Xia Li

Thermokarst lakes are dynamic carbon sinks and sources in permafrost landscapes, but their organic carbon (OC) burial remains poorly constrained. We analyzed ²¹⁰ Pb‐dated sediment cores from 12 thermokarst lakes on the Qinghai–Tibet Plateau to quantify sedimentary OC, total nitrogen (TN), C:N stoichiometry, and recent sediment mass accumulation rates (MAR) and OC burial rates (OCBR). Chronology‐dependent comparisons were restricted to sediment intervals with CRS‐derived calendar years of 1900 or later. Across all sediment layers, OC concentrations (1.12–250 g/kg), TN (0.094–26.1 g/kg), and C:N ratios (1.5–188) exhibited pronounced within‐ and among‐lake variability but with no consistent vertical trends shared across lakes. In post‐1900 sediments, MAR (113–21,964 g/m ² /yr) and OCBR (6.1–1,167 gC/m ² /yr) also showed substantial within‐core and among‐lake variability. Lake‐level median MAR and OCBR varied from 496 to 5,831 g/m ² /yr and from 17.8 to 191 gC/m ² /yr, respectively. MAR and OCBR were positively related in most lakes, but the slopes differed markedly, suggesting that similar increases in sediment accumulation translated into different increments of OC burial among lakes. Across lakes, median MAR decreased with mean annual precipitation (MAP), whereas median OCBR increased with MAP; no significant relationships were detected with mean annual temperature, elevation, lake area, or lake depth. These results show that recent OC burial cannot be inferred from sediment accumulation alone, but reflects differences in both sediment supply and the OC content of deposited material, with postdepositional preservation representing an additional potential influence.

Hydrological connectivity regulates river–sea exchange by controlling freshwater delivery, salinity intrusion, sediment transport, and biogeochemical fluxes that sustain deltaic ecosystems. In distributary river systems, connectivity is mediated by multiple alternative downstream pathways, yet most assessments rely on network‐averaged indices and overlook cumulative regulation along interacting routes. Here, we develop a pathway‐inclusive, probabilistic framework to quantify river‐sea hydrological connectivity in distributary networks by retaining all feasible pathways and cumulative low‐head barrier effects. This formulation distinguishes structural redundancy from functional connectivity by accounting for the unequal contributions of different pathways to river–sea exchange. We apply the framework to the Guangdong–Hong Kong–Macao Greater Bay Area using multi‐decadal river network and barrier data sets from the 1980s to the 2010s. Total river network length increased from 7.9 × 10 ⁶ m to 8.8 × 10 ⁶ m, while sluice number increased more than sixfold and density twentyfold. Despite network expansion, functional river–sea connectivity declined by approximately 40% across maximum, average, and minimum connectivity metrics. The sharpest decline in average connectivity occurred between the 1990s and 2000s, indicating a system‐wide transition toward reduced pathway effectiveness. Connectivity losses were most pronounced along high‐capacity pathways that historically supported disproportionate exchange, whereas increased pathway number and small‐channel construction failed to offset network‐scale degradation. Spatially, connectivity declined most strongly upstream, despite barrier clustering near estuarine outlets, revealing non‐local, system‐wide regulatory effects. These results demonstrate that explicitly evaluating all feasible downstream pathways is necessary to diagnose connectivity loss, identify critical routes for river–sea exchange, and avoid misleading inferences based on network‐averaged indices.

Artificial wetlands are widespread in urbanized coastal landscapes, but their role as managed assets for restoring habitat-network connectivity remains poorly quantified. We developed a scenario-based decision framework for prioritizing artificial wetlands in connectivity-oriented environmental management, using the Guangdong–Hong Kong–Macao Greater Bay Area as a model coastal megadelta. The framework diagnoses scale-dependent structural transitions in remnant natural wetlands, identifies stepping-stones, hubs and cut points in natural-only and natural–artificial wetland networks, and simulates progressive restoration of priority artificial wetlands with Condatis under alternative corridor directions, dispersal distances and restoration intensities. The natural wetland network showed three transition domains: fragmentation reduction at 1.59–8.89 km, local patch-cluster formation at 9.46–12.08 km and dominant-component expansion at 17.81–25.53 km. Most connectivity-critical natural nodes occurred outside protected areas, revealing a management gap not captured by area-based protection. Artificial wetlands substantially reshaped network structure: reservoirs and constructed ponds dominated priority-node counts, whereas aquaculture ponds contributed most priority-node area. Restoration responses depended jointly on corridor direction and dispersal distance. Lateral cross-delta scenarios produced the largest simulated gains, while longitudinal inland–coastal scenarios remained weak. Averaged response curves yielded geometric knee points at 9.4–10.7% of existing artificial-wetland area, marking the restored-area range where fitted connectivity gains shifted from weak to more rapidly increasing responses. The framework translates connectivity diagnosis into actionable restoration prioritization and can support adaptive wetland management in human-dominated coastal deltas.

