Sediment dredging is a widely used strategy to mitigate eutrophication and internal phosphorus (P) loading in urban lakes. However, a comprehensive understanding of its long-term effectiveness and the underlying mechanisms controlling internal P loading remains limited. Furthermore, the effects of dredging on sediment microbial community structure, functional diversity, and subsequent recovery processes have not been systematically investigated. To address this gap, a simulated dredging experiment was conducted on sediments from a shallow eutrophic urban lake (Xuanwu Lake, XWL). To comprehensively track the post-dredging dynamics of phosphorus and the sediment microbiome, we integrated high-resolution geochemical profiling using diffusive gradients in thin films and porewater samplers. We analyzed microbial community structure using 16S rRNA gene sequencing and metabolic functional diversity through community-level physiological profiling. We found that following an initial disturbance period, dredging effectively reduced the release flux of soluble reactive phosphorus (SRP) from sediments by an average of 27.4%. Concurrently, dredging significantly altered P speciation, which was characterized by a 29.4% decrease in mobile P and a 40.3% increase in the stable aluminum-bound P fraction. Importantly, dredging restructured seasonal P dynamics, making them primarily governed by the iron (Fe) cycle. This led to a paradoxical effect: enhanced P sequestration during cold and oxic seasons but a substantially intensified P-release during warm anoxic summers. This summer P-release rebound was driven by blooms of the iron-reducing bacterium Geobacter and was further exacerbated by the redeposition of P-rich suspended particulate matter. Our findings reveal that dredging is not merely a physical removal process but initiates a long-term Fe-S-P biogeochemical succession centered at the sediment-water interface, fundamentally regulated by microbial community dynamics. These findings challenge the conventional view of dredging as a one-time measure and highlight the need for season-specific monitoring and long-term management strategies.

River ecosystems are significant sources of nitrous oxide (N2O). However, the patterns and regulating mechanisms of N2O emissions in coastal rivers under the combined influence of urbanization and salinity gradients remain insufficiently explored. Therefore, this study investigated coastal rivers in Tianjin, located in the temperate climate zone of northern China, and classified six rivers according to dominant land-use types. Through field sampling and monitoring across four seasons, a total of 296 water samples and 74 sediment samples were collected and analyzed. We systematically examined the intensity, spatiotemporal variations, and key factors controlling N2O emissions from these coastal rivers. N2O fluxes in coastal rivers followed the order of Sewage river (1.75 ± 1.44 μmol m-2 h-1) > Mixed rivers (0.88 ± 0.91 μmol m-2 h-1) > Urban river (0.63 ± 0.27 μmol m-2 h-1) > Agricultural rivers (0.55 ± 0.20 μmol m-2 h-1). The N2O fluxes in the Sewage river were considerably higher than those in the other river types (p < 0.05), and all rivers exhibited the lowest N2O fluxes in autumn. Water nitrogen and phosphorus concentration primarily controlled the N2O emissions in the coastal rivers of Tianjin, whereas salinity played a significant regulatory role. Thus, the default emission factor recommended by the Intergovernmental Panel on Climate Change overestimated the N2O emission potential of the coastal rivers by 2.36-4.33 folds. This highlighted the need to revise this factor for systems strongly influenced by urbanization and human activities to improve the accuracy of regional and global riverine N2O emission inventories. These findings provide crucial empirical monitoring data for N2O emissions across different coastal river typologies, serving as an essential step toward correcting systematic overestimations and fundamentally improving future GHG inventories.

