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Animals are a critical component of biogeochemical cycles. While animal mediated fluxes of nutrients and energy have received considerable attention, the impacts of these fluxes on microbial community structure and function are comparatively understudied. Here, we investigated if freshwater mussel influences on biogeochemical cycling in stream sediment are accompanied by changes in sediment microbial community composition and ecoenzymatic activity, and if these relationships change under different nutrient regimes.We predicted that mussel effects on ecosystem function are reflected by modified sediment microbial communities. We hypothesized that if changes in either sediment ecoenzymatic function or microbial community composition are driven by mussel‐derived nutrient amendments, we should see muted changes in microbial community assemblages or function when a given nutrient is abundant. However, if microbial communities and function are influenced by other mussel functions, then we should see uniform changes regardless of nutrient availability.We transplanted freshwater mussels and natural river sediment to flow‐through mesocosms and monitored changes in microbial community composition over 1 week. Our results indicate that mussels always changed sediment microbial community composition, but the way communities changed was dependent on ambient nutrient concentrations.On the final day we measured the activity of ecoenzymes known to correlate to microbial function and nutrient availability. Mussels homogenized the stoichiometric ratios of ecoenzyme activities, indicating a consistent function of sediment microbes associated with freshwater mussels. Our results suggest that mussels may promote functional redundancy in sediment microbial communities and highlight the importance of animals in controlling biogeochemical transformations under changing nutrient conditions. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

More than 70,000 large dams have been built worldwide. With growing water stress and demand for energy, this number will continue to increase in the foreseeable future. Damming greatly modifies the ecological functioning of river systems. In particular, dam reservoirs sequester nutrient elements and, hence, reduce downstream transfer of nutrients to floodplains, lakes, wetlands, and coastal marine environments. Here, we quantify the global impact of dams on the riverine fluxes and speciation of the limiting nutrient phosphorus (P), using a mechanistic modeling approach that accounts for the in-reservoir biogeochemical transformations of P. According to the model calculations, the mass of total P (TP) trapped in reservoirs nearly doubled between 1970 and 2000, reaching 42 Gmol y(-1), or 12% of the global river TP load in 2000. Because of the current surge in dam building, we project that by 2030, about 17% of the global river TP load will be sequestered in reservoir sediments. The largest projected increases in TP and reactive P (RP) retention by damming will take place in Asia and South America, especially in the Yangtze, Mekong, and Amazon drainage basins. Despite the large P retention capacity of reservoirs, the export of RP from watersheds will continue to grow unless additional measures are taken to curb anthropogenic P emissions.

Headwater streams drain > 70% of global land areas but are poorly monitored compared with large rivers. The small size and low water buffering capacity of headwater streams may result in a high sensitivity to local hydrological alterations and divergent carbon transport dynamics relative to large rivers. To assess these aspects, here we carry out a benchmark investigation on the riverine carbon dynamics in a typical alpine headwater stream (Shaliu River) on the Qinghai-Tibetan Plateau based on annual flux monitoring, in-depth seasonal sampling and hydrological event monitoring. We show that riverine carbon in the Shaliu River was dominated by dissolved inorganic carbon, peaking in the summer due to high discharge brought by the monsoon. Combining seasonal sampling along the river and monitoring of soil-river carbon transfer during spring thaw, we also show that both dissolved and particulate forms of riverine carbon increased downstream in the pre-monsoon season due to increasing contribution of organic matter derived from thawed permafrost along the river. By comparison, riverine carbon fluctuated in the summer, likely associated with sporadic inputs of organic matter supplied by local precipitation events during the monsoon season. Furthermore, using lignin phenol analysis for both riverine organic matter and soils in the basin, we show that the higher acid-to-aldehyde (Ad/Al) ratios of riverine lignin in the monsoon season reflect a larger contribution of topsoil likely via increased surface runoff compared with the pre-monsoon season when soil leachate lignin Ad/Al ratios were closer to those in the subsoil than topsoil solutions. Overall, these findings highlight the unique patterns and strong links of carbon dynamics in alpine headwater streams with local hydrological events. Given the projected climate warming on the Qinghai-Tibetan Plateau, thawing of seasonal permafrost and alterations of precipitation regimes may significantly influence the alpine headwater carbon dynamics, with cascading effects on the biogeochemical cycles of the watersheds. The alpine headwater streams may also be utilized as sentinels for climate-induced changes in the hydrological pathways and/or biogeochemistry of the small basin. [ABSTRACT FROM AUTHOR]

In the past several decades, there has been considerable interest in groundwater–surface water interactions and their ability to regulate and cycle nutrients and pollutants. These interactions are spatially and temporally complex, but electrical resistivity imaging can be a useful tool for their characterization. Here, an electrical resistivity imaging monitoring array was installed laterally across a groundwater‐dominated Chalk river and into the adjacent riparian wetland; data were collected over a period of 1 year. Independent inversions of data from the entire transect were performed to account for the changing river stage and river water conductivity. Additionally, data from just the riparian zone were inverted using a temporally constrained inversion, and the correlation between the riparian zone resistivity patterns and river stage was assessed using time‐series analysis. The river stage and the Chalk groundwater level followed similar patterns throughout the year, and both exhibited a sharp drop following cutting of in‐stream vegetation. For the independent inversions, fixing the river resistivity led to artifacts, which prevented reliable interpretation of dynamics in the riverbed. However, the resistivity structure of the riparian zone coincided well with the intrusively derived boundary between the peat and the gravel present at the field site. Time‐series analysis of the inverted riparian zone models permitted identification of seven units with distinct hydrological resistivity dynamics (five zones within the peat and two within the gravel). The resistivity patterns in the gravel were predominantly controlled by up‐welling of resistive groundwater and the down‐welling of more conductive peat waters following the river vegetation cutting event. In comparison, although the vegetation cutting influenced the resistivity dynamics in the peat zones, the resistivity dynamics were also influenced by precipitation events and increasing pore‐water conductivity, likely arising from biological processes. It is evident that such approaches combining electrical resistivity imaging and time‐series analysis are useful for understanding the spatial extent and timing of hydrological processes to aid in the better characterization of groundwater‐surface water interactions. [ABSTRACT FROM AUTHOR]

Gas exchanges between streams and atmosphere strongly impact biogeochemical cycles, yet their quantification in space and time remains challenging. We propose a new method to measure gas exchange rate coefficients in headwater streams. The method is based on simultaneous slug injections of a conservative tracer and a volatile tracer, coupled to in situ semicontinuous measurements. Its originality lies in the mathematical exploitation of the tracers' breakthrough curves. Taking advantage of the entire breakthrough curves, we infer the gas exchange rate coefficient from a comparative analysis of the shape of the volatile and the conservative breakthrough curves. Tests on synthetic datasets confirmed the validity of the mathematical framework for the whole range of gas exchange rate coefficients found in headwater streams. Field experiments further attested to the real‐world applicability of the method. Salt and helium injections were performed in a first‐order stream and the helium breakthrough curves were obtained using in situ membrane inlet mass spectrometry. This new method allows to determine the gas exchange rates directly in the field, without any external calibration of the data. Its fast implementation enables a wider spatial coverage of the measurements of gas exchange rates, and thus offers an opportunity to improve the large‐scale estimates of CO2 emissions from headwater streams. More generally, our study highlights the informative potential of semicontinuous datasets and encourages further investigation of possible use of temporal signals. [ABSTRACT FROM AUTHOR]

Extreme temperatures have increased in intensity, duration and frequency in the last century, with potential consequences on key ecological processes such as organic matter breakdown. Many stream ecosystems are fueled by the breakdown of terrestrial leaf litter, which is exposed to atmospheric conditions for certain periods of time before entering the stream. Thus, extreme warming or freezing events may affect the litter physicochemical structure, which could translate into altered breakdown within the stream. The above prediction was tested by exposing litter of common riparian tree species in southern Chile to freezing (−20 ºC; dry or wet litter) or heating (40 ºC) and comparing breakdown with control litter exposed to room temperature (20 ºC), separating the effects of different breakdown agents (i.e., leaching, microorganisms and detritivores). The greatest effects were found in wet litter subjected to freezing; this treatment significantly increased leaching in the short term (48 h) and slowed down breakdown in the long term (30 days), mostly due to the inhibition of microbial breakdown. Heating also retarded microbial breakdown, but the effect was smaller. Our results suggest that short-term extreme temperatures—particularly cold ones—have the potential to slow down litter breakdown in streams, which will most likely impact global biogeochemical cycles where streams play a key role. [ABSTRACT FROM AUTHOR]

