechanism, such as heterotrophic demand by microbial assemblages or adsorption onto sediments, could also have contributed to PO43− uptake in the study stream. In contrast, NH4+ uptake clearly varied throughout the day. Up to 48% of the observed NH4+ uptake rate could be explained by NH4+ estimated from GPP, and NH4+ demand was positively associated with GPP, indicating a high dependence on photoautotrophic demand. An increase of nitrite (NO2−) concentration during additions (representing up to 70% of the added mass of NH4+) suggests that nitrification contributed to the diel pattern of NH4+ uptake. Our results indicate that nutrient uptake does not always rely on autotrophic demand in open-canopy streams and that other abiotic and dissimilatory mechanisms may explain the diel patterns of nutrient retention. In addition, our study highlights the need to measure uptake metrics throughout the day to obtain an accurate estimate of nutrient retention on a daily scale. [ABSTRACT FROM AUTHOR]

Stream hydro‐morphology refers to the heterogeneous distribution of hydrologic conditions that occur above a complex benthic surface such as a streambed.We hypothesised that hydro‐morphological conditions will influence the retention, re‐distribution, and microbial‐driven decomposition of leaf litter inputs in stream ecosystems because each process varies with overlying water velocity.We tested this hypothesis using: (1) the spatial distribution of water velocity within a stream reach as a surrogate of stream hydro‐morphology; (2) leaf tracer (i.e. Ginkgo biloba L.) additions with serial recovery to examine the relationship between benthic retention and overlying velocity; and (3) measurements of leaf litter decomposition (i.e. Alnus glutinosa [L.] Gaertn.) under different water velocity conditions.Results demonstrate that water velocity exerts a significant influence on the retention and re‐distribution of leaf litter inputs within the reach. The observed range of water velocity (from c. 0 to 0.92 cm/s) also strongly influences the range of leaf litter decomposition rates (0.0076–0.0222/day).Our findings illustrate that water velocity influences leaf litter dynamics in streams by controlling leaf litter transport, retention and re‐distribution as well as how leaves decompose within recipient stream reaches. Ultimately, the results show that the efficiency of leaf litter inputs in supporting stream ecosystem function is dependent on the hydro‐morphological characteristics of the receiving stream ecosystems. [ABSTRACT FROM AUTHOR]

Robinia pseudoacacia L. occupies large areas of Mediterranean riparian zones of the northeast of the Iberian Peninsula. This study investigates the influence of the invasive alien nitrogen-fixing R. pseudoacacia on leaf litter nitrogen (N) inputs and soil N availability in a mixed riparian forest in NE Spain. We measured annual leaf litter N inputs, decomposition rates, soil N processes, and soil N concentrations at three sections (near-stream, intermediate, and hillslope) across a riparian forested zone. Moreover, we explored changes in soil N availability associated with the presence of R. pseudoacacia by means of an empirical forest floor model. Leaf litter N content was higher for R. pseudoacacia than for the native non-fixing species. Although the contribution of R. pseudoacacia to annual leaf litter N inputs increased from the near-stream to the hillslope section, soil N mineralization, nitrification, and N availability were similar among sections. Simulations suggest that soil N availability was higher at the near-stream than at the hillslope section without the presence of R. pseudoacacia. However, this pattern smoothed down as R. pseudoacacia spread across the riparian forest. Overall, our results suggest that the spreading of R. pseudoacacia across the riparian zone contributed to homogenize soil N availability over time, and highlight that an integrated spatiotemporal view of the invasive process is needed to assess its impact on soil N biogeochemistry. [ABSTRACT FROM AUTHOR]

A comprehensive set of measurements and calculated metrics describing physical, chemical, and biological conditions in the river corridor is presented. These data were collected in a catchment-wide, synoptic campaign in Lookout Creek within the H.J. Andrews Experimental Forest (Cascade Mountains, Oregon, USA) in summer 2016 during low discharge conditions. Extensive characterization of 62 sites including surface water, hyporheic water, and streambed sediment was conducted spanning 1st through 5th order reaches in the river network. The objective of the sample design and data acquisition was to generate a novel data set to support scaling of river corridor processes across varying flows and morphologic forms present in a river network. The data are available at http://www.hydroshare.org/resource/f4484e0703f743c696c2e1f209abb842 (Ward, 2019). [ABSTRACT FROM AUTHOR]