Anthropogenic eutrophication and global warming have rendered shallow lakes a critical natural source of atmospheric CH4. However, critical knowledge gaps remain regarding the vertical and seasonal dynamics, response to eutrophication, and regulatory mechanisms of aerobic methane oxidation in shallow lake waters, limiting our ability to predict CH4 cycling and develop effective emission mitigation strategies. Seasonally on-board studies were carried out to examine the methane oxidation rate (MOR) and the specific methane oxidation rate (SMOR) in the water, as well as the abundance and community structure of methanotrophs in Lake Chaohu, located in the lower reaches of the Yangtze River in China. Annual pmoA gene copy numbers across water layers ranged from 246 ± 148 to 742 ± 473 copy mL-1 at the eight study sites. The two dominant methanotroph genera were Methylocystis and Methylosinus, belonging to Type II, and together accounting for over 90% of the methanotrophs on annual average. The MOR and SMOR ranged from 1.32 ± 0.30 to 31.9 ± 6.0 nmol L-1 h-1 and from 0.40 ± 0.09 d-1 to 2.75 ± 0.51 d-1, respectively. The highest MOR and SMOR values were observed in summer, and the bottom water exhibited higher MOR and SMOR than the surface water. Water temperature, CH4, pmoA gene, suspended particulate matter, and chlorophyll a were identified as the main driving factors. Seasonally, aerobic methane oxidation reduced 62.3 ± 14.9% - 76.6 ± 9.1% of the diffusive CH4 emissions in the lake. This study underscores the crucial role of aerobic methane oxidation in reducing diffusive CH4 emissions in shallow eutrophic lakes with the limited water depth.

Wastewater treatment plant (WWTP) effluents are an important source of disinfection by-products (DBPs) in urban waters; however, their potential influence on riverine methane (CH4) dynamics remains unclear. By combining field investigations with microcosm experiments, this study examined the associations between typical DBPs and dissolved CH4 concentrations and diffusive fluxes in WWTP-receiving rivers and evaluated the effects of haloacetic acids (HAAs) on sediment CH4 production. Field monitoring showed that the annual average concentrations of haloacetic acids (HAAs), trihalomethanes (THMs), and haloacetonitriles (HANs) at WWTP outfalls were 13.62 ± 3.17, 15.62 ± 4.27, and 2.09 ± 0.62 μg L-1, respectively. Dissolved CH4 concentrations (annual average 0.14 ± 0.05 μmol L-1) and diffusive fluxes (annual average 4.72 ± 1.36 μmol m-2 h-1) exhibited negative spatial correlations with DBP concentrations, particularly along upstream-downstream transects near the outfalls. The 28-day microcosm experiments showed an overall decline in sediment CH4 production with increasing HAA concentrations. Across the A1-A4 treatments, corresponding to 10, 50, 100, and 200 μg L-1 HAAs, cumulative CH4 concentrations decreased from 77.90 to 43.35 μmol L-1, while the methane production rate (MPR) decreased from 0.39 to 0.26 μg CH4 g-1 wet sediment d-1. CH4 production at the lowest HAA concentration of 10 μg L-1 was also lower than that in the control. The abundance of the methanogenic functional gene mcrA also declined with increasing HAA concentrations. Mantel tests and partial least squares structural equation modeling (PLS-SEM) further supported negative associations of HAA concentration with mcrA abundance and CH4 production. Overall, the microcosm results provide preliminary evidence that HAAs may inhibit sediment CH4 production potential, whereas the field observations indicate that DBPs are associated with lower dissolved CH4 concentrations and diffusive CH4 fluxes near WWTP outfalls. These findings highlight the potential relevance of DBPs to riverine CH4 dynamics and the suggest that DBPs may represent an overlooked factor associated with CH4 cycling in wastewater-impacted river systems.

Urban lakes suffer from eutrophication fueled by excessive phosphorus (P) accumulation; however, comprehensive understanding of P dynamics under intense anthropogenic pressure remains unclear. We elucidate these processes in a typical heavily utilized urban lake (XWL), in Nanjing City, China. Using a year-long monthly sampling campaign, we integrated DGT and HR-Peeper with sediment P fractionation analysis. This approach allowed for the systematic assessment of Fe, S, and P speciation and mobility within the sediments and porewater, thereby elucidating the mechanisms governing P transformation. We observed distinct temporal patterns and strong correlations among the concentrations of CDGT-Fe, CDGT-P, and CDGT-S. These findings confirm that P cycling in XWL is predominantly governed by Fe redox processes, with the S cycle playing a significant secondary role. Critically, we identified an anomalous seasonal pattern for internal P loading: SRP flux across the SWI varied annually between-1.10 and 5.25 mg/(m2 d), exhibiting a seasonal switch from a net P source in summer and autumn to a net sink in winter. Statistical analysis indicated that the SRP flux correlated positively with TWand porewater SRP, but negatively with sediment Fe-P and Org-P. Corroborating evidence from DGT-induced flux in sediments model simulations demonstrated increased sediment P resupply capacity in autumn. We emphasize the critical need to address the amplifying effect of synergistic Fe-S interactions on internal P loads in urban lakes, providing new insights for dynamic P management strategies.