Eutrophication driven by excessive total nitrogen (TN) and total phosphorus (TP) poses a major threat to China's aquatic ecosystems. Precise identification of nutrient hotspots is essential for effective mitigation but remains constrained by hydrologically mediated transport processes across multiple spatial scales. Process-based models such as the Soil and Water Assessment Tool (SWAT) are widely used, yet their coarse subbasin divisions obscure intra-subbasin heterogeneity and limit source apportionment. To address this, we developed a process-informed, multi-parameter downscaling framework integrating SWAT-derived subbasin outputs with high-resolution land use, topographic, and hydrological datasets. This approach downscaled nutrient source tracing to a 1km×1km grid, enabling fine-scale spatial patterns of TN and TP sources and revealing intra-subbasin heterogeneity that was not resolved by SWAT alone. The downscaling framework demonstrated robust performance, with adjusted determination coefficients (R2adj) and the Nash-Sutcliffe efficiency coefficient (NSE) for log-transformed TN and TP exceeding 0.90. Variation analysis further revealed substantial heterogeneity within the high-contributing subbasins, with 9 out of 11 for TN and 2 out of 13 for TP exhibiting internal variability over 30%. These results indicate that subbasin-scale aggregation can mask critical spatial variability in nutrient source distributions. Targeted interventions, such as upgrading sewage treatment and establishing constructed wetlands, should be directed to the precise internal hotspots responsible for disproportionate nutrient export. By bridging the gap between subbasin scale modeling and fine scale source delineation, this approach provides a more precise spatial representation of nutrient distributions and establishes a transferable framework for high-resolution nutrient management in diverse global watersheds.

Understanding and predicting climate change impacts on the terrestrial water cycle is essential for water resources management and hazard prevention. This study aims to project future runoff of a densely-populated river basin, the Pearl River Basin (PRB), under different Shared Socioeconomic Pahway (SSP) scenarios, by combining the Soil and Water Assessment Tool (SWAT) model and the CMIP6 climate projections. Results show that climate change will significantly increase the runoff of the PRB, with changing rates of 0.21, 0.20, 0.11, and 0.17 mm/month/year for low- to high-emission scenarios SSP126, SSP245, SSP370, and SSP585, respectively. Future runoff exhibits strong seasonal and spatial variability due to complex changes in precipitation and potential evapotranspiration across the basin. The PRB may experience higher flood risks during the wet season under all SSP scenarios, driven by a ~15% increase in runoff during the wettest month during 2061–2100 relative to that of 2021–2060. Conversely, drought risks may escalate in the East River Sub-basin of the PRB during the dry season under the high-emission scenarios (SSP370 and SSP585), with a ~20% reduction in runoff during the driest month during 2061–2100 relative to that of 2021–2060. The highest-emission scenario (SSP585) may lead to the most drastic hydrological changes, including increased risks of flooding and drought across different parts of the PRB. Our findings suggest intensified water cycling and increased hydrological risks in the PRB under a changing climate, highlighting the necessity of future water resource management to consider potential climate change impacts to mitigate the risks of floods and droughts effectively.

The accumulation of antibiotic resistance genes (ARGs) in aquatic systems jeopardizes public health and ecological environments. This study investigates ARGs dissemination in freshwater and seawater, focusing on the sources, prevalence and influencing factors. In freshwater, ARGs primarily originate from medical/pharmaceutical wastewaters, industrial operations, agriculture, and livestock sectors. By contrast, in addition to the above sources, seawater is contaminated by mariculture and terrestrial runoff. Comparative analysis indicates that fresh water hosts multidrug resistance, bacitracin resistance, sulfonamides, aminoglycosides, and beta-lactams, whereas seawater exhibits a wider range of ARGs encompassing sulfonamides, tetracyclines, aminoglycosides, beta-lactams, quinolones, macrolides, and chloramphenicol resistance genes. There was a stronger correlation between antibiotics and ARGs in seawater than in freshwater, especially in farmed waters. Human activities significantly contribute to ARGs pollution in both freshwater and seawater. Urbanization influences ARGs pollution in freshwater, while offshore distance and coastal economic development dictate ARGs selection pressure in seawater. This study shed lights on the current ARGs pollutant status in marine and freshwater ecosystems in China, providing a scientific foundation for water health preservation and ecosystem safeguarding measures.