Streams are important sources of carbon to the atmosphere, though knowing whether they merely outgas terrestrially derived carbon dioxide or mineralize terrestrial inputs of dissolved organic matter (DOM) is still a big challenge in ecology. The objective of this study was to investigate the influence of riparian groundwater (GW) and instream processes on the temporal pattern of stream DOM concentrations and quality in a forested headwater stream, and whether this influence differed between the leaf litter fall (LLF) period and the remaining part of the year (non-LLF). The spectroscopic indexes (fluorescence index, biological index, humification index, and parallel factor analysis components) indicated that DOM had an eminently proteinlike character and was most likely originated from microbial sources and recent biological activity in both stream water and riparian GW. However, paired samples of stream water and riparian GW showed that dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as the spectroscopic character of DOM differed between the two compartments throughout the year. A simple mass balance approach indicated that in-stream processes along the reach contributed to reducing DOC and DON fluxes by 50 and 30 %, respectively. Further, in-stream DOC and DON uptakes were unrelated to each other, suggesting that these two compounds underwent different biogeochemical pathways. During the LLF period, stream DOC and DOC:DON ratios were higher than during the non-LLF period, and spectroscopic indexes suggested a major influence of terrestrial vegetation on stream DOM. Our study highlights that stream DOM is not merely a reflection of riparian GW entering the stream and that headwater streams have the capacity to internally produce, transform, and consume DOM. [ABSTRACT FROM AUTHOR]

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 habitats. [ABSTRACT FROM AUTHOR]

Abstract: In‐stream nutrient retention results from the interaction between hydrological and biogeochemical processes involved in downstream transport. While hydrological processes set the opportunity for nutrient retention to occur, metabolic activity and abiotic processes determine the potential biogeochemical reactivity of streams. Yet, a comprehensive assessment of the relevance of hydrological opportunity versus biogeochemical reactivity on the variability of nutrient retention across streams is still missing. We compiled an extensive data set of existing studies on nutrient retention for ammonium, nitrate, and soluble reactive phosphorus to explore how variability in hydrological opportunity and biogeochemical reactivity explain nutrient retention. We quantified the relative contribution of hydrological opportunity and biogeochemical reactivity to the observed variability in stream nutrient retention using a linearization of the retention equation, which allows for an exact partitioning of the variance associated with residence time and nutrient uptake rate. Finally, we explored potential patterns of nutrient retention along the river network resulting from the interaction between hydrological opportunity and biogeochemical reactivity. Our results show that biogeochemical reactivity has a more relevant role on nutrient retention variability than previously thought, explaining over 66% of the variability in nutrient retention. Among the studied nutrients, retention variability of ammonium was the most subjected to biogeochemical reactivity controls. Furthermore, our results provide insights on controls of longitudinal patterns of nutrient retention along river networks, indicating that retention per unit length for the three nutrients will most likely decrease from headwaters to river mouth because of a decrease in water residence time. [ABSTRACT FROM AUTHOR]

The present study aims to understand how microbial decomposition of leaf litter from two riparian tree species differing in their quality varies among streams covering a gradient of nutrient concentrations. We incubated leaf litter from alder ( Alnus glutinosa) and sycamore (Platanus × hispanica) in 3 streams with low human pressure and 2 streams influenced by wastewater treatment plant effluents. We quantified leaf litter decomposition rates ( k) and examined the temporal changes in the leaf litter concentrations of carbon (C) and nitrogen (N) throughout the incubation period. We measured the extracellular enzyme activities involved in degradation of C (i.e., cellobiohydrolase) and organic phosphorus (i.e., phosphatase). Results showed that alder k decreased with increasing nutrient concentrations, while sycamore decomposed similarly among streams. For both species, leaf litter N concentrations were positively related to in-stream dissolved N concentrations. However, we found different temporal patterns of leaf litter N concentrations between species. Finally, we found relevant differences in the enzymatic activities associated to each leaf litter species across the nutrient gradient. These results suggest that the intrinsic characteristics of the leaf litter resources may play a relevant role on the microbially driven leaf litter decomposition and mediate its response to dissolved nutrient concentrations across streams. [ABSTRACT FROM AUTHOR]