Lakes are a significant source of N2O emissions, and the spatial and temporal variability of lake N2O emissions contributes to the uncertainty in global N2O budget estimates. Currently, researchers have a limited understanding of N2O emission patterns and controlling factors in different habitats of large, shallow lakes. This study focused on the phytoplankton-dominated zone (PDZ), submerged plants-dominated zone (SDZ), and emergent plants-dominated zone (EDZ) of Lake Taihu, China. The N2O fluxes in each lake zone were recorded monthly for one year using the static floating chamber method, to explore the spatio-temporal variations in N2O emissions and their controlling factors in lakes with complex habitat types. Additionally, a microcosm experiment was used to identify the effect of algal addition on the water column nitrogen (N) load and N2O emissions. Results showed that the annual average N2O fluxes at the water-air interface were 0.66 +/- 0.62 mu mol m(-2) h(-1), 0.09 +/- 0.06 mu mol m(-2) h(-1), and 0.56 +/- 0.93 mu mol m(-2) h(-1), from the PDZ, SDZ, and EDZ, respectively. The N2O fluxes in the PDZ were significantly (p < 0.05) higher than those from the SDZ (p < 0.05), while those in the EDZ did not differ significantly from the other two zones. During summer, the N2O concentrations and fluxes in all lake zones were higher than those in the other seasons. Statistical analysis indicated that water TN, temperature (T-W), and TP are crucial factors regulating N2O emissions. The algae addition experiment demonstrated that phytoplankton aggregation promoted N2O emissions by altering DO content and water column N load. Overall, this study emphasizes the importance of considering habitat differences in regional and global lake N2O emission estimates.

The comprehensive effects of environmental dredging on heavy metals (HM) are still uncertain. This study comprehensively evaluates the long-term effects of dredging on the environmental risk and bioavailability of HM (Cu, Ni, Zn, Pb, Cd, Cr, and As) in Lake Taihu, China, by comparing simulated dredged treated (D) and undredged (UD) sediment cores under in-situ conditions for one year. Threshold effect level (TEL), geological accumulation index (Igeo), potential ecological risk index (RI), and ratios of secondary phase and primary phase (RSP) methods were used to assess the environmental risk of sediment HM; and the diffusive gradient in thin-films (DGT) technique was applied to assess the bioavailability of sediment HM. The results indicate that Cd was the most polluted metal assessed by the Igeo and RI method, and that dredging significantly reduced the total content of sediment HM, particularly for Cu, Zn, and Cd, and its Igeo and RI index, but caused a slight effect on its fractionation and distinct effect on RSP index. These indices changed independently and seasonally. Porewater analysis suggested higher HM concentrations in summer and winter may cause corresponding deterioration in overlying water. DGT analysis suggested a large proportion of metal-DOM complexes and showed that dredging reduced the bioavailability of Ni, Cd, and As but had a mixed impact (effective and/or ineffective impact varied with seasons) on other metals. These findings highlight the complexity of dredging effects on sediment HM dynamics, underscoring the importance of seasonal monitoring and multi-geoengineering techniques targeted at total and specific metals.

