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Hydrodynamic and morphodynamic forces interacting across the sediment‐water interface control the biogeochemistry in the hyporheic zone. When investigating the redox zonation within streambeds, dissolved oxygen (O2) is considered a key solute to the understanding of river ecosystems. However, no field studies have measured the spatiotemporal O2 distribution linked to bedform celerity induced by changes in stream water velocity. Therefore, we developed and tested an innovative in situ setup in the River Erpe, Germany. The setup combines a planar O2 optode and O2 flow‐through cells for eight sediment depths to capture the variability in O2 dynamics, and a laser scanner to capture bedform morphodynamics, which was used to calculate bedform celerity. The setup was tested under different stream water velocities between 0.1 and 0.5 m/s. We found that O2 patterns in the streambed depend on stream water velocity. At low velocities, bedforms were stationary and a stable redox zonation with limited O2 penetration in the streambed (up to 4 cm) was observed. As we increased the velocity up to 0.3 m/s, the spatiotemporal variability of O2 distribution across the bedform increased, with anoxic patches moving along the migrating bedforms. At the highest velocity tested (0.5 m/s), the sediment bed was constantly oxygenated with deeper O2 penetration as compared to slower velocities. The present study provides proof of concept for in situ O2 measurements in small rivers, which helps to refine laboratory and mesocosm experiments, improve the knowledge of the processes involved in natural environments, and develop more sustainable river management strategies.

Dissolved oxygen (DO) level at the sediment–water interface is one key factor controlling redox-sensitive processes, such as nutrient cycling. Microcosm experiments with sediment collected from three reservoirs were performed to quantify the influences of water column oxygenation (oxic, anoxic, oxygen fluctuation), sediment characteristics (grain size distribution, total nitrogen and total phosphorus contents, microbial activities), and their interactions on nutrient fluxes from sediments to the water column. Algal growth bioassays were also performed using water from the microcosms to determine which conditions produced the most favorable growth conditions. Anoxic conditions increased the release of dissolved inorganic nitrogen (DIN), mainly as ammonium and phosphates, compared to the other DO conditions. Such effects were likely due to an inhibition of the nitrification–denitrification coupling process for DIN and a reductive dissolution of Fe (III) oxides for phosphates. Following this increased nutrient availability, algal growth in the bioassays was the highest in water collected from microcosms exposed to anoxic conditions. Under both oxic and anoxic conditions, the percentage of fine sediment particles led to decreasing DIN and phosphates fluxes by reducing the nutrient diffusion rate from sediments to the water column. Finally, both DO and sediment grain size controlled the contribution of sediments to reservoir eutrophication.

Poyang Lake, the largest freshwater lake in China and a globally significant wetland, is intricately connected to the hydrological dynamics of the Yangtze River via a complex river‐lake exchange system. This system generates to unique seasonal fluctuations, forming a distinctive seasonal lake system, which influences hydrological and hydrodynamic processes across floodplains. Recent years have witnessed significant alterations in the hydrological patterns of the Yangtze River, notably in water levels, thereby impacting the nutrient dynamics such as nitrogen transformation at the sediment‐water interface of Poyang Lake. This study establishes a coupled model integrating hydrodynamics and nitrogen transformation to elucidate the impacts of the hydrological regime of Yangtze River on nitrogen transformation in Poyang Lake after the operation of Three Gorges Dam. Findings reveal spatiotemporal variations in both hydraulics and nitrogen transformation within the seasonal lake system. Notably, the recharge rate between surface water and groundwater experiences a substantial shift, surpassing 60%. Furthermore, the nitrification rate at the sediment‐water interface escalates by 28.5%, and the denitrification rate increases by 21.3% owing to pronounced alterations in the hydrological regime. However, this intensified transformation does not translate to enhanced efficiency, as the nitrogen transformation efficiency declines to 72.3% of its original rate. This research provides a theoretical framework for understanding the ecological and environmental impacts of human interventions on Poyang Lake and highlights the implications for managing other floodplains and seasonal lakes globally, such as lakes on floodplains of Amazon River and Mekong River, which face similar challenges in hydrological dynamics and ecosystem health. Key Points Seasonal lake system hydraulics and nitrogen transforms with temporal and spatial patterns of change The hydrological regime of Yangtze River has seriously affected the exchange process at the surface water‐groundwater interface The nitrogen removal efficiency decreased to 72.3% of the original due to the change of hydrological regimes of Yangtze River