Variability in C and N stable isotopes has been acknowledged to hinder their use as tracers of food sources in the study of trophic interactions in ecosystems. This is particularly so whenever benthic primary production is substantial (variability in δ13C) and the ecosystem under study is affected by human impacts (variability in δ15N) in aquatic ecosystems., In this study, we aim to better understand the large and often unexplained variability in the natural abundance of δ13C and δ15N signatures of aquatic plants by analyzing the isotopic composition of plants from 81 lentic systems from NE Spain in relation to extrinsic (alkalinity, pH, nutrient concentrations, water body typology and basin land use) and intrinsic (functional group, carbon assimilation metabolism, elemental composition) predictors., We have encountered significant plasticity in isotopic signatures of aquatic plants associated with the variation in local conditions at the regional scale. The δ13C signature varied from −43.1‰ to −7.5‰ (35.7‰ range) and drivers were both intrinsic and extrinsic. The functional group was the most important factor as it is influenced by different carbon sources. Aquatic plants with leaves in contact with the atmosphere (helophytes, free floating and floating attached; −34.8‰ to −14.6‰) responded in a similar way as terrestrial plants. This contrasted with the enriched mean values of rooted submerged plants (−16.7‰ to −10.5‰) that were more enriched than the described terrestrial C3 range (−34‰ to −22‰) and completely overlapped the terrestrial C4 range (−20‰ to −8‰). Concentration of DIC and pH also emerged as important extrinsic factors driving δ13C variability., The δ15N signature ranged from −5.2‰ to 20.1‰ (25.2‰ range) and the variability was mostly associated with extrinsic factors such as water body type and basin land use, as they influence both the δ15N signature and concentration of the dissolved inorganic nitrogen in the aquatic ecosystems., Only one multifactorial model including the functional group (with the largest contribution), DIC and pH was selected as the best model explaining the variability in δ13C signatures of aquatic plants. The final model had a relatively large explained deviance and was consistent with the previous unifactorial results. Two different models were selected as the best models explaining variability in δ15N signatures of aquatic plants. The models included the geomorphological type of water body as the variable with the largest contribution, and the percentage of either natural or agricultural coverage in the basin. These results are summarized in a conceptual model showing the predictors and their range and direction of variation., This study shows that extrinsic factors are of greater importance in influencing the stable-isotope signatures of aquatic plants compared to terrestrial plants, because of varied sources and an often limited isotopic discrimination. [ABSTRACT FROM AUTHOR]

More than half of the global river network is composed of intermittent rivers and ephemeral streams (IRES), which are expanding in response to climate change and increasing water demands. After years of obscurity, the science of IRES has bloomed recently and it is being recognised that IRES support a unique and high biodiversity, provide essential ecosystem services and are functionally part of river networks and groundwater systems. However, they still lack protective and adequate management, thereby jeopardizing water resources at the global scale. This Action brings together hydrologists, biogeochemists, ecologists, modellers, environmental economists, social researchers and stakeholders from 14 different countries to develop a research network for synthesising the fragmented, recent knowledge on IRES, improving our understanding of IRES and translating this into a science-based, sustainable management of river networks. Deliverables will be provided through i) research workshops synthesising and addressing key challenges in IRES science, supporting research exchange and educating young researchers, and ii) researcher-stakeholder workshops translating improved knowledge into tangible tools and guidelines for protecting IRES and raising awareness of their importance and value in societal and decision-maker spheres. This Action is organized within six Working Groups to address: (i) the occurrence, distribution and hydrological trends of IRES; (ii) the effects of flow alterations on IRES functions and services; (iii) the interaction of aquatic and terrestrial biogeochemical processes at catchment scale; (iv) the biomonitoring of the ecological status of IRES; (v) synergies in IRES research at the European scale, data assemblage and sharing; (vi) IRES management and advocacy training. [ABSTRACT FROM AUTHOR]

