- Alba Argerich
Le colmatage du lit des cours d’eau a plusieurs conséquences écologiques, parmi lesquelles une altération des échanges nappe-rivière et des flux d’eau et de nutriments circulant dans la zone hyporhéique en réponse à une diminution de la porosité et de la perméabilité des sédiments. Cette altération se traduit par une modification des processus biogéochimiques se déroulant dans le milieu hyporhéique et essentiels à l’auto-épuration des cours d’eau, tels que les couples nitrification-dénitrification ou respirations aérobies/anaérobies, etc. A ce jour, il n’existe aucune relation quantitative ou qualitative entre caractéristiques physique des sédiments (porosité, perméabilité), échanges nappe-rivière et processus biogéochimiques à l’échelle du tronçon. Pourtant, l’échelle tronçon est l’échelle pertinente pour la gestion et restauration des cours d’eau. L’objectif de cette action est de produire ces premières relations à partir d’expérimentations de terrain. Dans un contexte DCE et restauration des cours d’eau, celles-ci seront un premier pas vers la mise au point d’un protocole pour déterminer la capacité auto-épuratrice d’un tronçon de cours d’eau, vers la création d’indicateurs fonctionnels hyporhéiques et la quantification des effets du colmatage sur les processus biogéochimiques.
MEDSOUL aims to contribute to the needs of the prioritized challenge about climate change and the efficiency in resource use by providing scientific knowledge and tools to face the management of freshwater resources under a "One Health" approach. MEDSOUL project takes into account both biogeochemistry, microbiological activity, and climate and land use changes impacts to determine river health and water availability for society.
Research Items (89)
Wastewater treatment plant (WWTP) effluents are sources of dissolved organic carbon (DOC) and inorganic nitrogen (DIN) to receiving streams, which can eventually become saturated by excess of DIN. Aquatic plants (i.e., helophytes) can modify subsurface water flowpaths as well as assimilate nutrients and enhance microbial activity in the rhizosphere, yet their ability to increase DIN transformation and removal in WWTP-influenced streams is poorly understood. We examined the influence of helophytes on DIN removal along subsurface water flowpaths and how this was associated with DOC removal and labile C availability. To do so, we used a set of 12 flow-through flumes fed with water from a WWTP effluent. The flumes contained solely sediments or sediments with helophytes. Presence of helophytes in the flumes enhanced both DIN and DOC removal. Experimental addition of a labile C source into the flumes resulted in a high removal of the added C within the first meter of the flumes. Yet, no concomitant increases in DIN removal were observed. Moreover, results from laboratory assays showed significant increases in the potential denitrifying enzyme activity of sediment biofilms from the flumes when labile C was added; suggesting denitrification was limited by C quality. Together these results suggest that lack of DIN removal response to the labile C addition in flumes was likely because potential increases in denitrification by biofilms from sediments were counterbalanced by high rates of mineralization of dissolved organic matter. Our results highlight that helophytes can enhance DIN removal in streams receiving inputs from WWTP effluents; and thus, they can become a relevant bioremediation tool in WWTP-influenced streams. However, results also suggest that the quality of DOC from the WWTP effluent can influence the N removal capacity of these systems.
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.