Sediment dredging efficacy for controlling internal nitrogen (N) and phosphorus (P) loadings in deep-water ecosystems remains elusive. To provide a scientific basis for potential future dredging plans, this study employed an in situ dredging simulation experiment in a seasonally stratified reservoir. Based on intact sediment core incubation, high-resolution Peeper (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to quantify the impact of dredging on internal N and P loading. The results indicated that TN and TP contents decreased by 48.4 % and 50.1 %, respectively, with declines observed across all N and P forms after dredging. In addition, the microbial community structure did not cause any long-term damage. NH4+-N fluxes were enhanced owing to the exposure of the ammonia-rich layer in the early stage (86.38 mg m-2 d-1 and 193.88 mg m-2 d-1 in non-dredging (ND) and dredging (D) treatments, respectively). Progressive organic matter depletion coupled with sustained pore water diffusion following dredging collectively contributed to the transition into an effective control period for NH4+-N release after 110 days. SRP fluxes ranged from -1.82 ± 0.45 to 1.32 ± 0.48 mg m-2 d-1 and -0.62 ± 0.45 to 2.76 ± 0.62 mg m-2 d-1 over the experimental period for the D and ND treatments, respectively, with a significant reduction (p < 0.05) after dredging. These results were mainly attributed to the reduction in Fe-P and Org-P after dredging, as well as the reduction in oxygen consumption in the nascent sediment-water interface and organic matter in the dredged sediments, collectively inhibiting Fe-P mobilization and Org-P mineralization. These findings highlight the feasibility of using sediment dredging to control internal loading in deep-water ecosystems, while also considering the potential environmental risks associated with elevated NH4+-N release in the early stages after dredging.

Sediment dredging and submerged macrophyte restoration are effective ways to improve water quality, especially in eutrophic shallow lakes. However, the impact of dredging on the interactions between dissolved organic matter (DOM) and microbial communities in lake surface sediments remains unexplored, especially in areas planting submerged macrophytes. In this study, DOM composition and microbial community composition were explored by fluorescence spectrum-parallel factor analysis, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), and high-throughput sequencing during the transition from agricultural production lake to ecological lake. Results showed that dredging reduced dissolved organic carbon in surface sediments. Dredging had significantly higher effects than submerged macrophyte decomposition (SMD) on microbial community composition (especially micro-eukaryotic) and biomarkers in surface sediments. Dredging and SMD had more impacts on micro-eukaryotic community assembly than prokaryotic community. The co-occurrence network reveals that dredging and SMD reshaped the ecological relationships between micro-eukaryotes and prokaryotes in microbial food webs. Particularly, SMD damaged the microbial food web structure in surface sediments. FT-ICR-MS further reveals that the relative abundances of tannins and lignin in DOM were 76 % and 35 % higher in dredged areas than in non-dredged areas, respectively. DOM molecular compositions affected the micro-eukaryotic community more than the prokaryotic community. The Mantel test shows that dredging changed nutrients and DOM fluorescence components in sediments. The microbial community was closely related to the DOM molecular composition. This study highlights the importance of proper implementation of dredging combined with submerged macrophyte to restore eutrophic water bodies.

Internal phosphorus (P) loading in sediments can cause or exacerbate lake eutrophication. Limited information is available on the coupling of P, sulfur (S) and iron (Fe) in the sediments of eutrophic urban lakes and the mechanisms regulating P release. Field samples from Lake Tangxun, the largest urban lake in China, were collected quarterly over one year from three sites with varying levels of P loading. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) were employed to determine soluble reactive phosphorus (SRP), soluble Fe, DGT-labile P/Fe/S, as well as SRP fluxes. The findings indicated that the trends of DGT-labile P and S during the warm seasons were synchronized and significantly (p < 0.05) positively correlated in all sites, indicating that P remobilization was mainly controlled by sulfur cycling. Strong dissimilatory sulfate reduction (DSR) contributed significantly to the generation of high SRP fluxes at the sediment-water interface (SWI) in the warm seasons. The SRP fluxes demonstrated significant (p < 0.05) variability across the study sites, ranging from 0.18 to 49.41 mg m-2 d-1, with the highest SRP fluxes in areas with higher sediment P content. SRP fluxes and DGT-labile P/Fe/S were significantly (p < 0.05) higher in summer and autumn than in winter. The statistical results indicate that dissolved oxygen (DO), and sediment available P content were crucial controlling factors for P flux. Our results highlight the dominant role of S cycling in internal P dynamics in this urban lake. We suggest problematic SRP fluxes that lead to eutrophication can be reduced by covering or removing sediment in areas of high P loading to reduce available P pools, or implementing artificial oxygenation during critical warm periods to inhibit Fe/S reduction.