Streams are important sources of carbon to the atmosphere, though whether they merely outgas terrestrially derived carbon dioxide or mineralize terrestrial inputs of dissolved organic matter (DOM) is still a big challenge in ecology. The objective of this study was to investigate the influence of riparian groundwater (GW) and in-stream processes on the temporal pattern of stream DOM concentrations and quality in a forested headwater stream, and whether this influence differed between the leaf litter fall period (LLF) and the remaining part of the year (no-LLF). The spectroscopic indexes (fluorescence index, biological index, humification index, and PARAFAC components) indicated that DOM had an eminently protein-like character and was most likely originated from microbial sources and recent biological activity in both stream water and riparian GW. However, paired samples of stream water and riparian GW showed that dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as the spectroscopic character of DOM differed between the two compartments throughout the year. A simple mass balance approach indicated that in-stream processes along the reach contributed to reduce DOC and DON fluxes by 50 % and 30 %, respectively. Further, in-stream DOC and DON uptake were unrelated to each other, suggesting that these two compounds underwent different biogeochemical pathways. During the LLF period, stream DOC and DOC : DON ratios were higher than during the no-LLF period, and spectroscopic indexes suggested a major influence of terrestrial vegetation on stream DOM. Our study highlights that stream DOM is not merely a reflex of riparian GW entering the stream and that headwater streams have the capacity to internally produce, transform, and consume DOM. [ABSTRACT FROM AUTHOR]

The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often 'hot spots' of ecological, biogeochemical, and hydrological processes and may provide refuge for biota during extreme events. Ecohydrological interfaces can have significant impact on global hydrological and biogeochemical cycles, biodiversity, pollutant removal, and ecosystem resilience to disturbance. The organizational principles (i.e., the drivers and controls) of spatially and temporally variable processes at ecohydrological interfaces are poorly understood and require the integrated analysis of hydrological, biogeochemical, and ecological processes. Our rudimentary understanding of the interactions between different drivers and controls critically limits our ability to predict complex system responses to change. In this paper, we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analyzing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their nonlinear process dynamics, ecohydrological interfaces need to be conceptualized as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems. [ABSTRACT FROM AUTHOR]

Stream hydraulics control flux into and out of slow-moving water transient storage (WTS) zones and, thus, hydrologic retention in stream channels. In-stream nutrient uptake is thought to depend on hydrologic retention, so stream hydraulics could influence the extent to which in-stream nutrient biogeochemistry affects nutrient export downstream. Our goals were to: 1) characterize WTS with an emphasis on water residence time and 2) evaluate its influence on nutrient uptake. We analyzed data from 2 y of monthly solute-tracer injections with accompanying nutrient-uptake estimates in 2 hydrogeomorphically different streams. We fit the conservative tracer breakthrough curves to 2 hydrodynamic models: the one-dimensional transport with inflow and storage (OTIS) and the stochastic mobile-immobile model (SMIM), which allows for a wide distribution of water residence times. The 2 streams differed hydraulically, especially in water residence-time distributions in WTS zones. SMIM parameters depended less on discharge than did OTIS parameters, indicating that SMIM may be influenced more by local features of channel morphology than by hydrologic conditions. NH4+ uptake differed between streams, was correlated with all SMIM hydraulic parameters, and was weakly correlated with only 1 OTIS parameter. Based on SMIM correlations, the parameters related to the exchange of free-flowing water with water storage zones and the in-stream retention times explained 43 and 41%, respectively, of the variation in NH4+ uptake in the streams. Soluble reactive P (SRP) uptake was similar between streams and was not correlated with hydraulic parameters. These results indicate that hydraulics and residence time of water can be important regulators of WTS zones and nutrient uptake in headwater streams, but other environmental factors must be considered for complete understanding of in-stream nutrient processing capacity. [ABSTRACT FROM AUTHOR]

Stream microbial communities and associated processes are influenced by environmental fluctuations that may ultimately dictate nutrient export. Discharge fluctuations caused by intermittent stream flow are increasing worldwide in response to global change. We examined the impact of flow cessation and drying on in-stream nitrogen cycling. We determined archaeal ( AOA) and bacterial ammonia oxidizer ( AOB) abundance and ammonia oxidation activity in surface and deep sediments from different sites along the Fuirosos stream (Spain) subjected to contrasting hydrological conditions (i.e., running water, isolated pools, and dry streambeds). AOA were more abundant than AOB, with no major changes across hydrological conditions or sediment layers. However, ammonia oxidation activity and sediment nitrate content increased with the degree of stream drying, especially in surface sediments. Upscaling of our results shows that ammonia oxidation in dry streambeds can contribute considerably (~50%) to the high nitrate export typically observed in intermittent streams during first-flush events following flow reconnection. Our study illustrates how the dry channels of intermittent streams can be potential hotspots of ammonia oxidation. Consequently, shifts in the duration, spatial extent and severity of intermittent flow can play a decisive role in shaping nitrogen cycling and export along fluvial networks in response to global change. [ABSTRACT FROM AUTHOR]