Summary: Despite the essential functions of sedimentary bacterial and fungal communities in biogeochemical cycling, little is known about their biogeographic patterns and driving processes in large rivers. Here we investigated the biogeographic assemblies and co‐occurrence patterns of sedimentary bacterial and fungal communities in the Jinsha River, one of the largest rivers in southwestern China. The mainstream of river was divided into upstream, midstream and downstream. The results showed that both bacterial and fungal communities differed significantly among three sections. For both communities, their composition variations in all sites or each river section were controlled by the combination of dispersal limitation and environmental selection, and dispersal limitation was the dominant factor. Compared with bacteria, fungi had stronger dispersal limitation. Co‐occurrence network analyses revealed higher network connectivity but a lower proportion of positive interaction in the bacterial than fungal network at all sites. In particular, the keystone species belonging to bacterial phyla Proteobacteria and Firmicutes and fungal phyla Ascomycota and Chytridiomycota may play critical roles in maintaining community function. Together, these observations indicate that fungi have a stronger dispersal limitation influence and less network connectivity than bacteria, implying different community assembly mechanisms and ecological functions between bacteria and fungi in large rivers. [ABSTRACT FROM AUTHOR]

Anthropogenic activities have altered biogeochemical cycles and the fate of organic carbon (OC) and nutrients in freshwater ecosystems. To investigate coupled OC and nutrient cycling and fate in streams, we compared the spiraling lengths (S) and uptake velocities (vf) of OC and nitrate (NO3−) in headwater streams (n = 72) across different land uses (i.e., agricultural, urban, natively vegetated) and regions (n = 8) in the United States. We did these comparisons with data collected for the second Lotic Intersite Nitrogen eXperiment (LINX II; Mulholland et al. 2009). OC spiraling lengths (S OC) in reference (21–4180 m) were shorter than in agricultural streams (89–37,156 m) and urban streams (104–12,605 m). OC mineralization velocities (v f −OC) and NO3− uptake velocities (v f −NO3) were weakly positively correlated across all sites (r = 0.33, p = 0.008). The strongest correlations between OC mineralization and NO3− uptake were in streams with gross primary production (GPP) similar to ecosystem respiration (ER) and human-altered streams (all agricultural streams: r = 0.56, p = 0.008, GPP∶ER = 0.6; all urban streams: r = 0.73, p < 0.001, GPP∶ER = 0.5). Additionally, the distances traveled by OC and NO3− before they were permanently removed from the stream by mineralization (S OC) or denitrification (Sw–den) were similar in magnitude, although S OC was shorter than Sw–den in most streams. This study demonstrates how OC and NO3− spiraling can be used to investigate how region and land use influence coupled OC-NO3− interactions and fate. [ABSTRACT FROM AUTHOR]

River sediments, functioning as sink and source for nutrients, are one of the most diverse bacteria habitats in freshwater ecosystems. Recently, more attention has been paid to the sediment bacteria because of their important roles in biogeochemical cycling. Our study addresses the question of how sediment bacterial community varies taxonomically and functionally along one of the largest plateau river in the world (the Yarlung Tsangpo River) subject to anthropogenic pressures, including wastewater discharge and river damming, as they may lead to adverse impacts on river ecosystems. The results indicated that river damming retained biogeochemical nutrients of carbon, silica and nitrogen in the reservoir, while phosphorus was more sensitive to wastewater discharge. Besides, wastewater discharge resulted in asynchronous change of biomass and diversity in sediment bacteria while they were both inhibited by the construction of dam. Moreover, significant alterations in the community composition were observed in dam-associated sites, while according to exploratory bioinformatics prediction tool of PICRUSt, functional variation was not detected between upstream and downstream of the dam due to functional redundancy. In addition, revealed by General Additive Model (GAM) and Mantel test results, elevation was the endemic driver shaping the sediment bacterial community diversity and composition along with pH, while nutrient concentrations were responsible for functional variation. Furthermore, little synchronized dynamics of co-occurrence was exhibited in the sediment bacteria in the Yarlung Tsangpo River. Therefore, our study proposed that sediment bacterial community should be defined taxonomically and functionally when studying its variation in river ecosystems. [ABSTRACT FROM AUTHOR]

While nitrogen (N2) fixation is an important process in nitrogen (N) biogeochemical cycling, supplying a significant portion of the N in natural ecosystems, few quantitative constraints exist concerning its contribution to the N enrichment and export from river ecosystems. This study estimates the N2 fixation rates of urban rivers in the Yangtze Estuary area using acetylene reduction. The results demonstrate that the prominent spatiotemporal variability of river N2 fixation rates is driven by various environmental factors. River N2 fixation rates are significantly higher in the summer (90.57 ± 14.60 ngN·L−1·h−1) than in the winter (57.98 ± 15.73 ngN·L−1·h−1). Spatially, rivers draining urban and suburban areas have higher N2 fixation rates than those draining rural areas. The N2 fixation rates are positively correlated with the N2 fixing cyanobacteria density, water temperature, light, and the water phosphorus (P) concentration, but they are negatively correlated with the dissolved N concentration (NH4+-N and NO3−-N). The N2 fixation rates annually range from 53.20 to 89.24 ngN·L−1·h−1 for all of the sampling rivers, which is equivalent to a depth integrated (0–0.6 m) N input of 0.163–0.274 gN·m−2·a−1. The determined annual N input via N2 fixation is generally higher than that of marine systems, but it is lower than that of eutrophic lakes. This study provides robust evidence that N2 fixation can supply a substantial portion of the N input to human-impacted river ecosystems, which has not been sufficiently accounted for when determining the N mass balance of riverine ecosystems. A high N2 fixation rate may increase the ratio of N to P input to river systems, and therefore render P the limiting factor in aquatic eutrophication. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential micronutrient for aquatic organisms. Despite its importance, our current knowledge of the biogeochemical cycling of dissolved Se in tropical estuaries is limited, especially in Southeast Asia. To gain insights into Se cycling in tropical peat-draining rivers and estuaries, samples were collected from the Rajang, Maludam, Sebuyau, Simunjan, Sematan, Samunsam and Lunda rivers and estuaries in western Sarawak, Malaysia, in March and September 2017 and analysed for various forms of Se (dissolved inorganic and organic). Mean total dissolved Se (TDSe), dissolved inorganic Se (DISe) and dissolved organic Se concentrations (DOSe) were 2.2 nmolL-1 (range: 0.7 to 5.7 nmolL-1), 0.18 nmolL-1 (range: less than the detection limit to 0.47 nmolL-1) and 2.0 nmolL-1 (range: 0.42 to 5.7 nmolL-1), respectively. In acidic, low-oxygen, organic-rich blackwater (peatland-draining) rivers, the concentrations of DISe were extremely low (near or below the detection limit, i.e. 0.0063 nmolL-1), whereas those of DOSe were high. In rivers and estuaries that drained peatland, DOSe / TDSe ratios ranged from 0.67 to 0.99, showing that DOSe dominated. The positive relationship between DISe and salinity and the negative relationship between DOSe and salinity indicate marine and terrestrial origins of DISe and DOSe, respectively. The positive correlations of DOSe with the humification index and humic-like chromophoric dissolved organic matter components in freshwater river reaches suggest that peat soils are probably the main source of DOSe. The DOSe fractions may be associated with high molecular weight peatland-derived aromatic and black carbon compounds and may photodegrade to more bioavailable forms once transported to coastal waters. The TDSe flux delivered by the peat-draining rivers exceeded those reported for other small rivers and is quantitatively more significant than previously thought. [ABSTRACT FROM AUTHOR]

Pampean streams constitute a drainage network which discharges continental freshwater into the associated marine coastal zone. The present study concerned a set of streams within General Pueyrredón district, Southeastern province of Buenos Aires, Pampa's region, Argentina, which deserves particular attention, since they flow in the area of influence of Mar del Plata, one of the most important tourist destination in the country. Different physical–chemical (i.e., temperature, conductivity, pH, dissolved oxygen), hydrographical and eco-physiological parameters were studied within five streams along an annual cycle, searching for seasonal variations and possible relations with land use. The behavior of physical–chemical parameters was consistent with previous reports on similar environments; inorganic nutrients presented high levels along the whole period, and have never been completely depleted. Within N nutrients, nitrate demonstrated to be the dominant along the whole period, with global mean values of ~ 18 µM and reaching values up to ~ 50 µM (total discharge of N ~ 1.23 Ton DIN y−1). A very high load of organic matter (from ~ 2000 mg Cm−3 in the fall up to ~ 4000 mg Cm−3 during summer) is discharged from these streams into the marine coastal zone (which represents ~ 214 Ton POC y−1) and, even until now, response effects have not been recorded there. The obtained results allowed us to sustain that anthropic activities (i.e., agriculture, cattle raising), continental runoff and climate conditions seem to be the main drivers of the hydrographical/biogeochemical behavior of the studied streams. Therefore, changes concerning them should also be included in future monitoring programs. [ABSTRACT FROM AUTHOR]