Dissolved inorganic nitrogen (DIN) in streams is mostly available as two different species: nitrate (NO3-) and ammonium (NH4+). These two DIN species undergo specific dissimilatory uptake pathways and show distinct preference during biological assimilation. These differences ultimately dictate how DIN is cycled within streams and its further export to downstream ecosystems. Here, we provide a synthesis analysis of the uptake of NO3- and NH4+ at the reach scale and on the contribution of dissimilatory and assimilatory uptake pathways. We combined 15N-tracer experiments in a single stream with compiled results of NO3- and NH4+ uptake from the literature. As expected, streams were more efficient in processing NH4+ than NO3- at the reach scale. These results were partially explained by the fact that, on average, dissimilatory NO3- uptake (i.e., denitrification and DNRA) had a low incidence on total NO3- uptake, whereas dissimilatory NH4+ uptake (i.e., nitrification) contributed to a high proportion of total NH4+ uptake thereby increasing in-stream NO3- concentration. Furthermore, assimilatory uptake by in-stream biotic compartments dominated the total uptake of the two DIN species and was generaly higher for NH4+ than for NO3-. Overall, results from this study indicate that assimilatory uptake by biotic compartments rather than permanent removal dominates total NO3- uptake in streams. In contrast, both assimilatory and dissimilatory uptake can contribute similarly to total NH4+ uptake. Our findings have strong implications for a better understanding of N cycling within the context of widespread increases in DIN concentration and changes in the NO3-:NH4+ ratio driven by human activities.
Efficient NH4⁺ oxidation is a critical issue in human-impaired streams receiving high N loads from the effluent of wastewater treatment plants (WWTP). Archaeal (AOA) and bacterial (AOB) ammonia oxidizers are strongly photoinhibited in laboratory cultures, so we expected that light availability would affect the distribution of AOA and AOB and NH4⁺ oxidation rates at the reach scale. We selected 2 contiguous reaches downstream of a WWTP input in La Tordera river (northeastern Spain) that strongly differed in canopy cover (open and shaded). Against expectations and despite significant differences in light availability, the 2 reaches showed similar abundance of AOA and AOB and similar daily rates of ecosystem respiration, gross primary productivity, and NH4⁺ oxidation. The abundance of ammonia oxidizers was not correlated with biomass in biofilms protected from light, whereas a positive relationship was found for light-exposed biofilms. This result suggests that biomass accrual could provide light protection to ammonia oxidizers in light-exposed biofilms. The contribution of NH4⁺ oxidation to whole-reach NH4⁺ uptake reached up to 89%, indicating a high potential for NH4⁺ oxidation in the 2 reaches. NH4⁺ oxidation rates were similar between night and day, but their contribution to whole-reach NH4⁺ uptake tended to be higher at night than during the day. Altogether, these findings highlight that environmental factors other than irradiance drive reach-scale NH4⁺ oxidation in this urban stream.
Humans affect streams by changing land uses in the catchment, altering stream channel morphology, and delivering nutrients, emergent pollutants and pathogens from urban and industrial activities into the streams. These activities clearly threaten the ecosystem health, but also can constitute a human health risk, since streams and rivers are a key water resource for human activity. In Mediterranean streams, these problems are additionally compounded by the scarcity of water. MEDSOUL is a coordinated project, which aims at contributing to the needs of the prioritized challenge of climate change and the resource use efficiency, by providing scientific knowledge and tools to face the management of freshwater resources. A multidisciplinary and collaborative approach has been implemented, using the combination of expertises from a) forest ecology, catchment modeling, and fluvial hydrology b) in-stream biogeochemistry and aquatic plant ecology and c) microbial indicators and dissemination and persistence of waterborne pathogens in the freshwater environment. Here, using the concept of One-Health approach, we present the Riera de Cànoves catchment as a case study. This catchment has a medium-size 12 km2 and the stream flow is intermitent, which remains dry over three to four months in Summer, and is highly influenced by the Waste Water Treatment Plant from the area. The catchment covers different human activities such as forestry, agriculture, and urban settlements. The goal of this work is the evaluation of blue water’s health under current and different climate change scenarios.