Dead wood is an important component of natural stream ecosystems, but its abundance has been reduced by human actions. Therefore, in many stream restoration projects dead wood is introduced into stream channels to improve physical habitat, biological communities, and ecosystem functioning. We added dead wood to 4 northern Iberian headwater streams, ranging in mean discharge from 0.026 to 2.5 m3/s, to enhance retention of nutrients, sediments, and organic matter. We placed logs and branches in the stream channels to simulate the amount and orientation found in natural streams and assessed the effects of restoration following a Before-After-Control-Impact design. We measured water and nutrient retention by experimental additions of PO43- and NH4+, with NaCl as a hydrological tracer, and leaf retention by releasing Ginkgo biloba leaves. Introduction of large wood significantly reduced water velocity, especially during high-flow periods, but the overall effects on average nutrient travel distance were not significant. When analyzed individually by stream, effects were significant only for PO43- in 1 stream. Wood addition increased average leaf-litter travel distance overall, but when analyzed individually by stream had no effect in the 2 smaller streams despite the fact that previously reported measures of benthic organic matter increased many fold. This discrepancy between ginkgo measurements and benthic storage raises concerns about the relevance of short-term release measurements and suggests that the effects of wood introduction on nutrient retention might also be stronger than reported here. Overall, wood addition decreased water velocity at all sites, but effects on retention of nutrients and organic matter were variable among sites. Nevertheless, the fact that all statistically significant differences found showed enhanced retention, which can be the basis of important ecosystem services, suggests that wood restoration produces functional benefits beyond the improvement of stream habitat. [ABSTRACT FROM AUTHOR]

Riparian 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. [ABSTRACT FROM AUTHOR]

We investigated changes in respiration across nighttime and daytime in a headwater stream. For this, we conducted consecutive nighttime and daytime experiments injecting the bioreactive tracer resazurin in two reaches with different riparian canopy densities (different levels of photosynthetically active radiation) to compare respiration rate coefficients. We found that even though stream water temperatures measured above the streambed at day and night (half-day timescale) were different within each reach and across reaches (95% confidence level), apparent respiration rate coefficients were not different across nighttime and daytime conditions (95% confidence level). A likely explanation for this is that the bulk of stream respiration takes place in the hyporheic zone, where diel fluctuations of stream temperature and photosynthetically active radiation are considerably attenuated and where temperature is not measured in routine investigations of stream metabolism. Our results suggest that community respiration in headwater streams may not need to be 'corrected' for temperature between daytime and nighttime, even though instantaneous changes in respiration are expected to occur from a pure biological perspective. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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 of patterns and controls of fine-scale stream nitrogen (N) 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 downstream site, the highest day-night variations occurred in early spring, when stream NO3¯ concentrations were 13% higher at night than at daytime. 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 downstream site, no diel variations in NO3¯ concentration occurred at the upstream site, likely because water temperature was colder (10°C vs. 12°C) and light availability was lower (4 vs. 9 mol·m−2·d−1). Although daily GPP was between 10- and 100-fold lower than daily respiration, photoautotrophic N uptake contributed to a 10% reduction in spring NO3¯ loads at the downstream 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. [ABSTRACT FROM AUTHOR]

Nitrate (NO) and ammonium (NH) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO:NH 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 NO or NH. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO and +NH) in five streams. Biomass-specific uptake rates ( U) of NO and NH were then measured using in situ additions of N-labeled NO and NH. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U-NO 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-NO was one order of magnitude greater and more variable than U-NH, 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., +NO and +NH) 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-NO and for the +NH than for the +NO treatments. In particular, enrichment with NH reduced biofilm U-NO uptake, which has important implications for N cycling in high NH streams. [ABSTRACT FROM AUTHOR]