Viruses drive microbial diversity, function and evolution and influence important biogeochemical cycles in aquatic ecosystems. Despite their relevance, we currently lack an understanding of their potential impacts on stream biofilm structure and function. This is surprising given the critical role of biofilms for stream ecosystem processes. Currently, the study of viruses in stream biofilms is hindered by the lack of an optimized protocol for their extraction, concentration and purification. Here, we evaluate a range of methods to separate viral particles from stream biofilms, and to concentrate and purify them prior to DNA extraction and metagenome sequencing. Based on epifluorescence microscopy counts of viral-like particles (VLP) and DNA yields, we optimize a protocol including treatment with tetrasodium pyrophosphate and ultra-sonication to disintegrate biofilms, tangential-flow filtration to extract and concentrate VLP, followed by ultracentrifugation in a sucrose density gradient to isolate VLP from the biofilm slurry. Viromes derived from biofilms sampled from three different streams were dominated by Siphoviridae, Myoviridae and Podoviridae and provide first insights into the viral diversity of stream biofilms. Our protocol optimization provides an important step towards a better understanding of the ecological role of viruses in stream biofilms. [ABSTRACT FROM AUTHOR]

Dendritic network models have proven valuable in predicting variation in strontium concentrations [Sr] and isotope ratios (87Sr/86Sr) across large river systems. These models also support inference about sources and processes controlling Sr fluxes from such systems, although this capacity has not been exploited in previous work. Here, we apply these methods to model [Sr] and 87Sr/86Sr values of a large river draining accreted volcanic arc terranes of the northern Cordillera of North America. The [Sr] and 87Sr/86Sr river models have excellent fit to measured river waters with RMSE = 0.05: r 2 = 0.67 and RMSE = 0.0004: r 2 = 0.87, respectively. The results strongly indicate that mixed siliciclastic sedimentary units dominate the Sr weathering flux within these systems. Extrapolation using global arc sediment distributions suggests arc sedimentray units could source ∼30% of the modern riverine Sr flux, constituting a major source of juvenile Sr to the ocean. Because the weathering of arc sediments can lag arc growth and may persist over 100's of millions of years, we suggest that the preservation and accretion of mixed siliciclastic units formed in these settings should be considered when scaling arc weathering contributions through geologic time. Contributions from such sediments could amplify or smooth the arc-derived rock weathering and 87Sr/86Sr flux over long timescales, altering interpretations of biogeochemical cycles and the silicate weathering feedback. • Northern Cordillera arc sediments: a major juvenile Sr source. • Weathering of global arc sediments can contribute 30% of riverine Sr-flux to the ocean - a major sorce of juvinile Sr. • Arc-derived sediments: a new metric for evaluating arc weathering, the silicate feedback,and global Sr cycling through deep time. [ABSTRACT FROM AUTHOR]

• Bacterial community composition and diversity in Koshi River was characterised. • Difference in bacterial communities along the mainstream and tributaries was observed. • Geographical distance as an individual factor explained for the highest variation in bacterial community composition of PA and FL bacteria. • Effect of particle environmental factors and dissolved environmental factor was remarkable in PA and FL bacteria, respectively. Large rivers serve as hotspots for the biogeochemical cycling of nutrients in which the role of microorganisms is crucial. Therefore, understanding the bacterial abundance, diversity, community composition, biogeography, and controlling environmental factors in such habitats is important. However, the information is still unexplored in Himalayan Rivers and their associated ecosystem towards the southern slopes, Nepal. Thus, composition and diversity of particle-attached (PA) and free-living (FL) bacteria was investigated along the mainstream and major tributaries of Koshi River i.e. the largest river of Nepal, using 16S rRNA gene-based Illumina MiSeq platform. Bacterial community composition significantly differed in habitat (mainstream and tributaries) and lifestyle (PA and FL), and the habitat variation was more pronounced than lifestyle variation. Likewise, bacterial communities were dissimilar in more geographically distant sites and vice versa. Distinct dominant taxa in mainstream and tributaries were indicative of different input sources and environmental conditions. Ten OTUs belonging to the phyla Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes contributed to more 50% of the bacterial community dissimilarity in two habitats. Geographical distance and water physicochemical properties explained for the most variation in PA and FL bacteria, respectively. Nonetheless, the individual effect of geographical distance was highest in both PA and FL bacteria. Effect of particulate environmental factor like TSS in variation of PA bacteria and that of dissolved environment factors like DOC and EC in FL bacteria was remarkable. [ABSTRACT FROM AUTHOR]

The transport and processing of nutrients and organic matter in streams are important functions that influence the condition of watersheds and downstream ecosystems. In this study, we investigated the effects of streambed sediment removal on biogeochemical cycling in Fawn River, a gravel-bottomed river in Indiana, U.S.A. We measured stream metabolism as well as nitrogen (N) and phosphorus (P) retention in both restored and unrestored reaches of Fawn River to examine how sediment removal affected multiple biogeochemical functions at the reach scale. We also assessed the properties of restored and unrestored streambed sediments to elucidate potential mechanisms driving observed reach-scale differences. We found that sediment removal led to lower rates of primary productivity and ecosystem respiration in the restored reach, likely due to macrophyte removal and potentially due to changes to sediment organic matter quality. We found minimal differences in N and P retention, suggesting that these processes are controlled at larger spatial or temporal scales than were examined in this study. Denitrification enzyme activity was lower in sediments from the restored reach compared to the unrestored reach, suggesting that restoration may have decreased N removal. Our results indicate that most near-term changes in biogeochemical function following restoration could be attributed to macrophyte removal, although effects from sediment removal may emerge over longer timescales. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential micronutrient for many organisms. Despite its importance, our current knowledge of the biogeochemical cycling of dissolved Se in tropical estuaries is limited, especially in Southeast Asia. To gain insights into Se cycling in tropical peat-draining rivers and estuaries, samples were collected from the Rajang, Maludam, Sebuyau, Simunjan, Sematan, Samunsam, and Lunda rivers and estuaries in western Sarawak, Malaysia, in March and September 2017 and analysed for various forms of Se (dissolved inorganic and organic). Mean total dissolved Se (TDSe), dissolved inorganic Se (DISe), and dissolved organic Se concentrations (DOSe) were 2.2 nmol L-1 (range: 0.7 to 5.7 nmol L-1), 0.18 nmol L-1 (range: less than the detection limit to 0.47 nmol L-1), and 2.0 nmol L-1 (range: 0.42 to 5.7 nmol L-1), respectively. In acidic, low-oxygen, organic-rich blackwater (peatland-draining) rivers, the concentrations of DISe were extremely low, whereas those of DOSe were high. In rivers and estuaries that drained peatland, DOSe / TDSe ratios ranged from 0.67 to 0.99, showing that DOSe dominated. The positive relationship between DISe and salinity and the negative relationship between DOSe and salinity indicate marine and terrestrial origins of DISe and DOSe, respectively. The positive correlations of DOSe with the humification index and humic-like chromophoric dissolved organic matter components in freshwater river reaches suggest that peat soils are probably the main source of DOSe. Discharges of water enriched with DOSe fractions associated with peatland-derived high-molecular-weight, high-aromaticity dissolved organic matter discharged from estuaries may promote productivity in the adjoining oligotrophic coastal waters. The results of this study suggest that the impacts of Se discharges on coastal ecosystems should be evaluated in the future. [ABSTRACT FROM AUTHOR]