Resumen En la riera de Cànoves, al pie del Parque Natural y Reserva de la Biosfera del Montseny, se está llevando a cabo un proyecto piloto para analizar la influencia de los cambios morfo-hidráulicos en el cauce del río, asociados a distintas técnicas de restauración fluvial, sobre la capacidad de autodepuración de nutrientes de estos ecosistemas. Los resultados de este estudio aportan información sobre los mecanismos que regulan la retención de nutrientes en ríos intermitentes afectados por la entrada de efluentes de estaciones de depuración de aguas residuales urbanas (EDARs). El estudio de estos ríos es importante debido a que las entradas de efluentes de EDARs constituyen el 100% de las cargas de nutrientes que transporta el río durante los periodos de estiaje, empeorando la calidad del agua. Palabras clave: ríos, caudal intermitente, autodepuración de nutrientes, restauración fluvial, técnicas de bioingeniería. Abstract This study focuses on the influence of the morpho-hydraulical changes in river channels on the self-purification capacity of nutrients in intermittent rivers. In particular, we tested different bioengineering techniques at Cànoves stream (NE Catalonia). Since WWTP effluents can constitute 100% of the river nutrient loads during low flow periods, the results of this study will provide information on the mechanisms that regulate nutrient retention capacity in order to improve stream quality.
The poster presents the priliminary results of the experiment that had the aim to examine the temporal development of denitrifying bacterial populations in newly created biofilms and how it is affected by different environmental and habitat specific conditions.
Large-scale factors associated with the environmental context of streams can explain a notable amount of variability in patterns of stream N cycling at the reach scale. However, when environmental factors fail to accurately predict stream responses at the reach level, focusing on emergent properties from small-scale heterogeneity in N cycling rates may help understand observed patterns in stream N cycling. To address how small-scale heterogeneity may contribute to shape patterns in whole-reach N uptake, we examined the drivers and variation in microbial N uptake at small spatial scales in two stream reaches with different environmental constraints (i.e., riparian canopy). Our experimental design was based on two N-15 additions combined with a hierarchical sampling design from reach to microhabitat scales. Regardless of the degree of canopy cover, small-scale heterogeneity of microbial N uptake ranged by three orders of magnitude, and was characterized by a low abundance of highly active microhabitats (i.e., hot spots). The presence of those hot spots of N uptake resulted in a nonlinear spatial distribution of microbial N uptake rates within the streambed, especially in the case of epilithon assemblages. Small-scale heterogeneity in N uptake and turnover rates at the microhabitat scale was primarily driven by power relationships between N cycling rates and stream water velocity. Overall, fine benthic organic matter (FBOM) assemblages responded clearly to changes in the degree of canopy cover, overwhelming small-scale heterogeneity in its N uptake rates, and suggesting that FBOM contribution to whole-reach N uptake was principally imposed by environmental constraints from larger scales. In contrast, N uptake rates by epilithon showed no significant response to different environmental influences, but identical local drivers and spatial variation in each study reach. Therefore, contribution of epilithon assemblages to whole-reach N uptake was mainly associated with emerging properties from small-scale heterogeneity at lower spatial scales.
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 (Uspec) 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 Uspec-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. Uspec-NO3− was one order of magnitude greater and more variable than Uspec-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 Uspec-NO3− and for the +NH4+ than for the +NO3− treatments. In particular, enrichment with NH4+ reduced biofilm Uspec-NO3− uptake, which has important implications for N cycling in high NH4+ streams.
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−1 and 0.05–0.15 mg N L−1) 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−1 and 0.37 mg N L−1) 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−1 and 0.39 mg N L−1), 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.