The implications of stream flow intermittency for dissolved organic matter (DOM) are not well understood despite its potential significance for water quality and ecosystem integrity. We combined intensive sampling with liquid chromatography and spectroscopic techniques to follow changes in DOC and DON concentrations as well as in DOM size fractions and spectroscopic properties in a temporary stream during an entire contraction-fragmentation-expansion hydrological cycle. DOC and DON concentrations remained low (range = 1.4-5.2 mg C L and 0.05-0.15 mg N L) during hydrological contraction and fragmentation, with concomitant increases in the proportion of high molecular weight substances (HMWS) during contraction and of DOM aromaticity during fragmentation. DOC and DON concentrations abruptly increased (up to 8.8 mg C L and 0.37 mg N L) at the end of the fragmentation phase, with a concomitant increase in the non-humic, microbial and aquatic character of DOM. Upon rewetting, the DOC and DON concentrations reached their highest values (up to 12.7 mg C L and 0.39 mg N L), with concomitant increases in the proportion of HMWS and in the humic, aromatic and terrestrial character of DOM. Subsequently, DOC and DON concentrations recovered to values similar to those at the contraction phase, while DOM composition variables indicated the prevalence of a DOM of humic and terrestrial character during the whole expansion phase. Overall, our results emphasize the importance of hydrological transitions for DOM dynamics in temporary streams, and point to the potential response of perennial streams under future water scarcity scenarios. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Streams affected by wastewater treatment plant (WWTP) effluents are hotspots of nitrification. We analyzed the influence of WWTP inputs on the abundance, distribution, and composition of epilithic ammonia-oxidizing (AO) assemblages in five Mediterranean urban streams by qPCR and amoA gene cloning and sequencing of both archaea (AOA) and bacteria (AOB). The effluents significantly modified stream chemical parameters, and changes in longitudinal profiles of both NH and NO indicated stimulated nitrification activity. WWTP effluents were an allocthonous source of both AOA, essentially from the Nitrosotalea cluster, and mostly of AOB, mainly Nitrosomonas oligotropha, Nitrosomonas communis, and Nitrosospira spp. changing the relative abundance and the natural composition of AO assemblages. Under natural conditions, Nitrososphaera and Nitrosopumilus AOA dominated AO assemblages, and AOB were barely detected. After the WWTP perturbation, epilithic AOB increased by orders of magnitude whereas AOA did not show quantitative changes but a shift in population composition to dominance of Nitrosotalea spp. The foraneous AOB successfully settled in downstream biofilms and probably carried out most of the nitrification activity. Nitrosotalea were only observed downstream and only in biofilms exposed to either darkness or low irradiance. In addition to other potential environmental limitations for AOA distribution, this result suggests in situ photosensitivity as previously reported for Nitrosotalea under laboratory conditions. [ABSTRACT FROM AUTHOR]

Leaf litter decomposition plays a major role in nutrient dynamics in forested streams. The chemical composition of litter affects its processing by microorganisms, which obtain nutrients from litter and from the water column. The balance of these fluxes is not well known, because they occur simultaneously and thus are difficult to quantify separately. Here, we examined C and N flow from streamwater and leaf litter to microbial biofilms during decomposition. We used isotopically enriched leaves (C and N) from two riparian foundation tree species: fast-decomposing Populus fremontii and slow-decomposing Populus angustifolia, which differed in their concentration of recalcitrant compounds. We adapted the isotope pool dilution method to estimate gross elemental fluxes into litter microbes. Three key findings emerged: litter type strongly affected biomass and stoichiometry of microbial assemblages growing on litter; the proportion of C and N in microorganisms derived from the streamwater, as opposed to the litter, did not differ between litter types, but increased throughout decomposition; gross immobilization of N from the streamwater was higher for P. fremontii compared to P. angustifolia, probably as a consequence of the higher microbial biomass on P. fremontii. In contrast, gross immobilization of C from the streamwater was higher for P. angustifolia, suggesting that dissolved organic C in streamwater was used as an additional energy source by microbial assemblages growing on slow-decomposing litter. These results indicate that biofilms on decomposing litter have specific element requirements driven by litter characteristics, which might have implications for whole-stream nutrient retention. [ABSTRACT FROM AUTHOR]