Agricultural runoff from the Mississippi‐Atchafalaya River Basin delivers nitrogen (N) and phosphorus (P) to the Gulf of Mexico, causing hypoxia, and climate drives interannual variation in nutrient loads. Climate phenomena such as El Niño–Southern Oscillation may influence nutrient export through effects on river flow, nutrient uptake, or biogeochemical transformation, but landscape variation at smaller spatial scales can mask climate signals in load or discharge time series within large river networks. We used multivariate autoregressive state‐space modeling to investigate climate signals in the long‐term record (1979–2014) of discharge, N, P, and SiO2 loads at three nested spatial scales within the Mississippi‐Atchafalaya River Basin. We detected significant signals of El Niño–Southern Oscillation and land‐surface temperature anomalies in N loads but not discharge, SiO2, or P, suggesting that large‐scale climate phenomena contribute to interannual variation in nutrient loads through biogeochemical mechanisms beyond simple discharge‐load relationships. Plain Language Summary: Runoff of excess nutrients from crop fertilizers applied throughout the Mississippi‐Atchafalaya River Basin, particularly nitrogen (N) and phosphorus (P), pollute freshwater and coastal ecosystems such as the Gulf of Mexico. Though agriculture is the main source, year‐to‐year variation in the size of nutrient loads is largely controlled by precipitation and river flow, which mobilize nutrients from the landscape. Additional climate variables, such as temperature, influence nutrient loads by controlling rates of nutrient uptake or transformation by plants, algae, and microbes, but these processes may be difficult to detect in a nearly continental‐scale river network with heterogeneous subbasins. We identified signals of multiple large‐scale climate phenomena in the long‐term record (1979–2014) of nutrient loads from the Mississippi River and its major tributaries. Climate effects on nutrient loads, particularly N, were different and often stronger than on river flow, indicating that long‐term patterns in nutrient loads were influenced by processes beyond simple precipitation‐driven runoff. Variable effects of climate on nutrient export present challenges for reducing nutrient loads to the Mississippi River and Gulf of Mexico. Adjustments to targeted reductions may be needed as global and regional climates change. Key Points: Significant signals of large‐scale climate phenomena appear in N loads but not discharge, SiO2, or P loads in the Mississippi River Basin at multiple spatial scalesEffects of climate variables differ among nutrients (N, P, SiO2) and nutrient forms (nitrate, ammonium)Climate‐driven processes independent of river flow contribute to temporal variation in nutrient loads within the Mississippi River Basin [ABSTRACT FROM AUTHOR]

Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events.We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live–dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths.The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live–dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters.Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size.Our findings show two important functions of the hyporheic zone during drought: (1) deeper (−50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone. [ABSTRACT FROM AUTHOR]

Heilongjiang is a "browning" river that receives substantial terrestrial organic matter, where reactivity of dissolved organic matter (DOM) may have important effect on ecosystem function and carbon biogeochemical cycle. However, little is known about microbial transformations of different DOM components, which could provide valuable insight into biogeochemical reactivity of DOM. In this study, bioavailability experiments were conducted for 55 days to determine changes of different DOM components by microbial transformations. Labile matter (C1) was detected only in initial DOM, and tryptophan-like substances (C4) were observed from day 5 onwards. Thus, three individual components were identified at each sampling time of the bioavailability experiment. The increase of Fmax in DOM components revealed that microbial humic-like substances (C2), terrestrial humic-like substances (C3), and C4 were produced by microbial transformation, especially in the spring samples. Further, two-dimensional correlation spectroscopy (2D-COS) indicated that shorter wavelength tryptophan-like and microbial humic-like substances can be degraded by microbes or transformed into longer wavelength complex substances. Relatively simple microbial humic-like substances were preferentially produced compared to complex terrestrial humic-like substances. The results make sense to understand the biogeochemical cycling and environmental effects of DOM in the Heilongjiang River. [ABSTRACT FROM AUTHOR]

Abstract Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ 74 Ge , and δ 30 Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean. Dissolved δ 74 Ge composition of rivers studied thus far range from 0.9 to 5.5‰ with a discharge-weighted global average of 2.6 ± 0.5‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (Δ 30 Si s e c) ranges from −2.7 to −0.2‰, which removes between 19–79% of the initial dissolved Si, while between 12–54% is incorporated by biota. For Ge, modeling indicates that 79–98% of the dissolved load is incorporated into secondary mineral phases with a Δ 74 Ge s e c ranging from −4.9 to −0.3‰. The fractionation induced by biological uptake is calculated to range from −2.6 to −1.3‰ for Δ 30 Si b i o and −0.7 ± 0.7‰ for Δ 74 Ge b i o. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters. Highlights • Ge and Si isotope composition of water and sediments in various rivers is presented. • Dissolved δ 74 Ge range 0.9–5.5‰, river sediment δ 74 Ge range 0.5–0.7‰. • Dissolved δ 74 Ge is strongly fractionated due to low chemical Ge mobility. • Ge/Si-isotope multi-proxy provides quantitative constraints on secondary vs biological fractionation. [ABSTRACT FROM AUTHOR]

Research at the interface between terrestrial, riverine, estuarine and marine environments is frequently constrained by significant disciplinary and geographical boundaries. This article outlines an international initiative, DANUBIUS-RI, which aims to address these problems by facilitating biogeochemical monitoring and interdisciplinary research on river-sea systems. The scope of the project spans the environmental, social and economic sciences and was accepted into the European Strategy Forum on Research Infrastructures roadmap in 2016. When operational, DANUBIUS-RI will offer researchers access to interdisciplinary expertise, facilities and European river-sea systems, providing a comprehensive platform for multidisciplinary research and training. [ABSTRACT FROM AUTHOR]

Biogeochemical maps of coastal regions can be used to identify important influences and inputs that define nearshore environments and biota. Biogeochemical tracers can also track animal movement and their diet, monitor human coastal development, and evaluate the condition of habitats and species. However, the beneficial applications of spatial biogeochemical analysis are hindered by a limited understanding of how tracer distribution is affected by different land and ocean–based influences. To help address these knowledge gaps, we determined the spatial trends of three stable isotopes (δ13C-carbon, δ15N-nitrogen, δ34S-sulfur) and 13 major and trace elements in an urbanized coastal embayment (Moreton Bay, Australia), as incorporated into the muscle tissue of a marine consumer, the eastern king prawn Melicertus plebejus. Results were used to identify unique biochemical regions within the bay and to discuss how spatial patterns in tracers could be used to indicate the relative importance of catchment, urban and offshore drivers in coastal bays. Discriminant analysis identified seven biogeochemical regions that were likely distinguished by variation in catchment, urban, and offshore input, and habitat type. δ13C and δ15N patterns suggested nearshore areas could be distinguished by increased sediment resuspension and higher wastewater inputs from catchments. High inshore lead (Pb) and copper (Cu) concentrations were likely the result of urban input. Arsenic (As) and cadmium (Cd) increased further from shore. This trend implied oceanic influences were a significant control over As and Cd bioavailability. Cobalt (Co) and rare earths were also used to differentiate some nearshore areas, but incongruent distribution patterns in Co suggested it may be less reliable. Overall, results indicated that δ15N, δ13C, Cd, Cu, Pb and rare earth elements were the most reliable tracers to differentiate nearshore and offshore environments, and catchment–based effects. We encourage future studies to consider using a similar multivariate approach in coastal spatial analysis, and to include unrelated tracers that reflect distinct coastal influences. [ABSTRACT FROM AUTHOR]

River watersheds are among the most complex terrestrial features in Alaska, performing valuable ecosystem functions and providing services for human society. Rivers are vital to both estuarine and aquatic biota and play important roles in biogeochemical cycles and physical processes. The functions of watersheds have been used as vulnerability indicators for ecosystem and socioeconomic resilience. Despite a long history of human activity, the Yukon River has not received the holistic and interdisciplinary attention given to the other great American river systems. By using hypothesis-based monitoring of key watershed functions, we can gain insight to regime-shifting stresses such as fire, toxins, and invasive species development. Coupling adaptive risk management practices involving stakeholders with place-based education, especially contaminants and nutrition related, can maintain resilience within communities. The Yukon watershed provides a broadscale opportunity for communities to monitor the environment, manage resources, and contribute to stewardship policy formation. Monitoring keystone species and community activities, such as citizen science, are critical first steps to following changes to resiliency throughout the Yukon watershed. Creating a policy environment that encourages local experimentation and innovation contributes to resilience maintenance during development-imposed stress. [ABSTRACT FROM AUTHOR]