Headwater streams are recipients of water sources draining through terrestrial ecosystems. At the same time, stream biota can transform and retain nutrients dissolved in stream water. Yet studies considering simultaneously these two sources of variation in stream nutrient chemistry are rare. To fill this gap of knowledge, we analyzed stream water and riparian groundwater concentrations and fluxes as well as in-stream net uptake rates for nitrate (NO3−), ammonium (NH4+), and soluble reactive phosphorus (SRP) along a 3.7 km reach on an annual basis. Chloride concentrations (used as conservative tracer) indicated a strong hydrological connection at the riparian–stream interface. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high in-stream biogeochemical processing. In-stream net nutrient uptake (Fsw) was highly variable across contiguous segments and over time, but its temporal variation was not related to the vegetative period of the riparian forest. For NH4+, the occurrence of Fsw > 0 μg N m−1 s−1 (gross uptake > release) was high along the reach, while for NO3−, the occurrence of Fsw < 0 μg N m−1 s−1 (gross uptake < release) increased along the reach. Within segments and dates, Fsw, whether negative or positive, accounted for a median of 6, 18, and 20% of the inputs of NO3−, NH4+, and SRP, respectively. Whole-reach mass balance calculations indicated that in-stream net uptake reduced stream NH4+ flux up to 90%, while the stream acted mostly as a source of NO3− and SRP. During the dormant period, concentrations decreased along the reach for NO3−, but increased for NH4+ and SRP. During the vegetative period, NH4+ decreased, SRP increased, and NO3− showed a U-shaped pattern along the reach. These longitudinal trends resulted from the combination of hydrological mixing with terrestrial inputs and in-stream nutrient processing. Therefore, the assessment of these two sources of variation in stream water chemistry is crucial to understand the contribution of in-stream processes to stream nutrient dynamics at relevant ecological scales.
In a set of streamside mesocosms, stream ecosystem respiration (ER) increased with biofilm biomass and flow heterogeneity (turbulence) generated by impermeable bedforms, even though those bedforms had no hyporheic exchange. Two streamside flumes with gravel beds (single layer of gravel) were operated in parallel. The first flume had no bedforms, and the second flume had 10-cm-high dune-shaped bedforms with a wavelength of 1.0 m. Ecosystem respiration was measured via resazurin reduction to resorufin in each flume at three different biomass stages during biofilm growth. Results support the hypothesis that ER increases with flow heterogeneity generated by bedforms across all biofilm biomass stages. For the same biofilm biomass, ER was up to 1.9 times larger for a flume with 10-cm-high impermeable bedforms than for a flume without the bedforms. Further, the amount of increase in ER associated with impermeable bedforms was itself increased as biofilms grew. Regardless of bedforms, biofilms increased transient storage by a factor of approximately 4.
We investigated the intrinsic (i.e., metabolic character of autotrophs) and extrinsic (i.e., nutrients and light availability) controls on the variation in autotrophic nitrogen (N) cycling in stream ecosystems based on 15N isotopic incorporation into five autotrophic components (biofilm, filamentous algae, bryophytes, and submerged and emergent macrophytes) differing in structural complexity and metabolic character. Autotrophs from a stream site with depleted 15N signatures and relatively low concentrations of dissolved inorganic N (DIN) were translocated to three reaches downstream of the same stream with higher 15N–DIN signatures and DIN concentrations and different light availability. After the translocation, autotrophs showed an asymptotical increase in their 15N signatures, achieving isotopic equilibrium with the stream water, from which we calculated N uptake and turnover rates for each autotrophic compartment at the three reaches. Uptake rates were highest when both DIN and light availability were also highest. Differences in DIN uptake rates were greater across reaches than among autotrophs, suggesting that autotrophic DIN uptake at both organism and community level is principally controlled by extrinsic factors (e.g., DIN concentration and light incidence). In contrast, variation in N turnover rates was larger among the different autotrophs than among the study reaches, suggesting a stronger control by intrinsic rather than extrinsic factors.