Soluble reactive phosphorus (SRP) depletion can be observed during prolonged algal blooms in shallow lakes, yet the role of sediments in this process remains inadequately understood. To elucidate the regulatory role of sediments in SRP cycling during bloom development, we conducted field campaigns throughout the 2024 rainy season in Lake Dianchi-a highly eutrophic plateau lake characterized by persistently low SRP concentrations. Water quality, sediment properties, SRP fluxes, and adsorption characteristics were analyzed at 30 sites to assess spatiotemporal shifts in sediment source-sink behavior. Water column SRP declined significantly from 15 ± 14 μg l-1 in the early rainy season to 4 ± 1 μg l-1 in the late rainy season (p < 0.001), coinciding with a pronounced rise in sediment Fe-P content (R2 = 0.442, p < 0.001), suggesting adsorption induced by external P inputs. Despite high sediment total P (2142 ± 833 mg kg-1) and a large reactive P fraction (44.0% ± 16.3%), adsorption capacity remained substantial (Qmax: 1223-3643 mg kg-1), attributed to calcareous sediments enriched with exogenous Fe-Ti minerals. Incubation experiments captured a dynamic transition in sediment SRP release flux (RSRP) from sink to source across Lake Dianchi, with mean RSRP increasing from 0.72 ± 5.02 to 1.74 ± 1.69 mg m-2d-1 from the early to late rainy season. RSRP was governed by water SRP concentration, reactive sediment P pool size, and P-binding strength, and was further amplified by temperature. These findings demonstrate the dual regulatory role of sediments: they buffer external P pulses while storing reactive P that fuels internal loading and sustains biological uptake. Such tightly coupled water-sediment feedback during the rainy season poses a major challenge for the restoration of eutrophic lakes such as Dianchi.

Algae and macrophytes in lake ecosystems regulate nitrous oxide (N2O) emissions from eutrophic lakes. However, knowledge of diurnal N2O emission patterns from different habitats remains limited. To understand the diurnal patterns and driving mechanisms of N2O emissions from contrasting habitats, continuous in situ observations (72 h) of N2O fluxes from an algae-dominated zone (ADZ) and reed-dominated zone (RDZ) in Lake Taihu were conducted using the Floating Chamber method. The results showed average N2O emission fluxes of 0.15 ± 0.06 and 0.02 ± 0.04 μmol m-2 h-1 in the ADZ and RDZ in autumn, respectively. The significantly higher (p < 0.05) N2O fluxes in the ADZ were mainly attributed to differences in nitrogen (N) levels. The results also showed significant diurnal differences (p < 0.05) in the N2O emission fluxes within the ADZ and RDZ, and daytime fluxes were significantly higher (p < 0.05) than nighttime fluxes. The statistical results indicated that N2O emissions from the ADZ were mainly driven by diurnal variations in N loading and the dissolved oxygen (DO) concentration, and those from the RDZ were more influenced by DO, redox potential, and pH. Finally, we determined the proper time for routine monitoring of N2O flux in the two habitats. Our results highlight the importance of considering diverse habitats and diurnal variations when estimating N2O budgets at a whole-lake scale.