We assessed stream ecosystem-level response to a drought-induced defoliation event by gypsy moth caterpillars (Lymantria dispar) with high-frequency water quality sensors. The defoliation event was compared to the prior year of data. Based on long-term records of precipitation and drought indices, the drought of 2015-2016 in Rhode Island, USA was an extreme climatic event that preceded and likely precipitated the defoliation from insect infestation. Canopy cover in the riparian area was reduced by over 50% increasing light availability which warmed the stream and stimulated autotrophic activity. Frass and leaf detritus contributed particulate carbon and organic nutrients to the stream. Based on locally calibrated s::can spectro::lyser data, nitrate concentration and flux did not significantly increase during defoliation while orthophosphate concentration and flux did significantly increase during part of the defoliation period. Lower mean daily dissolved oxygen (DO) levels and wider diel cycles of DO indicated higher biological activity during the defoliation event. Stream metabolism metrics were also significantly higher during defoliation and pointed to heterotrophic activity dominating in the stream. The increases in stream metabolism were low compared to other studies; in streams with higher nutrient levels (e.g., in agricultural or urban watersheds) the increase in light and temperature could have a stronger influence on stream metabolism. The in-stream metabolic processes and nutrient fluxes observed in response to the drought-driven defoliation event resulted from the long-term deployment of high-frequency water sensors. The proliferation of these water sensors now enable studies that assess ecosystem responses to stochastic, unusual disturbances. [ABSTRACT FROM AUTHOR]

The biogeochemical sulfur cycle is intimately linked to the cycles of carbon, iron, and oxygen, and plays an important role in global climate via weathering reactions and aerosols. However, many aspects of the modern budget of the global sulfur cycle are not fully understood. We present new δ 34 S measurements on sulfate from more than 160 river samples from different geographical and climatic regions—more than 46% of the world's freshwater flux to the ocean is accounted for in this estimate of the global riverine sulfur isotope budget. These measurements include major rivers and their tributaries, as well as time series, and are combined with previously published data to estimate the modern flux-weighted global riverine δ 34 S as 4.4 ± 4.5‰ (V-CDT), and 4.8 ± 4.9‰ when the most polluted rivers are excluded. The sulfur isotope data, when combined with major anion and cation concentrations, allow us to tease apart the relative contributions of different processes to the modern riverine sulfur budget, resulting in new estimates of the flux of riverine sulfate due to the oxidative weathering of pyrites (1.3 ± 0.2 Tmol S/y) and the weathering of sedimentary sulfate minerals (1.5 ± 0.2 Tmol S/y). These data indicate that previous estimates of the global oxidative weathering of pyrite have been too low by a factor of two. As pyrite oxidation coupled to carbonate weathering can act as a source of CO 2 to the atmosphere, this global pyrite weathering budget implies that the global CO 2 weathering sink is overestimated. Furthermore, the large range of sulfur isotope ratios in modern rivers indicates that secular changes in the lithologies exposed to weathering through time could play a major role in driving past variations in the δ 34 S value of seawater. [ABSTRACT FROM AUTHOR]

The turbulent surfaces of rivers and streams are natural hotspots of biogeochemical exchange with the atmosphere. At the global scale, the total river-atmosphere flux of trace gasses such as carbon dioxide depends on the proportion of Earth’s surface that is covered by the fluvial network, yet the total surface area of rivers and streams is poorly constrained. We used a global database of planform river hydromorphology and a statistical approach to show that global river and stream surface area at mean annual discharge is 773,000 ± 79,000 square kilometers (0.58 ± 0.06%) of Earth’s nonglaciated land surface, an area 44 ± 15% larger than previous spatial estimates. We found that rivers and streams likely play a greater role in controlling land-atmosphere fluxes than is currently represented in global carbon budgets. [ABSTRACT FROM AUTHOR]

Stream solute signals arise from the convolution of catchment delivery, hydraulic transport, and biogeochemical processing. Drawing inference from such signals requires analytical or experimental tools to isolate the signal attributable to stream processing. We used Lexan® chambers open to the air and inserted into the sediment (benthic chambers) and high-frequency in situ sensors to measure metabolism and nutrient processing rates for a small (0.36 m2) footprint across a variety of benthic cover types. We estimated gross primary production (GPP) and ecosystem respiration (ER) based on high-resolution dissolved O2 measurements, and we estimated N retention via both autotrophic assimilation (UA) and dissimilatory removal (UD) from high-resolution NO3− measurements. We observed marked spatial variation in metabolism and nutrient retention, principally in response to autotroph cover and light during thirty-three 1-wk deployments in Gum Slough, a spring-fed river (discharge ≈ 1.5 m3/s) in northern Florida. Spatially weighted mean chamber rates were commensurate with, but slightly underpredicted, open-channel measurements. Moreover, the stoichiometry (i.e., molar C∶N from GPP and UA) matched expectations based on tissue stoichiometry and autotroph composition. Our approach enabled disaggregated measurements of metabolic processing across heterogeneous riverine settings, albeit principally where sediments were sufficiently fine-grained to limit hydraulic exchange (assessed here with a Cl− tracer). In addition, the small chamber volume allowed manipulation of myriad environmental factors, including solute concentrations via both active enrichment, or, in our case, passive depletion. We observed negligible GPP and minimal UA responses to NO3− depletion, in contrast to strong UD responses. [ABSTRACT FROM AUTHOR]

A common assumption in phosphorus (P) load apportionment studies is that P loads in rivers consist of flow independent point source emissions (mainly from domestic and industrial origins) and flow dependent diffuse source emissions (mainly from agricultural origin). Hence, rivers dominated by point sources will exhibit highest P concentration during low-flow, when flow dilution capacity is minimal, whereas rivers dominated by diffuse sources will exhibit highest P concentration during high-flow, when land-to-river hydrological connectivity is maximal. Here, we show that Soluble Reactive P (SRP) concentrations in three forested catchments free of point sources exhibited seasonal maxima during the summer low-flow period, i.e. a pattern expected in point source dominated areas. A load apportionment model (LAM) is used to show how point sources contribution may have been overestimated in previous studies, because of a biogeochemical process mimicking a point source signal. Almost twenty-two years (March 1995–September 2016) of monthly monitoring data of SRP, dissolved iron (Fe) and nitrate-N (NO3) were used to investigate the underlying mechanisms: SRP and Fe exhibited similar seasonal patterns and opposite to that of NO3. We hypothesise that Fe oxyhydroxide reductive dissolution might be the cause of SRP release during the summer period, and that NO3 might act as a redox buffer, controlling the seasonality of SRP release. We conclude that LAMs may overestimate the contribution of P point sources, especially during the summer low-flow period, when eutrophication risk is maximal. [ABSTRACT FROM AUTHOR]

Abstract: Rivers deliver large amounts of terrestrially derived materials (such as nutrients, sediments, and pollutants) to the coastal ocean, but a global quantification of the fate of this delivery is lacking. Nutrients can accumulate on shelves, potentially driving high levels of primary production with negative consequences like hypoxia, or be exported across the shelf to the open ocean where impacts are minimized. Global biogeochemical models cannot resolve the relatively small‐scale processes governing river plume dynamics and cross‐shelf export; instead, river inputs are often parameterized assuming an “all or nothing” approach. Recently, Sharples et al. (2017), https://doi.org/10.1002/2016GB005483 proposed the SP number—a dimensionless number relating the estimated size of a plume as a function of latitude to the local shelf width—as a simple estimator of cross‐shelf export. We extend their work, which is solely based on theoretical and empirical scaling arguments, and address some of its limitations using a numerical model of an idealized river plume. In a large number of simulations, we test whether the SP number can accurately describe export in unforced cases and with tidal and wind forcings imposed. Our numerical experiments confirm that the SP number can be used to estimate export and enable refinement of the quantitative relationships proposed by Sharples et al. We show that, in general, external forcing has only a weak influence compared to latitude and derive empirical relationships from the results of the numerical experiments that can be used to estimate riverine freshwater export to the open ocean. [ABSTRACT FROM AUTHOR]

Abstract: Rivers deliver large amounts of fresh water, nutrients, and other terrestrially derived materials to the coastal ocean. Where inputs accumulate on the shelf, harmful effects such as hypoxia and eutrophication can result. In contrast, where export to the open ocean is efficient riverine inputs contribute to global biogeochemical budgets. Assessing the fate of riverine inputs is difficult on a global scale. Global ocean models are generally too coarse to resolve the relatively small scale features of river plumes. High‐resolution regional models have been developed for individual river plume systems, but it is impractical to apply this approach globally to all rivers. Recently, generalized parameterizations have been proposed to estimate the export of riverine fresh water to the open ocean (Izett & Fennel, 2018, https://doi.org/10.1002/2017GB005667; Sharples et al., 2017, https://doi.org/10.1002/2016GB005483). Here the relationships of Izett and Fennel ( ), https://doi.org/10.1002/2017GB005667 are used to derive global estimates of open‐ocean export of fresh water and dissolved inorganic silicate, dissolved organic carbon, and dissolved organic and inorganic phosphorus and nitrogen. We estimate that only 15–53% of riverine fresh water reaches the open ocean directly in river plumes; nutrient export is even less efficient because of processing on continental shelves. Due to geographic differences in riverine nutrient delivery, dissolved silicate is the most efficiently exported to the open ocean (7–56.7%), while dissolved inorganic nitrogen is the least efficiently exported (2.8–44.3%). These results are consistent with previous estimates and provide a simple way to parameterize export to the open ocean in global models. [ABSTRACT FROM AUTHOR]