Headwater streams have a strong capacity to transform and retain nutrients, and thus, a longitudinal decrease in stream nutrient concentrations would be expected from in-stream nutrient removal alone. Yet, a number of other factors within the catchment, including biogeochemical processing within the riparian zone and export to streams, can contribute to stream nutrient concentration, which may overcome the effect of in-stream biogeochemical processing. To explore this idea, we analyzed the longitudinal patterns of stream and riparian groundwater concentrations for chloride (Cl−), nitrate (NO3−), ammonium (NH4+), and phosphate (PO43−) along a 3.7 km reach at an annual scale. The reach showed a gradual increase in stream and riparian width, riparian tree basal area, and abundance of riparian N2-fixing tree species. Concentrations of Cl− indicated a~strong hydrological connection at the riparian-stream edge. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high biogeochemical processing at the riparian-stream edge and within the stream. A mass balance approach along the reach indicated that, on average, in-stream net nutrient uptake prevailed over release for NH4+ and PO43−, but not for NO3−. On an annual basis, in-stream processes contributed to change stream input fluxes by 11%, 26%, and 29% for NO3−, NH4+, and PO43−, respectively. Yet, longitudinal trends in concentration were not consistent with the prevailing in-stream biogeochem ical processes. During the riparian dormant period, stream concentration decreased along the reach for NO3−, but increased for NH4+ and PO43−. During the riparian vegetative period, NO3− and PO43− increased along the reach while NH4+ showed no clear pattern. These longitudinal trends were partially related to riparian forest features and groundwater inputs, especially for NO3− and PO43−. Our study suggests that even though in-stream biogeochemical processing was substantial, the riparian zone can modulate the longitudinal variation in stream nutrient chemistry in this headwater stream.
Understanding the variability of the natural abundance in nitrogen stable isotopes (expressed as δ(15)N) of primary uptake compartments (PUCs; e.g. epilithon or macrophytes) is important due to the multiple applications of stable isotopes in freshwater research and can give insights into environmental and anthropogenic factors controlling N dynamics in streams. While previous research has shown how δ(15)NN of PUCs varies with δ(15)N of dissolved inorganic N (DIN) among streams, less is known about how δ(15)N of PUCs varies over time. Here, we examined monthly variation of δ15N of PUCs and of DIN species (nitrate and ammonium) over a year, and compared it among streams with contrasting human impacts and PUC types. Our results showed no evidence of isotopic seasonal patterns. Temporal variability in δ(15)N-PUCs increased with human impact, being the highest in the urban stream, probably influenced by the high variability of δ(15)N-DIN. Among compartments, in-stream PUCs characterized by fast turnover rates, such as filamentous algae, showed the highest temporal variability in δ(15)N values (from -3.6 to 23.2 ‰). Our study elucidates some of the the environmental and biological controls of temporal variability of δ(15)N in streams, which should be taken into account when using stable isotopes as an ecological tool.
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.
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 15N natural abundance of DIN forms and of four different macrophyte species in five different streams influenced by inputs from wastewater treatment plants (WWTP) and over time within one of these streams. Results showed that 15N signatures were similar in species of submersed and amphibious macrophytes and in stream water DIN, whereas 15N signatures of the riparian species were not. 15N signatures of macrophytes were generally closer to 15N signatures of nitrate, regardless of the species considered. Our results showed significant relationships between 15N 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.
High variability in natural abundance of nitrogen stable isotopes (δ(15)N) has been reported for primary uptake compartments (PUCs; e.g. epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems but the origin of this variability is not well understood yet. We examined how δ15N of different PUC types relate to δ(15)N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km(2)), encompassing a gradient of human pressures from headwaters to the river valley. δ(15)N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ(15)N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel, and thus using stream water as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ(15)N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ(15)N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool.
Effluents from wastewater treatment plants (WWTPs) containing microorganisms 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, 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. © 2013 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.
We investigated how dissolved inorganic N (DIN) inputs from a wastewater treatment plant (WWTP) effluent are processed biogeochemically by the receiving stream. We examined longitudinal patterns of NH4+ and NO3- concentrations and their N-15 signatures along a stream reach downstream of a WWTP. We compared the delta N-15 signatures of epilithic biofilms with those of DIN to assess the role of stream biofilms in N processing. We analyzed the delta N-15 signatures of biofilms coating light-and dark-side surfaces of cobbles separately to test whether light constrains functioning of biofilm communities. We sampled during 2 contrasting periods of the year (winter and summer) to explore whether changes in environmental conditions affected N biogeochemical processes. The study reach had a remarkable capacity for transformation and removal of DIN, but the magnitude and relevance of different biogeochemical pathways of N processing differed between seasons. In winter, assimilation and nitrification influenced downstream N fluxes. These processes were spatially segregated at the microhabitat scale, as indicated by a significant difference in the delta N-15 signature of light-and dark-side biofilms, a result suggesting that nitrification was mostly associated with dark-side biofilms. In summer, N processing was intensified, and denitrification became an important N removal pathway. The delta N-15 signatures of the light-and dark-side biofilms were similar, a result suggesting less spatial segregation of N cycling processes at this microhabitat scale. Collectively, our results highlight the capacity of WWTP-influenced streams to transform and remove WWTP-derived N inputs and indicate the active role of biofilms in these in-stream processes.