Lakes are a crucial component of global carbon cycling and play a key role in the global CO2 and CH4 budgets. Intensified anthropogenic activity contributes to eutrophication, and reduces macrophytes while increasing phytoplankton in shallow lakes. However, the information on how the CO2 and CH4 fluxes at water-air interface temporally varies across habitats dominated by various primary producers in lakes and how CH4 emission counteracts CO2 adsorption is limited. In this study, in situ fluxes of CO2 and CH4 were measured monthly for one year at four sites in Lake Taihu, China. These sites had contrasting habitats dominated by high-density phytoplankton, low-density phytoplankton, floating-leaved macrophytes, and submerged macrophytes. We found that the CO2 fluxes ranged from - 439 f 385 to - 106 f 348 mu mol m- 2h- 1 and the CH4 fluxes ranged from 1.96 f 3.79 to 10.7 f 14.8 mu mol m- 2h- 1. The fluxes of CO2 and CH4 were mainly regulated by water pH, phosphorus levels, and sediment organic carbon. All four habitats acted as sinks of atmospheric CO2 and sources of atmospheric CH4, and the habitats with high-density phytoplankton showed the highest CO2 absorption and strongest CH4 emission. Generally, the CH4 emission counteracted 7-69 % of the atmospheric CO2 absorption in the four sites. These results collectively revealed that in a large, shallow eutrophic lake, habitats dominated by various primary producers consistently adsorb atmospheric CO2, even being counteracted by CH4 emission. These findings provide valuable insights for understanding carbon cycling and CO2 and CH4 dynamics across the water-air interface in shallow lakes.

Organic carbon decomposition in lake sediments contributes substantially to the global carbon cycle and is strongly affected by temperature. However, the magnitude of temperature sensitivity (Q10) of decomposition and the underlying factors remain unclear at the continental scale. Carbon quality temperature (CQT) hypothesis asserts that less reactive and more recalcitrant molecules tend to have higher temperature sensitivities, but its support is challenged by complex composition of organic matter and environmental constraints. Here, we quantified Q10 of the sediments across 50 freshwater ecosystems along a 3500 km north-south transect, and characterized the quality of sediment dissolved organic carbon with chemodiversity reflected in molecular richness, functional traits (i.e., molecular weight, bioavailability, etc.) and composition. We further included classic environmental variables, such as climatic, physicochemical and microbial factors, to explore how Q10 is constrained by these factors or carbon quality. We found that Q10 varied greatly across lakes, with the mean value of 1.78 +/- 0.62, but showed nonsignificant latitudinal pattern. Q10 was primarily predicted by chemodiversity and showed an increasing trend with the biochemical recalcitrance indicated by traits such as aromaticity and standard Gibb's Free Energy at both molecular and compositional levels. This suggests that carbon quality is the crucial determinant of Q10 in lakes, supporting the CQT hypothesis. Moreover, Q10 decreased linearly with the increase of molecular richness, implying that the resistance of decomposition to warming is associated with higher molecular diversity. Compared with the structural equation model containing only environmental variables, inclusion of chemodiversity increased 32.8% of the explained variation in Q10, and chemodiversity was the only driver showing direct effects. Collectively, this study illustrates the importance of chemodiversity in shaping the pattern of Q10, and has significant implications for accurately predicting the carbon turnover in lake ecosystems in the context of global warming. The mean value of temperature sensitivity of organic carbon decomposition in lake sediments is 1.78 +/- 0.62. The quantity of sediment organic carbon determines the absolute rate of decomposition, while the quality of organic carbon determines the sensitivity of decomposition to warming. At both molecular and compositional levels, functional traits of DOM revealed the positive correlation between Q10 and biochemical recalcitrance, thus supporting the carbon quality temperature hypothesis.image

Shallow lakes are an important natural source of atmospheric methane (CH4), and the input of autochthonous organic matter (OM) into their sediments encourages methanogenesis. Although algal- and macrophytic-originated OM in these lakes are expected to have different impacts on methanogenesis and methanogenic archaeal communities in lake sediments owing to their various properties, their specific influence and role in sediment remain unclear. In this study, a 148-day incubation was carried out by adding algal- and macrophytic-OM to the sediments of shallow eutrophic Lake Chaohu and Lake Taihu in China. CH4 was periodically monitored, while the methanogens were examined via qPCR and high-throughput sequencing at the end of incubation. Algal-OM stimulated CH4 production more than macrophytic-OM in both sediments, with the rates initially increasing and then decreasing before reaching a relative constant. Macrophytic-OM promoted CH4 production to a comparable extent in both lakes, while algal-OM promoted greater CH4 in Lake Chaohu than in Lake Taihu. However, algal-OM did not significantly increase mcrA gene copies, while macrophytic-OM did by 17.0-20.1-fold. Algal-OM potentially promoted the methylotrophic pathway in Lake Taihu but did not change the methanogenic structure in Lake Chaohu. Comparatively, macrophytic-OM promoted CH4 production mainly by acetoclastic methanogen proliferation in both lakes. More CH4 release with algal-OM compared to macrophytic-OM deserves further attention owing to the prevailing increasing algal blooms and the declining macrophyte population in lakes.