We examined the relevance of dissolved inorganic nitrogen (DIN) forms (nitrate and ammonium) in stream water as N sources for different macrophyte species. To do this, we investigated the variability and relationships between N natural abundance of DIN forms and of four different macrophyte species in five different streams influenced by inputs from wastewater treatment plants and over time within one of these streams. Results showed that N signatures were similar in species of submersed and amphibious macrophytes and in stream water DIN, whereas N signatures of the riparian species were not. N signatures of macrophytes were generally closer to N signatures of nitrate, regardless of the species considered. Our results showed significant relationships between N signatures of DIN and those of submersed Callitriche stagnalis and amphibious Veronica beccabunga and Apium nodiflorum, suggesting stream water DIN as a relevant N source for these two functional groups. Moreover, results from a mixing model suggested that stream water DIN taken up by the submersed and amphibious species was mostly in the form of nitrate. Together, these results suggest different contribution to in-stream N uptake among the spatially-segregated species of macrophytes. While submersed and amphibious species can contribute to in-stream N uptake by assimilation of DIN, macrophyte species located at stream channel edges do not seem to rely on stream water DIN as an N source. Ultimately, these results add a functional dimension to the current use of macrophytes for the restoration of stream channel morphology, indicating that they can also contribute to reduce excess DIN in streams. [ABSTRACT FROM AUTHOR]

Human activity has significantly increased dissolved inorganic N (DIN) availability and has modified the relative proportion of NO3- and NH4+ species in many streams. Understanding the relationship between DIN concentration and DIN uptake is crucial to predicting how streams will respond to increased DIN loading. Nonetheless, this relationship remains unclear because of the complex interactions governing DIN uptake. We aimed to evaluate how biofilms from 2 streams differing in background DIN concentration would respond to increases in availability and changes in speciation (NO3- or NH4+) of DIN. We measured DIN uptake by biofilms in artificial flumes in each stream, using separate 15N-NO3- and 15N-NH4+ additions in a graded series of increasing DIN concentrations. The ambient uptake rate (U) was higher for NO3- than for NH4+ in both streams, but only U for NH4+ differed between streams. Uptake efficiency (UN-specific) at ambient conditions was higher in the low-N than in the high-N stream for both DIN species. A Michaelis-Menten model of uptake kinetics best fit the relationship between uptake and concentration in the case of NH4+ (for both streams) but not in the case of NO3- (neither stream). Moreover, saturation of NH4+ uptake occurred at lower rates (lower Umax) in the low-N than in the high-N stream, but affinity for NH4+ was higher (lower Ks) in the low-N stream. Together, these results indicate that the response capacity of biofilm communities to short-term increases of DIN concentration is determined primarily by the ambient DIN concentrations under which they develop. Our study also shows that DIN uptake by benthic biofilms varies with DIN availability and with DIN speciation, which often is modified by human activities. [ABSTRACT FROM AUTHOR]

Hydrology is a key factor in the terrestrial landscape that connects upland and riparian ecosystems to streams. Hydrological connectivity through the catchment and along the stream network influences not only the timing and the magnitude of terrestrial inputs to stream ecosystems, but also regulates stream metabolism and biogeochemical cycling. In mediterranean climate regions (med-regions), there is a marked seasonal alternation of dry and wet periods, and streams experience a wide range of hydrological conditions from floods to droughts. Moreover, nutrient dynamics in med-streams are highly variable within the year and between years compared to temperate streams. In this review, we explore how seasonal changes in hydrological connectivity and hydrological extremes affect nutrient cycling and transport at different spatial scales in med-catchments. We show that during the dry period, the stream network contracts more severely in med-catchments than in temperate catchments. During the contraction phase, upland-stream disconnection decreases nutrient supply from terrestrial ecosystems, while stream fragmentation increases the spatial variation of stream nutrient concentrations. Moreover, the precipitation regime typical of med-regions (with irregular distribution of precipitation and episodic large rainfall events) decreases the ability of terrestrial and aquatic biota to retain nutrients and favours nutrient export to downstream ecosystems. Altogether, this hydrological setting confers to med-streams a characteristic temporal pattern in stream nutrient dynamics that is analogous to a particular biogeochemical heartbeat with higher amplitude, variation and unpredictability than that observed in temperate streams. [ABSTRACT FROM AUTHOR]