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Dissolved organic matter (DOM) is a complex mixture of organic compounds, which represents an essential source of carbon (C) and nutrients in aquatic ecosystems. In addition, DOM can play a key ecological role by modifying the optical properties of waters, mediating the availability of metals and influencing trophic food web structure. While the effects of drying and rewetting on DOM dynamics in terrestrial soils is a well studied subject, less is known about its effects in aquatic ecosystems, especially in streams. This is an important gap of knowledge since temporary streams that naturally cease to flow are found worldwide. Moreover, many streams with perennial flow are currently facing flow intermittency due to the effects of water extraction or changes in land-use and climate. The aim of this study was to evaluate the effects of stream flow intermittency on the spatial and temporal variability of DOM. The study was performed in a 300-m long reach of the Fuirosos stream (Catalonia, NE Spain) during the drying (June to July) and rewetting (October to November) phases. We sampled at several points along the study reach every 3 to 4 days. We assessed DOM amount by measuring the concentration of dissolved organic C and nitrogen (N). We characterized DOM composition using spectroscopic measurements, size-exclusion chromatography and C:N stoichiometry. Results showed two markedly distinct biogeochemical shifts between the drying and the rewetting phases. During the transition from continuous to fragmented flow we observed an increase in the magnitude and spatial variability of DOM concentrations and DOM was dominated by compounds from aquatic origin. After flow recovery, we also observed a pronounced increase in DOM concentration, but during this hydrologic phase DOM was dominated by compounds of terrestrial origin. Taken together, these results emphasize the relevance of flow intermittency in regulating stream DOM dynamics not only in terms of its availability but also in terms of its quality, which may affect metabolic and biogeochemical responses of the communities of these ecosystems. Finally, these results can help predict biogeochemical shifts associated to DOM in more temperate regions under future water scarcity scenarios.
Stream ecosystem respiration was quantified via resazurin reduction in a series of 6 flume experiments, with the purpose of understanding how respiration is influenced by biofilms and presence of bedforms that resulted in increased turbulence intensity. Two streamside flumes (40 m x 0.4 m) with were constructed and operated in parallel. The first flume had no bedforms and the second had 10-cm bedforms built of impermeable material with a wavelength of 1.0 m. The bedforms were covered with a monolayer of < 4 cm gravel. Stream water was flushed through the flumes with a velocity of ~0.069 m/s. Ecosystem respiration was measured via resazurin transformation to resorufin. Three sets of experiments were conducted in each flume, one with minimal biofilms, one with 10 days of biofilm growth, and one with 30 days of biofilm growth. Velocity measurements were made throughout the flumes with acoustic Doppler velocimetry. Ecosystem respiration varied by a factor of 2.25 among the experiments. Respiration was nearly perfectly correlated (r2 = 0.99) to a combination of biofilm mass and turbulence intensity, with biofilm mass explaining 91% of the variation and turbulence intensity explaining the remainder of the variation.
Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime, these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus (P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall, changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios. KeywordsTemporary streams–Flow intermittency–Drought–Nitrogen–Phosphorus–Spatial heterogeneity
Quantification of the transient storage zone (As) has become critical in stream biogeochemical studies addressed to examine factors controlling nutrient uptake. It is expected that higher As 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 As, the intrinsic physical and biological characteristics of stream structures that generate As 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 As. We estimated ammonium and phosphate uptake coefficients in both the main channel and As 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 As 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 As differed among them as well as the ratio between ammonium and phosphorus uptake coefficients in As, 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 As in nutrient uptake not only depends on the size of As but on the type of materials used to increase As, and thus, have biogeochemical implications on restoration projects aimed to modify channel morphology. KeywordsNutrient cycling–Transient storage–Nutrient assimilation–Ammonium–Phosphorus–Stream
The hyporheic zone is of great interest for stream ecologists because of its role in stream biogeochemical processing. Our study addresses the effects of leaf-litter inputs and varying discharge on surface–hyporheic water exchange and their possible consequences for the hyporheic zone biogeochemistry. Our study was conducted during autumn in Riera de Santa Fe (northeastern Iberian Peninsula), a stream with a well developed deciduous riparian canopy. We placed 15 wells spaced at 5-m intervals longitudinally down the study reach and measured surface and hyporheic nutrient and dissolved O2 (DO) concentrations on 23 sampling dates (15 during the leaffall period and 8 after a flood that washed out 65% of the accumulated leaf biomass). We assessed changes in surface-water exchange and in hyporheic NH4-N and soluble reactive P (SRP) uptake via coinjection of a conservative tracer and nutrients. Compared to surface water, hyporheic water had lower DO, higher SRP and NO3-N concentrations, and similar NH4-N concentration. Hyporheic water had higher DO saturation (p = 0.00) and higher NH4-N concentration (p = 0.00) in downwelling than in upwelling wells, whereas SRP and NO3-N concentrations did not differ significantly between well types (p > 0.05). Hydrologic connectivity was higher in downwelling than in upwelling wells and decreased with leaf-litter accumulation in the stream channel and increased with stream discharge. Increased connectivity after a flood reduced the difference in DO between surface and hyporheic compartments in upwelling and downwelling wells and in NO3-N in upwelling wells. NH4-N and SRP uptake responded differently to these changes. Hyporheic SRP uptake rate was controlled by hyporheic SRP concentration, which did not vary with changes in connectivity, whereas NH4-N uptake rate was indirectly affected by changes in connectivity through the influence of connectivity on DO availability. Last, although no NO3-N was added during the solute injections, we observed an increase in hyporheic NO3-N that probably was caused by nitrification. Together these results illustrate how the combination of stream hydrology and organic matter accumulation can dictate seasonal changes in hyporheic biogeochemistry.
A space-for-time substitution approach was used to evaluate potential effects of climate change on stream nutrient uptake by examining the relationship between stream environmental parameters and carbon (C), nitrogen (N) and phosphorus ( P) uptake along an altitudinal gradient. The study was carried out in 14 streams located in the Central Pyrenees ( NE Spain) draining calcareous catchments that cover an altitudinal range of 700-2,100 ma.s.l. In these streams, uptake of inorganic ( soluble reactive phosphorus (SRP), ammonium and nitrate) and organic ( acetate and glycine) nutrients was estimated. Additionally, several physical, chemical and biological parameters were measured. Results showed higher uptake for both SRP, a potentially limiting nutrient in these streams, and glycine, a labile source of dissolved organic N, than for the rest of the nutrients. Uptake of SRP, nitrate, glycine and acetate varied along stream environmental gradients associated with changes in stream hydromorphology, SRP availability and epilithic biomass. However, these gradients did not vary with altitude. These results indicate that climate change effects on stream nutrient uptake are more likely to be driven by indirect effects on hydromorphology and nutrient availability induced by shifts in the precipitation and run-off regime than by direct modifications in the thermal regime.
1. Due to the hierarchical organization of stream networks, land use changes occurring at larger spatial scales (i.e. the catchment) can affect physical, chemical and biological characteristics at lower spatial scales, ultimately altering stream structure and function. Anthropogenic effects on streams have primarily been documented using structural metrics such as water chemistry, channel alteration and algal biomass. Functional parameters, including metrics of nutrient retention and metabolism, are now being widely used as indicators of stream condition.
