Hydrological connectivity affects nitrogen migration and retention in the land‒river continuum

Ecological water conveyance-driven wetland hydrological connectivity and morphological changes in arid regions: An analysis of the Taitema Lake wetland

Wetland hydrological connectivity is essential for both providing the structure and function of wetland ecosystems and restoring them. The Taitema Lake wetland exhibit a high sensitivity to hydrological changes, with alterations in watershed dynamics having a significant impact on their ecosystem function and structure. There are notable gaps in the assessment studies of hydrological connectivity. This study examines the spatiotemporal changes and distribution characteristics of hydrological connectivity under ecological water conveyance using Landsat images taken before and after the conveyance from 2002 to 2023. Techniques used include the MSPA model and the landscape index-based hydrological connectivity index. The findings demonstrate that: 1. Ecological water transport enhances wetlands' hydrological connection; the Tarim River contributes 39.43 % to the water conveyance volume of the area, while the Cherchen River contributes 60.57 %. 2. Their respective impacts on hydrological connectivity are 93.72 % and 6.28 %. A favorable association exists between a water area and connectivity. Alterations in water areas can somewhat influence the connectedness of aquatic patches; however, they are not the primary determinant of hydrological connectivity.3. The primary patches with elevated important values can be found near the bridge across the Taitema Lake, which serves as the nucleus of the overall wetland hydrological connectivity. 4. Alterations in the core patches significantly influence hydrological connectivity, whereas bridging and branching serve a secondary function.This paper presents practical examples of examining ecohydrological processes in wetland ecosystems situated in arid regions. Additionally, it establishes a scientific foundation for researching ecological water conveyance and enhancing water resource management.

Hydrological connectivity shape the nitrogen pollution sources and microbial community structure in a river-lake connected system

Intensified agricultural and urban activities have exacerbated nitrogen pollution, posing a severe threat to freshwater ecosystems, particularly under intensified agricultural and urbanization activities. This study systematically examined Baiyangdian Lake (BYD) and its principal inflowing rivers, namely Fu River (FH), Baigouyin River (BGY), and Xiaoyi River (XY) to characterize the spatio-temporal distribution, primary nitrogen sources, and the impact on sediment microbial community structure. Results indicated pronounced seasonal variations in both nitrogen pollution loads and sources, with riverine nitrogen levels rising markedly from dry season (May) to wet season (August). Atmospheric deposition accounted for 43.9% of the nitrogen input dry season, whereas in wet season, agricultural fertilizers and sewage contributed 23.3 and 26.4%, respectively. Additionally, microbial communities exhibited distinct temporal and spatial patterns, with significantly higher diversity and species richness being during the wet season. The, microbial composition shifted, as evidenced by a decline in Proteobacteria and increases in Firmicutes and Actinobacteriota. River-lake connectivity emerged as a critical factor, with FH displaying a notably higher connectivity index in wet season compared to BGY and XY rivers. Structural equation modeling (SEM) analysis further revealed that river-lake connectivity was significantly and positively correlated with nitrogen pollution, was significantly and negatively correlated with microbial α-diversity. These findings demonstrated that river-lake connectivity directly influenced nitrogen concentrations, which in turn indirectly modulated microbial diversity.