Rivers link terrestrial ecosystems and marine ecosystems, and they transport large amounts of substances into oceans each year, including several forms of silicon (Si), carbon (C), and other nutrients. However, river damming affects the water flow and biogeochemical cycles of Si, C, and other nutrients through biogeochemical interacting processes. In this review, we first summarize the current understanding of the effects of river damming on the processes of biogeochemical Si cycle, especially the source, composition, and recycling process of biogenic silica (BSi). Then, we introduce dam impacts on the cycles of C and some other nutrients. Dissolved silicon in rivers is mainly released from phytolith dissolution and silicate weathering. BSi in suspended matter or sediments in most rivers mainly consists of phytoliths and mainly originates from soil erosion. However, diatom growth and deposition in many reservoirs formed by river interception may significantly increase the contribution of diatom Si to total BSi, and thus significantly influence the biogeochemical Si, C, and nutrient cycles. Yet the turnover of phytoliths and diatoms in different rivers formed by river damming is still poorly quantified. Thus, they should be further investigated to enhance our understanding about the effects of river damming on global biogeochemical Si, C and nutrient cycles. [ABSTRACT FROM AUTHOR]

With the development of hydropower in the karst area of Southwest China, a series of cascade canyon reservoirs have been formed through the construction of dams. Given that hydrodynamic conditions in canyon reservoirs play a pivotal role in controlling the spatiotemporal distribution of physical and chemical properties of the stored water, hydrodynamic characteristics are of great importance in understanding biogeochemical cycles in those reservoirs. To further this understanding, a field campaign was conducted in the Wujiangdu Reservoir of Guizhou Province. It was found that from the reservoir inlet to the front of the dam, velocity ( v) was negatively correlated and had a logarithmic relationship with distance along the ship track ( s) under dry-season flow conditions [ v = −0.104ln ( s) + 0.4756]. Analysis showed that dry-season flow velocity had no significant correlation with water temperature, pH, or dissolved oxygen (DO). However, when velocity decreased to 0.061 m/s, water depth increased abruptly. In addition, DO displayed a sudden drop and the trend in pH changed from increasing to decreasing, while water temperature showed an opposite trend, indicating the existence of a transition zone from the river to the reservoir. [ABSTRACT FROM AUTHOR]

Multiple processes support the significant efflux of carbon dioxide (CO2) from rivers and streams. Attribution of CO2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO2 production is closely related to O2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO2 and O2 concentrations and other water quality parameters were analyzed in two data sets: a synoptic field study and nationwide water quality monitoring data. CO2 and O2 concentrations were strongly negatively correlated in both data sets ( ρ = −0.67 and ρ = −0.63, respectively), although the correlations were weaker in high-alkalinity environments. In nearly all samples, the molar oversaturation of CO2 was a larger magnitude than molar O2 undersaturation. We used a dynamically coupled O2CO2 model to show that lags in CO2 air-water equilibration are a likely cause of this phenomenon. Lags in CO2 equilibration also impart landscape-scale differences in the behavior of CO2 between high- and low-alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO2 dynamics are primary steps in understanding the sources and magnitude of CO2 oversaturation in rivers and streams. [ABSTRACT FROM AUTHOR]

Sediment hypoxia is a growing problem and has negative ecological impacts on the aquatic ecosystem. Hypoxia can disturb the biodiversity and biogeochemical cycles of both phosphorus (P) and nitrogen (N) in water columns and sediments. Anthropogenic eutrophication and internal nutrient release from lakebed sediment accelerate hypoxia to form a dead zone. Thus, sediment hypoxia mitigation is necessary for ecological restoration and sustainable development. Conventional aeration practices to control sediment hypoxia, are not effective due to high cost, sediment disturbance and less sustainability. Owing to high solubility and stability, micro-nanobubbles (MNBs) offer several advantages over conventional water and wastewater treatment practices. Clay loaded oxygen micro-nanobubbles (OMNBs) can be delivered into deep water sediment by gravity and settling. Nanobubble technology provides a promising route for cost-effective oxygen delivery in large natural water systems. OMNBs also have the immense potential to manipulate biochemical pathways and microbial processes for remediating sediment pollution in natural waters. This review article aims to analyze recent trends employing OMNBs loaded materials to mitigate sediment hypoxia and subsequent pollution. The first part of the review highlights various minerals/materials used for the delivery of OMNBs into benthic sediments of freshwater bodies. Release of OMNBs at hypoxic sediment water interphase (SWI) can provide significant dissolved oxygen (DO) to remediate hypoxia induced sediment pollution Second part of the manuscript unveils the impacts of OMNBs on sediment pollutants (e.g., methylmercury, arsenic, and greenhouse gases) remediation and microbial processes for improved biogeochemical cycles. The review article will facilitate environmental engineers and ecologists to control sediment pollution along with ecological restoration. [Display omitted] • OMNBs can mitigate eutrophication induced anoxia and sediment pollution. • Cost-effective technology for environmental remediation and ecological restoration. • Positive influence of OMNBs on microbial processes and geochemical cycle. • Reduced emission of GHGs and sustainable restoration of aquatic biodiversity. [ABSTRACT FROM AUTHOR]

Bioactive trace metals play a significant role as micronutrients in the ocean and therefore it is important to evaluate their sources. Submarine groundwater discharge (SGD) has been recognized as an input of trace metals to the coastal sea. Here, we investigated the significance of SGD as a source of dissolved trace metals (dTM) to the coastal sea in a regional area such as the North Western (NW) Mediterranean Sea. We analysed dTM concentrations in SGD end-members and incorporate data on SGD dTM concentrations and water flows reported in previous studies carried out in this area, to estimate the following ranges of SGD-driven dTM fluxes (in 10 6 mol y − 1 ): Cd: 0.0007–0.03, Co: 0.004–0.11, Cu: 0.09–1.9, Fe: 1.8–29, Ni: 0.09–1.9, Pb: 0.002–0.06, Zn: 0.38–12. These fluxes were compared to dTM fluxes from riverine discharge and atmospheric deposition, demonstrating that SGD is a major source of dTM to the NW Mediterranean Sea. Whilst riverine inputs are limited to the surrounding of river mouths and atmospheric fluxes are distributed throughout the whole basin mainly during sporadic depositional events, SGD represents a permanent, albeit seasonally variable, source of metals to most of the coastal areas. SGD-driven dTM inputs may be even more significant, in relative terms, in other coastal regions of the Mediterranean Sea where rivers are scarce, as it is the case of the African coast and many islands. This study highlights the relevance of SGD as a source of dTM to the Mediterranean Sea and the need of its consideration in the calculation of metal budgets in the basin and in the investigation of biogeochemical cycles in coastal areas. [ABSTRACT FROM AUTHOR]

The contribution of phytoliths to total biogenic silica (BSi) volumes in rivers worldwide, and the associated implications for the biogeochemical cycle, require in-depth study. Based on samples from rivers in Peninsular Malaysia, this project investigated the source and characteristics of BSi found in Asian tropical rivers, as well as the process of reverse weathering taking place in these fluvial systems. Results indicated that BSi samples collected in sediments consisted of phytolith, diatom and sponge spicules. Phytoliths, predominantly of the elongate form, comprised 92.8%-98.3% of BSi in the Pahang River. Diatom BSi in this river consisted mainly of pennatae diatoms, but represented a relatively small proportion of the total BSi volume. However, diatom BSi (predominantly of the Centricae form) was more prevalent in the Pontian and Endau Rivers with shares of 68.8% and 79.3% of the total BSi volumes, respectively, than Pahang River. Carbon contents of the BSi particulates ranged from 1.85% to 10.8% with an average of 4.79%. These values are higher than those recorded in other studies to date, and indicate that BSi plays a major role in controlling permanent carbon burial. This study suggests that phytoliths from terrestrial plants are the primary constituents of BSi in the rivers of Peninsular Malaysia, and therefore represent a significant proportion of the coastal silica budget. [ABSTRACT FROM AUTHOR]