Freshwater reservoirs are known as important sources of atmospheric N2O; however, the patterns and controls of N2O emissions from these reservoirs remain poorly understood, particularly for reservoirs severely disturbed by aquaculture activities. This study aimed to bridge this knowledge gap by determining the spatiotemporal and magnitude variations of N2O concentrations, dynamics of the drivers of N2O concentration and diffusive flux, and N2O emission patterns from reservoirs severely affected by human activities. We measured N2O concentrations and estimated diffusive fluxes in a temperate reservoir-including the upstream river inflow zone-in northern China, which was experiencing acute aquaculture disturbance. A thin boundary layer equation was employed, and we analyzed the thermal and dissolved oxygen stratification conditions and other environmental auxiliary factors to identify crucial factors and underlying mechanisms that regulate N2O emissions. Our results showed that aquaculture activities led to the accumulation of high levels of organic matter and nutrients in sediments. The annual average water-dissolved N2O concentration and diffusive flux across the water-air interface in the reservoir were 24.97 +/- 11.16 nmol L-1 and 0.43 +/- 0.36 mu mol m(-2).h(-1), respectively. The water-dissolved N2O concentrations exhibited supersaturation, indicating the reservoir as a net source of atmospheric N2O. Both N2O concentrations and diffusive fluxes exhibited strong spatiotemporal variability during the study period. The lacustrine area of the reservoir was the hotspot for N2O emissions. The statistical results indicated that nitrogen loading, temperature, and dissolved oxygen (DO) were the key drivers of N2O emissions. Moreover, dam construction and intensive aquaculture disturbances have altered aquatic conditions and significantly affected N2O emissions. Our results indicated that N2O emission estimates for the entire reservoir should cover different reservoir zones and sampling seasons. We suggest measures such as sediment dredging to reduce sediment organic matter and nutrient loading for mitigating N2O emissions from the reservoir.

Clean soil is a potential capping material for controlling internal nutrient loading and helping the recovery of macrophytes in eutrophic lakes, but the long-term effects and underlying mechanisms of clean soil capping under in-situ conditions remain poorly understood. In this study, a three-year field capping enclosure experiment combining intact sediment core incubation, in-situ porewater sampling, isotherm adsorption experiments and analysis of sediment nitrogen (N) and phosphorus (P) fractions was conducted to assess the long-term perfor-mance of clean soil capping on internal loading in Lake Taihu. Our results indicate that clean soil has excellent P adsorption and retention capacity as an ecologically safe capping material and can effectively mitigate NH4+-N and SRP (soluble reactive P) fluxes at the sediment-water interface (SWI) and porewater SRP concentration for one year after capping. The mean NH4+-N and SRP fluxes of capping sediment were 34.86 mg m- 2 h-1 and-1.58 mg m- 2 h-1, compared 82.99 mg m- 2 h-1 and 6.29 mg m- 2 h-1 for control sediment. Clean soil controls internal NH4+-N release through cation (mainly Al3+) exchange mechanisms, while for SRP, clean soil can not only react with SRP due to its high Al and Fe content, but also stimulate the migration of active Ca2+ to the capping layer, thus precipitating as Ca-bound P (Ca-P). Clean soil capping also contributed to the restoration of macrophytes during the growing season. However, the effect of controlling internal nutrient loading only lasted for one year under in-situ conditions, after which the sediment properties returned to pre-capping conditions. Our results highlight that clean Ca-poor soil is a promising capping material and further research is needed to extend the longevity of this geoengineering technology.

