The construction of dams to intercept natural rivers constitutes the most severe human activity influencing the underlying surface. This study focuses on four cascade reservoirs of the Lancang River and explores their impact on the migration of organic matter in sediments. The research reveals significant spatial variations in total organic carbon (TOC) and total nitrogen concentrations in the sediments of the four reservoirs. The carbon and nitrogen isotopes indicate that terrigenous organic matter is the main source of TOC in the sediments, contributing an average of 66.80%. Endogenous algal-derived organic matter is the second significant source, contributing between 14.30% and 32.91%. The sources contributed from upstream organic matter are the lowest, ranging from 6.36% to 15.33%. Our study demonstrates that the construction of cascade reservoirs may significantly alter the processes of material sources in the river basin ecosystem, particularly in the large reservoir which increased more endogenous algal-derived organic matter.

Introduction: Microorganisms are essential for cycling phosphorus and nitrogen and play a crucial role in maintaining the health and stability of river ecosystems. The reservoir operation changes the river's hydrological processes, forming reservoir inundation areas and influencing the diversity of microorganisms and their environmental functions.Methods: To study the microbial composition and action mechanism in rivers affected by reservoir operation, Xiangxi River, the closest tributary to the Three Gorges Dam on the Yangtze River, was sampled to examine the spatiotemporal fluctuations of bacterial populations and functional genes in water and sediments. The physicochemical properties, microbial communities and functional genes were analyzed in August 2022 and June 2023.Results: Spatially, except for conductivity, the chlorophyll (Chl), dissolved oxygen (DO), and pH values in the upper reaches of the basin were higher than those in the mouth (where it joins the Yangtze River). Specific physicochemical gradients created by the reservoir operation drove spatial and temporal shifts in bacterial community structure. In water samples (W), dominant microbial species included Exiguobacterium and Candidatus Fonsibacter, contributing to organic matter degradation and nutrient transformation. Nitrospira indicated their roles as nitrifiers or denitrifiers in sediment samples (S), essential for nitrogen cycling. In the mouth zone, Methyloceanibacter dominated in the transition zone, and they were involved in methane or organic metabolism.Discussion: The dominance of Microcystis in the upstream region reflected its prevalence in nutrient-rich, algal-rich environments. Paralia in the middle of the river highlighted the favorable conditions of suitable light and moderate flow rates for diatom growth. Reservoir regulation also altered the functional gene composition, making it more similar to that found in lake ecosystems. The most abundant functional genes were those associated with Amino Acid Transport and Metabolism, while phosphorus-related genes predominantly involved energy production and conversion. The dominance of genes linked to electron transport underscored the pivotal role of microbial respiration and oxidative phosphorylation in energy metabolism, which was fundamental to ecosystem productivity.

Reservoir water-level fluctuation zones (WLFZs) are important in carbon budgets, but opaque chambers omit daytime photosynthesis. We measured CO₂ and CH₄ fluxes at the air–water and air–soil interfaces of Hanfeng Reservoir (water-level amplitude 4.72 m), with three-chamber partitioning in the WLFZ during the growing season. The air–water interface was a weak CO₂ source (2.1 ± 40.9 mg m⁻² h⁻¹) and a persistent CH₄ source (0.71 ± 1.36 mg m⁻² h⁻¹). In the WLFZ, daytime gross primary production (1665 mg m⁻² h⁻¹) exceeded ecosystem respiration (1280 ± 757 mg m⁻² h⁻¹; daytime net ecosystem exchange NEE, −385 ± 1180 mg m⁻² h⁻¹), whereas the estimated 24 h NEE indicated a weak net CO₂ source (+153 to +343 mg m⁻² h⁻¹). Air–soil CH₄ emissions averaged 0.70 ± 1.51 mg m⁻² h⁻¹, with no significant net plant-mediated transport. At this site, and under modeled night-time scenarios, the opaque chamber overestimated daily net CO₂ emissions 3.7 to 7.1-fold. In 24 h measurements of two dominant species at one site in August 2024, 09:00–13:00 sampling represented the daily mean CH₄ flux within 15%, but not the CO₂ flux. These site-specific results indicate that accounting for vegetation and diel variability can reduce bias in WLFZ carbon budgets.