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 significantly 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. [ABSTRACT FROM AUTHOR]

Effluents from wastewater treatment plants ( WWTPs) containing micro-organisms and residual nitrogen can stimulate nitrification in freshwater streams. We hypothesized that different ammonia-oxidizing ( AOB) and nitrite-oxidizing ( NOB) bacteria present in WWTP effluents differ in their potential to colonize biofilms in the receiving streams. In an experimental approach, we monitored biofilm colonization by nitrifiers in ammonium- or nitrite-fed microcosm flumes after inoculation with activated sludge. In a field study, we compared the nitrifier communities in a full-scale WWTP and in epilithic biofilms downstream of the WWTP outlet. Despite substantially different ammonia concentrations in the microcosms and the stream, the same nitrifiers were detected by fluorescence in situ hybridization in all biofilms. Of the diverse nitrifiers present in the WWTPs, only AOB of the Nitrosomonas oligotropha/ureae lineage and NOB of Nitrospira sublineage I colonized the natural biofilms. Analysis of the amoA gene encoding the alpha subunit of ammonia monooxygenase of AOB revealed seven identical amoA sequence types. Six of these affiliated with the N. oligotropha/ureae lineage and were shared between the WWTP and the stream biofilms, but the other shared sequence type grouped with the N. europaea/eutropha and N. communis lineage. Measured nitrification activities were high in the microcosms and the stream. Our results show that nitrifiers from WWTPs can colonize freshwater biofilms and confirm that WWTP-affected streams are hot spots of nitrification. [ABSTRACT FROM AUTHOR]

Inhibition by light potentially influences the distribution of ammonia oxidizers in aquatic environments and is one explanation for nitrite maxima near the base of the euphotic zone of oceanic waters. Previous studies of photoinhibition have been restricted to bacterial ammonia oxidizers, rather than archaeal ammonia oxidizers, which dominate in marine environments. To compare the photoinhibition of bacterial and archaeal ammonia oxidizers, specific growth rates of two ammonia-oxidizing archaea ( N itrosopumilus maritimus and N itrosotalea devanaterra) and bacteria ( N itrosomonas europaea and N itrosospira multiformis) were determined at different light intensities under continuous illumination and light/dark cycles. All strains were inhibited by continuous illumination at the highest intensity (500 μE m−2 s−1). At lower light intensities, archaeal growth was much more photosensitive than bacterial growth, with greater inhibition at 60 μE m−2 s−1 than at 15 μE m−2 s−1, where bacteria were unaffected. Archaeal ammonia oxidizers were also more sensitive to cycles of 8-h light/16-h darkness at two light intensities (60 and 15 μE m−2 s−1) and, unlike bacterial strains, showed no evidence of recovery during dark phases. The findings provide evidence for niche differentiation in aquatic environments and reduce support for photoinhibition as an explanation of nitrite maxima in the ocean. [ABSTRACT FROM AUTHOR]

Quantification of the transient storage zone (A) has become critical in stream biogeochemical studies addressed to examine factors controlling nutrient uptake. It is expected that higher A may enhance the interaction between nutrients and biota and thus, increase nutrient uptake. However, results from the literature are controversial. We hypothesized that besides of the size of A, the intrinsic physical and biological characteristics of stream structures that generate A are also relevant for nutrient uptake. We performed 24 additions of phosphate, ammonium, and chloride in four reaches of a man-made channel where we introduced three types of naturally colonized substrata packs (mud, sand and cobbles) to modify A. We estimated ammonium and phosphate uptake coefficients in both the main channel and A using a solute transport model (OTIS-P) and compared the results among reaches with different substrata types. The introduction of substrata packs decreased water velocity and increased A similarly among treatments. Nutrient uptake coefficients in the main channel were similar among reaches with different type substrata packs; however, nutrient uptake coefficients measured in A differed among them as well as the ratio between ammonium and phosphorus uptake coefficients in A, which were 1.6 in reaches with mud packs and 0.02 in reaches with sand or cobble packs. Results obtained in this study suggest that the contribution of A in nutrient uptake not only depends on the size of A but on the type of materials used to increase A, and thus, have biogeochemical implications on restoration projects aimed to modify channel morphology. [ABSTRACT FROM AUTHOR]