Temperature has an enhanced role in sediment N2O and N2 fluxes in wider rivers

Riverine N2O and N2 fluxes, key components of the global nitrogen budget, are known to be influenced by river size (often represented by average river width), yet the specific mechanisms behind these effects remain unclear. This study examined how environmental and microbial factors influenced sediment N2O and N2 fluxes across rivers with varying widths (2.8 to 2,000 m) in China. Sediment acted as sources of both N2O and N2 emissions, with both N2 (0.2 to 20.8 mmol m-2 d-1) and N2O fluxes (0.7-54.2 μmol m-2 d-1) decreasing significantly as river width increased. N2 fluxes were positively correlated with denitrifying bacterial abundance, whereas N2O fluxes, when normalized by the abundance of denitrifying bacteria, were negatively correlated with the abundance of N2O-reducing microbes. Water physicochemical factors, particularly temperature and nitrate, were more important drivers of these fluxes than sediment factors. Nitrate significantly increased denitrifying bacterial abundance, whereas higher temperatures enhanced cell-specific activity. Lower N2O and N2 emissions in wider rivers were attributed to decreased denitrifying microbial abundance and lower denitrification rates, in addition to the commonly assumed reduction in exogenous N2O and N2 inputs. Rolling regression analysis showed that nitrate concentration had a stronger effect on sediment N2O and N2 fluxes in narrower rivers, whereas temperature was more influential in wider rivers. This difference is attributed to more stable nitrate concentrations and decreased nitrogen removal efficiency in wider rivers, while temperature variation remained consistent across all river widths. Beyond sediments, temperature had a greater effect on excess N2O concentrations than nitrate in the overlying water of wider rivers (>165 m), highlighting its broader impact. This study provides new biogeochemical insights into how river width influences sediment N2O and N2 fluxes and highlights the importance of incorporating temperature into flux predictions, particularly for wider rivers.

Hydrological connectivity and dissolved organic matter impacts nitrogen and antibiotics fate in river-lake system before and after extreme wet season

The impact and mechanism of hydrological connectivity and dissolved organic matter on the fate of nitrogen and antibiotics are still lack off in a river-lake connected system under climate extreme events. This study examined the fate of NO3--N, 38 antibiotics, and dissolved organic matter (DOM) in Baiyangdian Basin, through dry and wet seasonal (after extreme rainfall) samplings at 2023. In the system, NO3--N and ∑antibiotics average concentrations were higher in the dry season, while the relative abundance of humic-like components was higher in the wet season. Spatial autocorrelation analysis showed that the high-high clusters of pollutants and DOM components were mainly distributed in rivers, and the temporal difference was significant. MixSIAR and PMF model were respectively applied to nitrogen and antibiotics sources apportionment. The results showed that non-point sources (NPS) of nitrogen and antibiotics exhibited an upward trend, while the point sources decreased from dry to wet seasons. Hydrological connectivity was characterized by using δ18O-H2O, which was higher in the wet season. Partial least squares path model revealed that hydrological connectivity directly impacted humic-like components, which were the direct influencing factor of the concentration and NPS for antibiotics and nitrogen in the connected system. Extreme rainfall weaken the impact of hydrological connectivity on the concentration and NPS of pollutants, while enhanced the impact of humic-like components on pollutants NPS. These findings clarified the impact mechanism of hydrological connectivity and DOM on nitrogen and antibiotics fate in the connected system, which plays an important role in future water quality management under extreme events.

Water quality improves with increased spatially surface hydrological connectivity in plain river network areas

Hydrological connectivity remarkably affects the water quality of river-lake systems, particularly in densely urbanized plain river network areas, where its impact remains unclear. The growing urbanization and rapid changes in hydrological networks make it more challenging to manage water quality effectively. Understanding how hydrological connectivity changes and the influence on key water quality variables is crucial for improving management strategies. We quantified hydrological connectivity between lakes in the northern Taihu Lake Basin using a connectivity topological model based on graph theory and landscape ecology. XG-Boost models were developed to elucidate the potential threshold effect of hydrological connectivity on key water quality parameters. These models were accompanied by linear mixed-effect (LME) models, which included land use types as a random effect to evaluate the response relationship between hydrological connectivity and water quality. Results indicated that the spatiotemporal dynamics of hydrological connectivity decreased over the last 20 years. Furthermore, changes in hydrological connectivity considerably influenced environmental variables in river-lake network areas. The XG-Boost models identified a Pij value of 0.02 as a potential threshold, at which spatial hydrological connectivity begins to impact water quality as concentrations change steadily above this threshold. The LME models confirmed that enhanced spatial hydrological connectivity was generally associated with reduced concentrations of TN, TP, NH3-N, and CODMn, and increased DO levels. In addition, hydrological connectivity was influenced by factors such as the shortest river path between lakes and hydraulic facilities along the path. This finding suggests that hydrological connectivity can be restored to improve water quality by refining river network topology, optimizing existing sluice schedules, or removing unnecessary dikes. These results highlight the potential of hydrological connectivity optimization to support water quality improvement strategies in complex urban river networks.

