Dynamic responses of DOC and DIC transport to different flow regimes in a subtropical small mountainous river

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

Exploring hydrological controls on dissolved organic carbon export dynamics in a typical flash flood catchment using a process-based model

The dynamics of dissolved organic carbon (DOC) export from headwater catchments are of critical importance for the global carbon balance and are driven by complex runoff processes. Most previous studies have used statistical relationships between runoff and DOC concentration to estimate DOC export dynamics. Thus, the coupling mechanisms between runoff generation and DOC export dynamics at the process level were obscured in the fitting parameters and have rarely been addressed. In this study, high-frequency (hourly) discharge and DOC export from a typical flash flood experimental headwater catchment with an area of 1.8 km2 were simulated using a process-based model (INCA-C). The results showed that the INCA-C model successfully captured the hourly dynamics of both discharge and DOC concentrations with a Nash-Sutcliffe efficiency (NSE) of 0.47-0.81 and 0.28-0.70 among moderate events and 0.81-0.85 and 0.19-0.90 among extreme events, respectively. The DOC was exported with distinct concentration dynamics, fluxes, and contributions from the four flow pathways under different storm intensities. At higher intensities, the DOC fluxes were exported by subsurface flows, particularly from shallow organic soil, with greater peaks and shorter time-to-peaks. Exported DOC is primarily sourced from subsurface runoff from the mineral layer (73 %-77 %) during moderate events, whereas it is primarily sourced from subsurface runoff from the organic layer (61 %-79 %) during extreme events. The two contrasting contributions suggest that hydrological pathway controls and DOC dynamic patterns can shift owing to runoff generation influenced by storm intensity. The distinct and variable controls of different flow pathways on DOC export highlight the need to explain the role of hydrology in regulating DOC storm exports through process-based modelling.

Influences of hydrodynamics on dissolved inorganic carbon in deep subtropical reservoir: Insights from hydrodynamic model and carbon isotope analysis

Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13CDIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13CDIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d-1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d-1, resulting in reservoir CO2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO2 emissions and optimizing reservoir management.

Transition of carbon-nitrogen coupling under different anthropogenic disturbances in subtropical small mountainous rivers

The commonly observed inverse relationship between dissolved organic carbon (DOC) and nitrate (NO₃^-) concentrations in aquatic systems can be explained by stoichiometric and thermodynamic principles regulating microbial assimilation and dissimilation processes. However, the interactive effects of human activities and dissolved oxygen (DO) on the DOC and DIN (dissolved inorganic nitrogen, mainly composed of NO₃^--N and NH₄⁺-N) relations are not well identified, particularly in subtropical small mountainous rivers (SMRs). Here, we investigated the exports and relations of DOC-DIN in 42 Taiwan SMRs under different anthropogenic disturbances. Results showed that the island-wide mean concentrations of the three solutes in streams are generally low, yet the abundant rainfall and persistent supply contrarily lead to disproportional high DOC and DIN yields. The inverse DOC-NO₃^--N relation does not appear under well‑oxygenated conditions, regardless of low or high human disturbance. However, a significant inverse relationship between DOC-NO₃^--N would emerge in highly-disturbed watersheds under low-oxygenated conditions (mean annual DO <6.5 mg L^-1), where excess N accumulates as NH₄⁺-N rather than NO₃^--N. The controlling mechanism of DOC-DIN relations would shift from energetic constraints to redox constraints in low-oxygenated conditions. Although riverine concentrations of DOC, NO₃^--N, and NH₄⁺-N could be elevated by human activities, the transition of DOC-DIN relation pattern is directly linked to DO availability. Understanding the mechanism that drives CN coupling is critical for assessing the ecosystem function in the delivery and retention of DOC and DIN in aquatic ecosystems.

The influence of the deep subtropical reservoir on the karstic riverine carbon cycle and its regulatory factors: Insights from the seasonal and hydrological changes

Reservoirs are widely established worldwide with considerable environmental impacts, especially on the riverine carbon cycle. However, the influence of reservoirs on the cycling of different forms of carbon and its regulation factors (e.g., seasonal variations versus hydrological management) have not been simultaneously studied. To fill this knowledge gap, seasonal water samples from the deep subtropical reservoir (Longtan reservoir) in the Pearl River were collected, and the concentrations and stable carbon isotopes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were determined. The variations in stable carbon isotopes of DIC (-11.4‰ to -5.2‰), DOC (-32.2‰ to -26.2‰), and POC (-38.9‰ to -25.3‰) in the river-reservoir system indicated active production and degradation processes in different layers. We estimated that up to 23.0% of DIC, 20.5% of DOC, and most POC were intercepted or degraded within the reservoir. Our results further illustrated that hydrological management (water storage regulation) and seasonal variations from different perspectives controlled the cycling of different forms of carbon in the reservoir. In addition, with the gradual increase in the number of reservoirs, hydrological management can be considered as a potentially effective strategy to adjust the carbon biogeochemical cycling of reservoirs in the future.