Northern rivers connect a land area of approximately 20.5 million km2 to the Arctic Ocean and surrounding seas. These rivers account for ~10% of global river discharge and transport massive quantities of dissolved and particulate materials that reflect watershed sources and impact biogeochemical cycling in the ocean. In this paper, multiyear data sets from a coordinated sampling program are used to characterize particulate organic carbon (POC) and particulate nitrogen (PN) export from the six largest rivers within the pan-Arctic watershed (Yenisey, Lena, Ob', Mackenzie, Yukon, Kolyma). Together, these rivers export an average of 3055 × 109 g of POC and 368 × 109 g of PN each year. Scaled up to the pan-Arctic watershed as a whole, fluvial export estimates increase to 5767 × 109 g and 695 × 109 g of POC and PN per year, respectively. POC export is substantially lower than dissolved organic carbon export by these rivers, whereas PN export is roughly equal to dissolved nitrogen export. Seasonal patterns in concentrations and source/composition indicators (C:N, δ13C, Δ14C, δ15N) are broadly similar among rivers, but distinct regional differences are also evident. For example, average radiocarbon ages of POC range from ~2000 (Ob') to ~5500 (Mackenzie) years before present. Rapid changes within the Arctic system as a consequence of global warming make it challenging to establish a contemporary baseline of fluvial export, but the results presented in this paper capture variability and quantify average conditions for nearly a decade at the beginning of the 21st century. [ABSTRACT FROM AUTHOR]

Ecosystem metabolism, that is, gross primary productivity (GPP) and ecosystem respiration (ER), controls organic carbon (OC) cycling in stream and river networks and is expected to vary predictably with network position. However, estimates of metabolism in small streams outnumber those from rivers such that there are limited empirical data comparing metabolism across a range of stream and river sizes. We measured metabolism in 14 rivers (discharge range 14-84 m s) in the Western and Midwestern United States (US). We estimated GPP, ER, and gas exchange rates using a Lagrangian, 2-station oxygen model solved in a Bayesian framework. GPP ranged from 0.6-22 g O m d and ER tracked GPP, suggesting that autotrophic production supports much of riverine ER in summer. Net ecosystem production, the balance between GPP and ER was 0 or greater in 4 rivers showing autotrophy on that day. River velocity and slope predicted gas exchange estimates from these 14 rivers in agreement with empirical models. Carbon turnover lengths (that is, the distance traveled before OC is mineralized to CO) ranged from 38 to 1190 km, with the longest turnover lengths in high-sediment, arid-land rivers. We also compared estimated turnover lengths with the relative length of the river segment between major tributaries or lakes; the mean ratio of carbon turnover length to river length was 1.6, demonstrating that rivers can mineralize much of the OC load along their length at baseflow. Carbon mineralization velocities ranged from 0.05 to 0.81 m d, and were not different than measurements from small streams. Given high GPP relative to ER, combined with generally short OC spiraling lengths, rivers can be highly reactive with regard to OC cycling. [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]

The biogeochemical cycles of phosphorus in rivers are intimately linked to the processes that occur in terrestrial ecosystems. Riverine networks hold a crucial role in the transfer of nutrients from the land and atmosphere to the coastal oceans and often act as pool for numerous inorganic and organic compounds. Biogeochemical transformation of elements in river network is extensively influenced by catchment alteration and anthropogenic inputs. By means of the rising consciousness of human impact on the excellence of rivers, emphasis is given on rivers, as an ecosystem by itself and also on the river-coast continuum. In this study, the major forcing functions that affect the riverine composition of phosphorus have been examined, in Indian context. An attempt has been made to study and inventorize phosphorus flux from major Indian rivers. Relatively high concentrations of dissolved PO43- (dissolved inorganic phosphorus - DIP) are observed in few of the Indian rivers, which may be due to modifications in river catchment. The flow of DIP and particulate inorganic phosphorus to the coastal ocean from Indian rivers is estimated to be about 190 × 10³ tonnes year-1 and 1367 × 10³ tonnes year-1 respectively. Suspended load is significant in Indian rivers and its cumulative flux is in the order of 1450 × 106 tonnes year-1. The DIP concentration in the Indian rivers is more than twice the concentration observed for the other rivers in the world. Such increased nutrient input into the riverine system reflects the imbalances and alterations in terrestrial sources. Thus, the quantity and quality of nutrient input to the rivers need to be monitored to cope with the existing and future climatic and environmental changes. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2010.03.001 Byline: Zaihua Liu, Wolfgang Dreybrodt, Haijing Wang Abstract: The magnitudes, variations, locations and mechanisms responsible for the global atmospheric CO.sub.2 sink are uncertain and under continuing debate. Previous studies have focused mainly on the sinks in the oceans, and soil and vegetation on the continents. Here, we show, based on theoretical calculations and field monitoring evidence, that there is an important but previously underestimated sink for atmospheric CO.sub.2 as DIC-dissolved inorganic carbon that results from the combined action of carbonate dissolution, the global water cycle and the photosynthetic uptake of DIC by aquatic organisms in ocean and land. The sink constitutes up to 0.8242Pg C/a, amounting to 29.4% of the terrestrial CO.sub.2 sink, or 10.4% of the total anthropogenic CO.sub.2 emission. 0.244 PgC/a are transferred to the sea via continental rivers and 0.2278Pg C/a by meteoric precipitation over the seas. 0.119Pg C/a is released back to the atmosphere again, and 0.2334Pg C/a is stored in the continental aquatic ecosystem. Therefore, the net sink is estimated as 0.7052Pg C/a. This sink may increase with an intensification of the global water cycle as a consequence of global warming, rising anthropogenic emissions of CO.sub.2 and carbonate dust in atmosphere, and afforestation, which increases the soil pCO.sub.2 and thus the carbonate dissolution. Fertilization with the elements N, P, C, Fe, Zn, and Si increases the organic matter storage/burial by aquatic organisms and thus decreases the CO.sub.2 return to the atmosphere. Based on the ensemble mean projection of global warming for the year 2100 by IPCC, it is estimated that the atmospheric CO.sub.2 sink will increase by 21%, or about 0.18Pg C/a. However, the uncertainty in the estimation of this sink needs further exploration. Author Affiliation: (a) The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, 46 Guanshui Road, Guiyang 550002, China (b) Karst Dynamics Laboratory, Institute of Karst Geology, Chinese Academy of Geological Sciences, 50 Qixing Road, Guilin 541004, China (c) Institute of Experimental Physics, University of Bremen, Bibliothekstra[sz]e 1, Bremen 28359, Germany (d) School of Geographical Sciences, Southwest University, 2 Tiansheng Road, Chongqing 400715, China Article History: Received 29 May 2008; Accepted 10 March 2010

Autumn is a season of dynamic change in forest streams of the northeastern United States due to effects of leaf fall on both hydrology and biogeochemistry. Few studies have explored how interactions of biogeochemical transformations, various nitrogen sources, and catchment flow paths affect stream nitrogen variation during autumn. To provide more information on this critical period, we studied (1) the timing, duration, and magnitude of changes to stream nitrate, dissolved organic nitrogen (DON), and ammonium concentrations; (2) changes in nitrate sources and cycling; and (3) source areas of the landscape that most influence stream nitrogen. We collected samples at higher temporal resolution for a longer duration than typical studies of stream nitrogen during autumn. This sampling scheme encompassed the patterns and extremes that occurred during base flow and stormflow events of autumn. Base flow nitrate concentrations decreased by an order of magnitude from 5.4 to 0.7 µmol L−1 during the week when most leaves fell from deciduous trees. Changes to rates of biogeochemical transformations during autumn base flow explained the low nitrate concentrations; in-stream transformations retained up to 72% of the nitrate that entered a stream reach. A decrease of in-stream nitrification coupled with heterotrophic nitrate cycling were primary factors in the seasonal nitrate decline. The period of low nitrate concentrations ended with a storm event in which stream nitrate concentrations increased by 25-fold. In the ensuing weeks, peak stormflow nitrate concentrations progressively decreased over closely spaced, yet similarly sized events. Most stormflow nitrate originated from nitrification in near-stream areas with occasional, large inputs of unprocessed atmospheric nitrate, which has rarely been reported for nonsnowmelt events. A maximum input of 33% unprocessed atmospheric nitrate to the stream occurred during one event. Large inputs of unprocessed atmospheric nitrate show direct and rapid effects on forest streams that may be widespread, although undocumented, throughout nitrogen-polluted temperate forests. In contrast to a week-long nitrate decline during peak autumn litterfall, base flow DON concentrations increased after leaf fall and remained high for 2 months. Dissolved organic nitrogen was hydrologically flushed to the stream from riparian soils during stormflow. In contrast to distinct seasonal changes in base flow nitrate and DON concentrations, ammonium concentrations were typically at or below the detection limit, similar to the rest of the year. Our findings reveal couplings among catchment flow paths, nutrient sources, and transformations that control seasonal extremes of stream nitrogen in forested landscapes. [ABSTRACT FROM AUTHOR]