The sediment–water interfaces (SWI) of streams serve as important biogeochemical hotspots in watersheds and contribute to whole-catchment reactive nitrogen budgets and water-quality conditions. Recently, the SWI has been identified as an important source of nitrous oxide (N₂O) produced in streams, with SWI residence time among the principal controls on its production. Here, we conducted a series of controlled manipulations of SWI exchange in an urban stream that has high dissolved N₂O concentrations and where we concurrently evaluated less-mobile porosity dynamics. Our experiments took place within isolated portions of two sediment types: a coarse sandy stream bed resulting from excess road-sand application in the watershed, and a coarse sand mixed with clay and organic particles. In these manipulation experiments we systematically varied SWI vertical-flux rates and residence times to evaluate their effect on the fate of nitrate and production rates of N₂O. Our experiments demonstrate that the fate and transport of nitrate and N₂O production are influenced by hydrologic flux rates through SWI sediments and associated residence times. Specifically, we show that manipulations of hydrologic flux systematically shifted the depth of the bulk oxic–anoxic interface in the sediments, and that nitrate removal increased with residence time. Our results also support the emerging hypothesis of a ‘Goldilocks’ timescale for the production of nitrous oxide, when transport and reaction timescales favor incomplete denitrification. Areal N₂O production rates were up to threefold higher during an intermediate residence-time experiment, compared to shorter or longer residence times. In our companion study we documented that the studied sediments were dominated by a long-residence-time less-mobile porosity domain, which could explain why we observed N₂O production even in bulk-oxic sediments. Overall, we have experimentally demonstrated that changes to SWI hydrologic residence times and SWI substrate associated with urbanization can change the biogeochemical function of the river corridor.

Lakes are exposed to anthropogenic pollution including the release of allochthonous bacteria into their waters. Antibiotic resistance genes (ARGs) stabilize in bacterial communities of temperate lakes, and these environments act as long-term reservoirs of ARGs. Still, it is not clear if the stabilization of the ARGs is caused by a periodical introduction, or by other factors regulated by dynamics within the water column. Here we observed the dynamics of the tetracycline resistance gene (tetA) and of the class 1 integron integrase gene intI1 a proxy of anthropogenic pollution in the water column and in the sediments of subalpine Lake Maggiore, together with several chemical, physical and microbiological variables. Both genes resulted more abundant within the bacterial community of the sediment compared to the water column and the water-sediment interface. Only at the inset of thermal stratification they reached quantifiable abundances in all the water layers, too. Moreover, the bacterial communities of the water-sediment interface were more similar to deep waters than to the sediments. These results suggest that the vertical distribution of tetA and intI1 is mainly due to the deposition of bacteria from the surface water to the sediment, while their resuspension from the sediment is less important.

Continuously feed-based aquaculture leads to excessive nitrogen and phosphate loads, degrading the culture environment. In this study, lotus ( Nelumbo nucifera Gaertn ) was planted in yellow catfish ( Pelteobagrus fulvidraco Richardson ) culture ponds to construct an integrated agriculture–aquaculture system to reduce nutrient accumulation and remediate the aquacultural environment. Results showed that additional lotus cultivation significantly reduced TN and TP concentrations in the pond water by 51.81% and 34.68%, respectively. Meanwhile, the concentrations of NH 4 + -N, TN, and TP in the sediment of co-culture ponds significantly decreased by 54.12%, 8.55%, and 10.85%, respectively, at the conclusion of the experiment period. Additionally, with the growth of lotus, the co-culture ponds exhibited significantly lower nitrogen and phosphorus fluxes across the sediment–water interface. Spearman correlation analysis revealed a significant positive correlation between the NH 4 + -N flux and TN concentration in the water and between TN flux and NO 3 − -N concentration in the water, respectively. This study suggests that the lotus-fish co-culture offers an optimal integrated agriculture–aquaculture system capable of mitigating excessive nutrient loads and improving the aquacultural environment.

Purpose This study aimed to provide in situ evidence for the simultaneous coupling of iron (Fe), sulfide (S(-II)) and phosphorus (P) in the sediment‒water interface (SWI) under anaerobic conditions, thereby enhancing the understanding of the process and mechanism of phosphorus dynamics in eutrophic lake sediments. Materials and methods A 300-day redox-cycle process of aerobic-anoxic-anaerobic was conducted in six periods. High-resolution methods, including diffusive gradient in thin films (DGT) and pore water sampling technology (HR-Peeper) were used to measure Fe, S(-II) and P in the vertical sediment. A 96-well microplate spectrophotometer was employed to detect the dissolved and labile concentrations of Fe(II) and P. The determination of S(-II) was performed using the computer imaging densitometry (CID) scanning method. The ratio of the labile to the soluble concentration ( R diff , C DGT /C SOL ) was calculated to characterize the kinetics of Fe and P release from sediments into the overlying water. Results and discussion The vertical distribution of labile and soluble P, Fe(II) and S(-II) in water and sediment was measured. Findings reveal concentration peaks of these elements at 10–40 mm depth in the SWI, with a significant positive correlation between them. The Fe–P coupling is the main driver of phosphorus release, with S(-II) participating in this process. Organic matter is utilized as an electron donor by sulfate-reducing bacteria, which reduces Fe(III) to Fe(II) and forms FeS precipitation, further promoting the release of Fe–P. The ratio of labile to dissolved concentration (C DGT /C SOL ) peaks (20–40 mm in the SWI) during the transition from aerobic to anaerobic conditions, indicating a strong replenishment ability of sediment to pore water at this stage. Conclusions Under anaerobic conditions, there is a synchronous vertical distribution of P, Fe(II), and S(-II), with the reduction of Fe oxides being the main cause of P release from sediments. Under highly anaerobic conditions, S(-II) indirectly affects the dynamic of P by combining with Fe(II), potentially becoming the dominant factor. Changes in R diff -P and R diff -Fe infer that in the SWI of Lake Taihu sediments, Fe and P have a high capacity to replenish from the sediment particles to the pore water.