Offset integrity reduces environmental risk: Using lessons from biodiversity and carbon offsetting to inform water quality offsetting in the catchments of the Great Barrier Reef

Environmental offsetting has been developed as a mechanism to facilitate the benefits from economic development while avoiding or minimizing environmental harm. This is achieved by compensating for environmental impacts at one location by generating equivalent environmental improvements elsewhere. However, experience with biodiversity and carbon offsetting indicates it can be difficult to ensure the integrity of offsets. Under recent legislation in the catchments of the Great Barrier Reef (GBR), Australia, it is mandatory for water quality emissions from new or expanded point source development to be offset by reducing pollution elsewhere, frequently through reducing non-point source pollution (NPSP). Therefore, informed by experience with biodiversity and carbon offsetting, we summarised sources of uncertainty in NPSP reduction that would influence water quality offset integrity; estimated the maximum potential demand for water quality offsets from sewage treatment plants, the largest point source emitter of total nitrogen (TN) in the GBR catchments, between 2018 and 2050; and discussed the implications of both on the ability of offsetting to counterbalance the impact of economic development in catchments where nitrogen loads have a large influence on the health of important GBR ecosystems. The catchments surrounding the population centres of Cairns and Mackay had both a potentially high future demand for nitrogen water quality offsets and nitrogen loads with a strong influence on the health of the GBR. Consequently, any low integrity water quality offsets in these catchments could jeopardise progress toward the water quality improvements needed to ensure the continued health of the GBR. Water quality offsetting has numerous strengths as a policy instrument however substantial uncertainties remain related to environmental outcomes. Until further research can reduce these uncertainties, water quality offsets that are implemented near increased point source emissions and have a high certainty of effectiveness may provide a balance between scientific rigour and policy workability.

High-frequency monitoring during rainstorm events reveals nitrogen sources and transport in a rural catchment

Rural areas lacking essential sewage treatment facilities and collection systems often experience eutrophication due to elevated nutrient loads. Understanding nitrogen (N) sources and transport mechanisms in rural catchments is crucial for improving water quality and mitigating downstream export loads, particularly during storm events. To further elucidate the sources, pathways, and transport mechanisms of N from a rural catchment with intensive agricultural activities during storm events, we conducted an analysis of 21 events through continuous sampling over two rainy seasons in a small rural catchment from the lower reaches of the Yangtze River. The results revealed that ammonia-N (NH4+-N) and nitrate-N (NO3--N) exhibited distinct behaviors during rainstorm events, with NO3--N accounting for the primary nitrogen loss, its load being approximately forty times greater than that of NH4+-N. Through examinations of the concentration-discharge (c-Q) relationships, the findings revealed that, particularly in prolonged rainstorms, NH4+-N exhibited source limited pattern (b = -0.13, P < 0.01), while NO3--N displayed transport limited pattern (b = -0.21, P < 0.01). The figure-eight hysteresis pattern was prevalent for both NH4+-N and NO3--N (38.1% and 52.0%, respectively), arising from intricate interactions among diverse sources and pathways. For NO3--N, the hysteresis pattern shifted from clockwise under short-duration rainstorms to counter-clockwise under long-duration rainstorms, whereas hysteresis remained consistently clockwise for NH4+-N. The hysteresis analysis further suggests that the duration of rainstorms modifies hydrological connectivity, thereby influencing the transport processes of N. These insights provide valuable information for the development of targeted management strategies to reduce storm nutrient export in rural catchments.