Headwater streams represent the key sites of nutrient retention, but little is known about temporal variation in this important process. We used monthly measurements over 2 years to examine variation in retention of soluble reactive phosphorus (SRP) and ammonium (NH 4+) in two Mediterranean headwater streams with contrasting hydrological regimes (that is, perennial versus intermittent). Differences in retention between streams were more evident for NH 4+, likely due to strong differences in the potential for nitrogen limitation. In both streams, nutrient-retention efficiency was negatively influenced by abrupt discharge changes, whereas gradual seasonal changes in SRP demand were partially controlled by riparian vegetation dynamics through changes in organic matter and light availability. Nutrient concentrations were below saturation in the two streams; however, SRP demand increased relative to NH 4+ demand in the intermittent stream as the potential for phosphorus limitation increased (that is, higher dissolved inorganic nitrogen:SRP ratio). Unexpectedly, variability in nutrient retention was not greater in the intermittent stream, suggesting high resilience of biological communities responsible for nutrient uptake. Within-stream variability of all retention metrics, however, increased with increasing time scale. A review of studies addressing temporal variation of nutrient retention at different time scales supports this finding, indicating increasing variability of nutrient retention with concomitant increases in the variability of environmental factors from the diurnal to the inter-annual scale. Overall, this study emphasizes the significance of local climate conditions in regulating nutrient retention and points to potential effects of changes in land use and climate regimes on the functioning of stream ecosystems.
This study examined the effect of increasing in-channel leaf standing stocks on hydrologic transient storage and nutrient retention in a Mediterranean mountain stream. A flood at the end of the leaf fall period provided the opportunity to examine the effect of abrupt removal of much of the leaf material. Twenty-one chloride additions were performed from October to December 2004. In 13 of these, we also added ammonium and phosphate to estimate nutrient uptake lengths and uptake velocities to assess nutrient retention. The one-dimensional transport with inflow and storage (OTIS) model was used to estimate transient water storage parameters. Although discharge remained constant during leaf fall, water residence time increased because of in-channel litter accumulation, as did nutrient uptake velocity. Flooding reduced leaf benthic standing stocks by 65% and dramatically altered hydraulic and nutrient retention properties of the channel. After recession, the stream rapidly recovered in terms of nutrient retention, especially for phosphate. Abrupt changes in discharge under flood conditions largely determined the variability in stream nutrient retention. However, leaf litter inputs played an important role in nutrient dynamics during constant flow. Because both the flood regime and the timing of leaf fall are being regionally altered by climate change, our results have implications for stream nutrient dynamics under climate change scenarios. Intense leaf fall from deciduous riparian vegetation is of major importance for both the community structure (Wallace et al. 1997) and metabolism (Crenshaw et al. 2002) of streams. Leaf litter inputs provide large quantities of energy to headwater streams that typically exhibit low levels of primary productivity (Fisher and Likens 1973). The ecological relevance of these inputs is well recognized,
A smart tracer is a tracer that provides, directly or through measurement of its concentration or in combination with another compound, at least 1 bit more information than a conservative tracer. In other words, the tracer provides information about conditions in the hydrologic system in addition to arrival time - location history, chemical conditions, biological activity, physical interactions, or other information. We have developed a smart tracer for quantifying biological activity and sediment-water interaction in streams. We will present a hands-on demonstration of the resazurin (Raz) test of biological activity and show results from an injection of the tracer in the Riera de Santa Fe de Montseny, Catalonia, Spain. In the presence of living bacteria (in many streams these are most common as biofilms on sediment), mildly fluorescent blue resazurin reduces irreversibly to strongly fluorescent resorufin. Using the information provided by this reaction along a 125- m stream reach, in conjunction with a chloride tracer, we were able to qualitatively identify bacterial growth and to quantify sediment-water interactions.