Oxidative stress of Microcystis aeruginosa induced by algicidal bacterium Stenotrophomonas sp. KT48

Cyanobacterial harmful algal blooms are a worldwide problem with substantial adverse effects on the aquatic environment as well as human health. Among the multiple physicochemical and biotic approaches, algicidal bacterium is one of the most promising and eco-friendly ways to control bloom expansion. In this study, Stenotrophomonas sp. KT48 isolated from the pond where cyanobacterial blooms occurred exhibited a strong inhibitory effect on Microcystis aeruginosa. However, the algicidal performance and mechanisms of Stenotrophomonas sp. remain under-documented. To explore the algicidal performance and physiological response againt M. aeruginosa, further works were implemented here. Our results indicated that the algicidal rate of strain KT48 cultured in 1/8 LB medium supplemented with 0.3% starch or glucose was about 30% higher than that in 1/8 LB medium. Strain KT48 culture, cell-free filtrate, and cells re-suspended were inoculated into the M. aeruginosa culture, and the Chl-a content was determined. Those results indicated that the algicidal activity of cells re-suspended was far higher than that of cell-free filtrate and culture. Thus, strain KT48 exhibited algicidal activity mainly through direct attacking M. aeruginosa rather than excretion of algicides. Furthermore, strain KT48 led to an increase in cellular reactive oxygen species (ROS) and caused lipid peroxidation as supported by the increase in malondialdehyde (MDA) levels. The ROS and MDA levels in algal cells treated with strain KT48 cells re-suspended were about 3.23-fold and 2.80-fold higher than those of untreated algal cells on day 11. And a further inhibition to the antioxidant system is suggested by a sharp decrease in the superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities. In addition, we also observed that the morphology of most algal cells changed from integrity to break. This study not only indicated strain KT48 with strong algicidal activity, but also explored the underlying algicidal mechanisms to provide a source of bacterial agent for the biocontrol of cyanobacterial blooms. KEY POINTS: • Strain KT48 exhibited strong algicidal activity mainly through direct attacking M. aeruginosa. • The addition of glucose could enhance the algicidal rate of strain KT48 by about 30%. • Strain KT48 led to an increase in cellular reactive oxygen species (ROS) level that causes membrane damage as supported by the increase in malondialdehyde (MDA) levels.

The impacts of reservoirs on the sources and transport of riverine organic carbon in the karst area: A multi-tracer study

Reservoirs have been constructed as clean energy sources in recent decades with various environmental impacts. Karst rivers typically exhibit high dissolved inorganic carbon (DIC) concentrations, whether and how reservoirs affect carbon cycling, especially organic carbon (OC)-related biogeochemical processes in karst rivers, are unclear. To fill this knowledge gap, multiple tracer methods (including fluorescence excitation-emission matrix (EEM), ultraviolet (UV) absorption, and stable carbon (δ¹³C) and radiocarbon (Δ¹⁴C) isotopes) were utilized to track composition and property changes of both particulate OC (POC) and dissolved OC (DOC) along river-transition-reservoir transects in the Southwest China karst area. The changes in chemical properties indicated that from the river to the reservoir, terrestrial POC is largely replaced by phytoplankton-derived OC, while gradual coloured dissolved organic matter (CDOM) removal and addition of phytoplankton-derived OC to the DOC pool occurred as water flowed to the reservoir. Higher primary production in the transition area than that in the reservoir area was observed, which may be caused by nutrient released from suspended particles. Within the reservoir, the production surpassed degradation in the upper 5 m, resulting in a net DIC transformation into DOC and POC and terrestrial DOM degradation. The primary production was then gradually weakened and microbial degradation became more important down the profile. It is estimated that ~3.1-6.3 mg L^-1 (~15.5-31.5 mg-C m^-2 (~10-21%)) DIC was integrated into the OC pool through the biological carbon pump (BCP) process in the upper 5 m in the transition and reservoir areas. Our results emphasize the reservoir impact on riverine OC transport, and due to their characteristics, karst areas exhibit a higher BCP potential which is sensitive to human activities (more nutrient are provided) than non-karst areas.

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet-dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣF_aq ). Here, we determined the NLCB of a lowland catchment (~140 km² ) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire-related C emissions and ΣF_aq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km^-2  year^-1 ), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (-56 Mg C km^-2  year^-1 ) and ΣF_aq (-28 Mg C km^-2  year^-1 ) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣF_aq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events-up to 39% of annual ΣF_aq was exported in one event. We hypothesize that wetter and hotter conditions in the wet-dry tropics in the future will increase ΣF_aq and fire emissions, potentially further reducing the current C sink in the region. More long-term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet-dry tropics where our result indicating a significant C sink may not hold.