Wenqing Shi

Dams trap organic carbon (OC) and create carbon dioxide (CO ₂ ) and methane (CH ₄ ) emission hotspots within reservoirs, underlying the ongoing debate over their role in carbon emissions. However, even without dams, this labile OC would still decompose and emit CO ₂ or CH ₄ during downstream transport and after deposition in estuaries. Thus, relying solely on dammed reservoir observations to assess these emissions may introduce considerable bias. Here, we simulated both dammed and non-dammed scenarios for 5346 global reservoir dams to assess their impact on carbon emissions from retained OC. The results show that reservoir-only approaches overestimate dam-induced carbon emissions by neglecting baseline emissions under no-dam conditions. After baseline correction, dams increase carbon emissions by an average of 21.5% in CO ₂ -equivalent (CO ₂ -eq), which represents only 18.6% of the emissions estimated using the conventional reservoir-based accounting approach. XGBoost identified reservoir depth and hydraulic residence time as the primary controls on dam-induced carbon emissions.

Emergency treatment of algal blooms in lake and reservoir source waters requires coagulants that remain effective under changing raw water conditions. This study compared polyaluminum chloride (PAC) and chitosan (CTS) in raw water collected from Yangku Reservoir during wet and dry seasons. Jar tests evaluated algal density, algal biomass, and nitrogen and phosphorus fractions across a 3~15 mg/L reagent-mass dosage range. Treatment performance differed between seasons. Mean algal density and soluble reactive phosphorus (SRP) removals in the wet season were 63.12% and 68.51%, respectively, and an apparent 70.05% decrease in measured NH₄⁺-N concentration was also observed. Because the fate of dissolved inorganic nitrogen was not resolved, the NH₄⁺-N decrease should not be interpreted as direct coagulative removal. Dry season water had higher algal density and a higher SRP/TP ratio. PAC maintained relatively stable algal biomass removal across seasons and showed stronger phosphorus removal, whereas CTS was more sensitive to seasonal changes in the raw water matrix. These findings support season-specific preliminary screening of coagulants while highlighting the need for residual-Al, pilot-scale, and process-mechanism validation before full-scale application.

In hydro‐fluctuation belts (HFBs), plants thrive during dry seasons, sequestering atmospheric carbon dioxide (CO ₂ ). When water levels rise in wet seasons, these plants become submerged, resulting in their die‐off and decomposition, which release CO ₂ and methane (CH ₄ ) into the atmosphere. To assess the climatic implications of this sequestration‐emission sequence, we conducted incubation experiments on HFB plants from Poyang Lake, China, to quantify changes in CO ₂ ‐equivalent (CO ₂ ‐eq) based on CH ₄ global warming potentials and to elucidate the underlying microbial mechanisms using quantitative polymerase chain reaction. The results demonstrated that, after submersion, the aboveground parts die and partially decompose, creating anaerobic conditions that promoted methanogenesis for CH ₄ production. This sequence from initial CO ₂ sequestration to the subsequent release of CO ₂ and CH ₄ increased CO ₂ ‐eq and contributed to the greenhouse effect. When CH ₄ global warming potentials are considered over 20‐, 100‐, and 500‐year time horizons, the CO ₂ ‐eq increases by 51.4, 15.9, and 2.7 g per gram of plant carbon, respectively. If plant desiccation induced by drought occurs prior to submersion, it may lead to a higher net CO ₂ ‐eq following subsequent inundation. These findings offer new insights into the ecological impacts of plant carbon sequestration in HFBs.

Reservoirs with thicker sediments generally exhibit elevated carbon emissions; however, this relationship may not necessarily persist when emissions are assessed per unit thickness. A comprehensive understanding of this relationship is essential for accurately evaluating carbon emissions across various sedimentation patterns. This study proposes the concept of carbon emission intensity (CEI), defined as carbon emissions per unit mass or volume of sediment, evaluates its correlation with sediment thickness (ST) using 58 global data sets and clarifies the impact of ST on CEI through controlled incubation experiments. The results revealed a strong negative logarithmic correlation between CEI and ST ( R ² = 0.88), indicating that increasing ST substantially reduces CEI values. The observed trend can be attributed to the potential of thicker sediment layers to restrict oxygen penetration, promoting organic carbon (OC) burial. The incubation experiments reinforced this mechanism, revealing that although thicker sediments generated higher absolute carbon emissions, their CEI values were consistently lower. Oxygen penetration and active OC decomposition were found to be limited to the top few centimeters of sediment, independent of the total ST. The findings indicate that dam construction in narrow river sections could be strategically favorable, as these areas are more likely to accumulate thicker sediments characterized by lower CEI, reducing overall carbon emissions.

Clarifying carbon (C) cycling in small ponds is vital for understanding C transport in lowland agricultural landscape. Quantifying C flux is crucial for learning C cycling, but is challenging due to its complex cycling and significant impacts from intensive human activities. Here, we developed a process‐based model (CDP) to achieve a daily estimation of C dynamics in agricultural ponds within lowland artificial watersheds (polders), and proposed a dual evaluation approach (concentration and flux) to assess the model's performance using two data sets obtained from eight typical polders in the Lake Taihu Basin. The developed model captured pond C dynamics, achieving a Nash‐Sutcliffe efficiency of 0.44 ± 0.27. Our C flux estimations based on the newly‐developed model showed large C emissions, primarily through carbon dioxide (CO₂) (497.5 g C m⁻² yr⁻¹), along with significant C burial (27.8 g C m⁻² yr⁻¹) with a hot moment in summer. Scenario simulations revealed the distinct impacts of pond C emissions and burial associated with the growth and death of phytoplankton and macrophytes. A 10% increase in macrophyte growth rates associated with a 1.8 g C m⁻² yr⁻¹ increase in CO₂ emission, while a similar increase in phytoplankton growth rates related to a 12.2–16.2% increase in C burial. This study revealed a quick response of C flux to phytoplankton‐macrophyte dominance, and highlighted the high potential of the process‐based model for high‐resolution (daily) quantification of C fluxes, thereby enhancing our understanding of C cycling in lowland agricultural ponds.