Background Riverine particles undergo a rapid transformation when they reach estuaries. The rapid succession of hydrodynamic and biogeochemical regimes forces the particles to flocculate, settle and enter the sediment pool. The rates and magnitudes of flocculation depend on the nature of the particles which are primarily affected by the types and quantities of organic matter (OM). Meanwhile, the OM characteristics vary widely between environments, as well as within a single environment due to seasonal climate and land use variability. We investigated the effect of the OM types and quantities through laboratory experiments using natural estuarine particles from the Mississippi Sound and Atchafalaya Bay as well as model mixtures of montmorillonite and organic molecules (i.e., biopolymers (guar/xanthan gums) and humic acid). Results Biopolymers promote flocculation but the magnitude depends on the types and quantities. Nonionic guar gum yields much larger flocs than anionic xanthan gum, while both of them exhibit a nonlinear behavior in which the flocculation is the most pronounced at the intermediate OM loading. Moreover, the effect of guar gum is independent of salinity whereas the effect of xanthan gum is pronounced at higher salinity. Meanwhile, humic acid does not affect flocculation at all salinity values tested in this study. These results are echoed in the laboratory manipulation of the natural estuarine particles. Flocculation of the humic acid-rich Mississippi Sound particles is unaffected by the OM, whereas that of biopolymerrich Atchafalaya Bay particles is enhanced by the OM. Conclusions Flocculation is positively influenced by the presence of biopolymers that are produced as the result of marine primary production. Meanwhile, humic acid, which is abundant in the rivers that drain the agricultural soils of Southeastern United States, has little influence on flocculation. Thus, it is expected that humic acid-poor riverine particles (e.g., Mississippi River, and Atchafalaya River, to a lesser degree) may be prone to rapid flocculation and settling in the immediate vicinity of the river mouths when mixed with biopolymer-rich coastal waters. It is also expected that humic acid-rich riverine particles (e.g., Pearl River) may resist immediate flocculation and be transported further away from the river mouth. [ABSTRACT FROM AUTHOR]

Spatial scale of processes is necessary information to design appropriate ecosystem research and management. In-stream ecological processes are tightly linked with their adjacent terrestrial environment and impacted by modifications of riparian environments, particularly in small, forested streams. Quantitative understanding of how the dynamic coupling of these adjacent ecosystems varies along stream continua is necessary to enhance stream research and management., In temperate, forest-dominated watersheds of British Columbia, Canada, we examined the stream-forest linkage along a stream-size gradient based on biogeochemical analysis of fine particulate organic matter retained in streambed sediment ( FPOMS). We analysed FPOMS at 48 stations in 14 cobble-bed streams from 2·2 to 39 m bankfull widths during summer low-flow conditions and tested their relationships with local, forest-related, environmental factors (light penetration, benthic coarse POM biomass, riparian-soil properties) along the stream-size gradient., Carbon/nitrogen ratio of FPOMS ( FPOMS-C/N) was distinctly lower in stream bankfull widths larger than an estimated threshold around 7·6 m and corresponded with decrease in N/chlorophyll a of FPOMS and inorganic nitrogen concentration. We found significant linkages of FPOMS-C/N with local streamside environments in streams narrower than the threshold (but not in widths <~3 m). For wider streams, reduced canopy cover was likely to enhance in-stream nitrogen cycling by algae and consistently lowered FPOMS-C/N regardless of streamside environment. These indicate a nonlinear attenuation of the strength of linkage between in-stream biogeochemical processes and forest-related environments along the stream-size gradient., Synthesis and applications. The relationship of biogeochemical proxies of in-stream fine particulate organic matter with local environments can be a practical tool for measuring the strength of stream-forest linkage. Furthermore, regional- and stream-specific threshold stream widths dividing in-stream biogeochemical regimes can be an important landmark for ecologists and managers to refine designs for stream research and management at appropriate spatial scales. For stream widths narrower than a threshold, reach-scale linkages between stream and terrestrial environments can be a major focus, and riparian conditions need to be carefully managed to protect stream habitats. For stream widths wider than a threshold, catchment impacts on streams at larger spatial scales would be a more important focus. [ABSTRACT FROM AUTHOR]

The significant implication of chromophoric dissolved organic matter (CDOM) for water quality and biogeochemical cycle leads to an increasing need of CDOM monitoring in coastal regions. Current ocean-color algorithms are mostly limited to open-sea water and have high uncertainty when directly applied to turbid coastal waters. This paper presents a semianalytical algorithm, quasi-analytical CDOM algorithm (QAA-CDOM), to invert CDOM absorption from Earth Observing-1 (EO-1) Hyperion satellite images. This algorithm was developed from a widely used ocean-color algorithm QAA and our earlier extension of QAA. The main goal is to improve the algorithm performance for a wide range of water conditions, particularly turbid waters in estuarine and coastal regions. The algorithm development, calibration, and validation were based on our intensive high-resolution underwater measurements, International Ocean Color Coordinating Group synthetic data, and global National Aeronautics and Space Administration Bio-Optical Marine Algorithm Data Set data. The result shows that retrieved CDOM absorption achieved accuracy (\root mean square error (RMSE) = 0.115\ \m^-1 and R^2 = 0.73) in the Atchafalaya River plume area. QAA-CDOM is also evaluated for scenarios in three additional study sites, namely, the Mississippi River, Amazon River, and Moreton Bay, where ag(440) was in the wide range of 0.01–15 \m^-1. It resulted in expected CDOM distribution patterns along the river salinity gradient. This study improves the high-resolution observation of CDOM dynamics in river-dominated coastal margins and other coastal environments for the study of land–ocean interactive processes. [ABSTRACT FROM AUTHOR]

The understanding of nutrient uptake in streams is impeded by a limited understanding of how geomorphic setting and flow regime interact with biogeochemical processing. This study investigated these interactions as they relate to transient storage and nitrate uptake in small agricultural and urban streams. Sites were selected across a gradient of channel conditions and management modifications and included three 180-m long geomorphically distinct reaches on each of two streams in north-central Colorado. The agricultural stream has been subject to historically variable cattle-grazing practices, and the urban stream exhibits various levels of stabilisation and planform alteration. Reach-scale geomorphic complexity was characterised using highly detailed surveys of channel morphology, substrate, hydraulics and habitat units. Breakthrough-curve modelling of conservative bromide (Br−) and nonconservative nitrate (NO3−) tracer injections characterised transient storage and nitrate uptake along each reach. Longitudinal roughness and flow depth were positively associated with transient storage, which was related to nitrate uptake, thus underscoring the importance of geomorphic influences on stream biogeochemical processes. In addition, changes in geomorphic characteristics due to temporal discharge variation led to complex responses in nitrate uptake. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Excitation-emission matrix (EEM) fluorescence was combined with parallel factor analysis (PARAFAC) to model base-extracted particulate (POM) and dissolved (DOM) organic matter quality in the Neuse River Estuary (NRE), North Carolina, before and after passage of Hurricane Irene in August 2011. Principle components analysis was used to determine that four of the PARAFAC components (C1-C3 and C6) were terrestrial sources to the NRE. One component (C4), prevalent in DOM of nutrient-impacted streams and estuaries and produced in phytoplankton cultures, was enriched in the POM and in surface sediment pore water DOM. One component (C5) was related to recent autochthonous production. Photoexposure of unfiltered Neuse River water caused an increase in slope ratio values (SR) which corresponded to an increase in the ratio C2:C3 for DOM, and the production of C4 fluorescence in both POM and DOM. Changes to the relative abundance of C4 in POM and DOM indicated that advection of pore water DOM from surface sediments into overlying waters could increase the autochthonous quality of DOM in shallow microtidal estuaries. Modeling POM and DOM simultaneously with PARAFAC is an informative technique that is applicable to assessments of estuarine water quality. [ABSTRACT FROM AUTHOR]