Spatial heterogeneity of the effects of river network patterns on water quality in highly urbanized city

River water quality deterioration is a serious problem in urban water environments. River network patterns affect water quality by influencing the flow, mixing, and other processes of water bodies. However, the effects of urban river network patterns on water quality remain poorly understood, thereby hindering the urban planning and management decision-making process. In this study, the geographically weighted regression (GWR) model was used to explore the spatial heterogeneity of the relationship between river network pattern and water quality. The results showed that the river network has a complex structure, high connectivity, and relatively even distribution and morphology. Important river structure indicators affecting water quality included the water surface ratio (Wp) and multifractal features (∆α, ∆f) while important river connectivity indicators included circuitry (α) and network connectivity (γ). River structure has a more complex effect on water quality than connectivity. This study recommends that the Wp should be increased in agricultural areas and appropriately reduced in urban built-up areas, and the number of river segments and nodes should be controlled within a rational configuration. Our study provides key insights for evaluating and optimizing the river network patterns to improve water quality of urban rivers. In the future, the land use intensity, hydrological processes, and human activities should be coupled with the river network pattern to deepen our understanding of urban river environment.

Storm-induced nitrogen transport via surface runoff, interflow and groundwater in a pomelo agricultural watershed, southeast China

The storm-induced export of nitrogen (N) from agricultural watersheds significantly impacts aquatic ecosystems, yet the mechanisms of source supply and transport behind N species remain unclear. Here, we investigated the hydrological factors influencing the timing and magnitude of river N species export in a Chinese pomelo agricultural watershed. We conducted continuous observations of watershed hydrology, N species, and their isotopic ratios along a soil-groundwater-river continuum during two storm events in 2018-2019. We found the export flux of river NO3-N covers ∼80% of the total N flux during storms, and the rest for other N species. Our results further revealed distinct pathways and timing of N transport among different N species, especially between ammonium N (NH4-N) and nitrate N (NO3-N). NH4-N in stormflow predominantly originates from sewage and soil leachate, rapidly transported via surface runoff and interflow. Orchard fertilization (contributed 41-56% based on SIAR analysis) was the major source of river NO3-N, which underwent initial dilution via surface runoff and subsequently became enriched through delayed discharge of soil leachate and groundwater. The variations in timing and magnitude of N transport between storms can be explained by antecedent conditions such as precipitation, soil N pools, and storm size. These findings emphasize the hydrological controls on N export from agricultural watersheds, and highlight the variations in source supply and transport pathways among different N species. The insights gained from this study hold significance for managing agricultural pollution and restoring impaired aquatic systems.

Source availability and hydrological connectivity determined nitrate-discharge relationships during rainfall events in karst catchment as revealed by high-frequency nitrate sensing

Karst terrain seasonal monsoonal rainfall is often associated with high concentrations of nitrate-N in streams draining agricultural land. Such high concentrations can pose problems for environmental and human health. However, the relationship between rainfall events that mobilize nitrate and resulting nitrate export remains poorly understood in karst terrain. To better understand the processes that drive nitrate dynamics during rainfall events, the characteristics of individual rainfall events were analyzed using sensor technology. Thirty-eight rainfall events were separated from the high-frequency dataset spanning 19 months at a karst spring site. The results revealed that nitrate-discharge (N-Q) hysteresis in 79% of rainfall events showed anticlockwise hysteresis loop patterns, indicating nitrate export from long distances within short event periods. Karstic hydrological connectivity and source availability were considered two major determining factors of N-Q hysteresis. Gradual increase in hydrological connectivity during intensive rainfall period accelerated nitrate transportation by karst aquifer systems. Four principal components (PCs, including antecedent conditions PC1&3 and rainfall characteristics PC2&4 explained 82% of the cumulative variance contribution to the rainfall events. Multiple linear regression of four PCs explained more than 50% of the variation of nitrate loading and amplitude during rainfall events, but poorly described nitrate concentrations and hydro-chemistry parameters, which may be influenced by other factors, e.g., nitrate transformation, fertilization time and water-rock interaction. Although variation of N concentration during event flow is evident, accounting for antecedent conditions and rainfall factors can help to predict rainfall event N loading during rainfall events. Pollution of the karstic catchment occurred by a flush of nitrate input following rainfall events; antecedent and rainfall conditions are therefore important factors to consider for the water quality management. Reducing source availability during the wet season may facilitate to reduction of nitrogen loading in similar karst areas.