Reservoirs are important sources of emissions of the greenhouse gas methane, but the temporal and spatial variability of methane emissions and the underlying mechanisms driving methane flux in cascade reservoirs remain poorly understood. In this study, we used floating chambers to study methane emission fluxes and their temporal and spatial variations in three temperate cascade reservoirs with different nutrient levels and morphologies in northern China. We measured the methane production rate, sediment properties and water quality to identify key factors driving methane emissions. The results showed that the three cascade reservoirs were persistent sources of atmospheric methane during the ice-free period, and the methane flux ranged from 1.01 to 107.17 mu mol m-2h- 1, with a mean of 11.17 mu mol m-2h- 1. The mean methane flux of the eutrophic reservoir was more than 2 and 4 times that of the two mesotrophic reservoirs. Overall, methane flux showed high spatial and temporal variability within and among reservoirs, and the three reservoirs showed different patterns of spatial and temporal variability. Statistical analysis showed that the three reservoirs had different key drivers, indicating that the methane emissions of the three reservoirs were regulated by different underlying mechanisms. When all three reservoirs were pooled, we found that Chl-a was the most important driver of methane emissions. Our results suggest that to obtain an accurate estimate of whole-reservoir methane flux, different reservoir zones and seasons must be considered. Our results also highlight that regional and global methane budget upscaling should include considerations of reservoir diversity (different climatic zones, morphology and trophic levels).

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH4) emissions. The temporal and spatial variability of CH4 emissions from different reservoirs results in uncertainty in the estimation of the global CH4 budget. In this study, surface water CH4 concentrations were measured and diffusive CH4 fluxes were estimated via a thin boundary layer model in a temperate river-reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH4 concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH4 flux across the water-air interface ranged from 0.66 to 3.61 μmol/(m2•hr), with an annual average of 1.67 ± 0.75 μmol/(m2•hr). During the study period, the dissolved CH4 concentration was supersaturated and was a net source of atmospheric CH4. Notably, CH4 concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH4 emissions, and its flux was significantly higher than that of the tributary and main basin; the CH4 flux in autumn was greater than that in other seasons. We also deduced that the CH4 concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH4 flux from temperate reservoirs to estimate the CH4 budget at regional and global scales.

Internal nutrient loadings pose a high risk of being an additional N and P source, exacerbating eutrophication and deteriorating water quality. In this study, we selected the Daheiting Reservoir (DHTR) in North China, with a pronounced water level gradient, to investigate internal N and P loadings, estimate N and P fluxes across the sediment‒water interface based on the pore water profiles, and reveal the potential effects of water discharge from an upstream reservoir and high-intensity cage aquaculture on the risks of internal N and P release. The results indicated that DHTR presented with severe internal nutrient loadings, and the N and P fluxes showed significant spatiotemporal variations. NH4+-N and soluble reactive phosphorus (SRP) fluxes were higher in deep areas (averages of 26.14 and 9.9 mgm-2d-1, respectively) than in shallow areas near inflows (averages of 5.0 and 1.24 mgm-2d-1, respectively). Unexpectedly, the estimated NH4+-N and SRP fluxes were the lowest in summer (averages of 3.94 and 0.33 mgm-2d-1, respectively), which may have been influenced by seasonal thermal stratification and copious discharge from the hypolimnion of the upstream reservoir (Panjiakou Reservoir). Comparison of annual internal and external N and P loadings revealed that water discharge from the upstream Panjiakou Reservoir was the dominant source of N and P to the reservoir, contributing up to 83.6% of N input and 55.4% of P input. The internal P loading also contributed to water eutrophication to a great extent, accounting for 34.7% of the total P input. Our results highlight the impact of upstream reservoir discharge operation on downstream reservoir water quality and the importance of controlling the internal nutrient loading in cascade reservoirs, and further provide theoretical and practical foundations for the development of policies and strategies to conserve reservoir ecosystems.