In recent years, the eutrophication of lakes has accelerated in cold arid regions; the release of nutrients from sediments is an important contributor. The sequential extraction method, high-resolution peeper (HR-Peeper), and diffusive gradients in thin films (DGT) techniques were used to study the occurrence characteristics, release risk, and release mechanism of phosphorus (P) at the sediment–water interface (SWI) of Ulanor Wetland in the Hulun Lake Basin, Inner Mongolia, China. The mean total P concentration in overlying water was lower in August than that in May. Dissolved organic P (DOP) or particulate P (PP) was the main form of P in the overlying water. PP dominates in May and DOP in August. Refractory P was the main form of P in sediments. The concentrations of soluble reactive P and DGT-active P in the pore water of the sediment column were higher than those in the overlying water, and the concentrations were higher in August than those in May. Release of P in the wetland sediments occurred during the non-frozen seasons, with a higher risk in August than in May. The good linear correlation between dissolved P, Fe, and Mn in the DGT profiles verified their co-release due to the anaerobic reduction of Fe/Mn oxides. Moreover, alkaline sediments are also conducive to the release of sediment P. This study can provide data and theoretical support for eutrophication control in Ulanor Wetland and other similar water bodies in cold and arid regions.

As an important component of inland waters, shallow macrophyte-dominated lakes significantly influence regional carbon budgets. By using the static chamber-gas chromatography method and the sediment in-situ simulation, continuous fixed-point observations of CO2 exchange fluxes (F(CO2)) at water-air and water-sediment interfaces of shallow macrophyte-dominated Hurleg Lake were conducted. Combination with watershed meteorological conditions and lake water environmental parameters, their influencing factors were explored. The results revealed significant diel variations in F(CO₂) at both interfaces, characterized by peaks in the early morning and troughs in the evening or late night-a common feature of shallow macrophyte-dominated lakes. The composition of submerged macrophyte communities considerably affected the relative contribution of sediment-released CO₂ to the net atmospheric flux. The maximum contribution was observed in areas dominated by Potamogeton, followed by Myriophyllum zones, while the minimum occurred in Chara beds. Nocturnal F(CO₂) played a critical role in sustaining the carbon source function of the lake, accounting for 22.65%–42.90% of the total daily flux at the water-air interface and 5.57%-64.54% at the water-sediment interface across different vegetated and unvegetated zones. Neglecting nocturnal F(CO₂) would substantially increase uncertainties in estimating the lake’s overall carbon budget. The F(CO₂) at the water-air interface was primarily regulated by water temperature, pH, dissolved oxygen, and atmospheric pressure, whereas F(CO₂) at the sediment-water was mainly driven by porewater CO₂ concentration, sediment porosity, and water temperature.

This study investigates the decomposition process of algal blooms (ABs) in eutrophic lakes and its impact on the labile endogenous nitrogen (N) cycle. In situ techniques such as diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) were employed to decipher the vertical distribution of N fractions within the sediment–water interface (SWI) in Taihu, China. Additionally, an annular flume was used to simulate regional differences in lake conditions and understand labile nitrogen transformation during AB decomposition. This study reveals that the NH4+-N fraction exuded from algae is subsequently converted into NO3-N and NO2-N through nitrification, resulting in a significant increase in the concentrations of NO3−-N and NO2−-N at the SWI. The decomposition of algae also induces a significant increase in dissolved organic matter (DOM) concentration, referring to humic acid and humus-like components; a seven-millimeter decrease in dissolved oxygen (DO) penetration depth; as well as a significant decrease in the pH value near the SWI, which consequently promotes denitrification processes in the sediment. Moreover, the decomposition process influences nitrogen distribution patterns and the role conversion of sediments between a “source” and a “sink” of nitrogen. This investigation provides evidence on the migration and/or transformation of N fractions and offers insights into the dynamic processes across the SWI in eutrophic lakes.