Aquaculture plays a crucial role in meeting the increasing demand for protein-rich food. However, aquaculture production also comes with a large carbon footprint, partly due to the substantial emission of greenhouse gases, particularly methane (CH₄), during the aquaculture production. Yet, our understanding on the magnitude, pathways, and drivers of CH4 emission from aquaculture ponds is limited, particularly in the Asian continent where more than 90% of global aquaculture production occurs. In this study, we quantified CH4 concentrations, air–water fluxes, production in anoxic sediments, and oxidation in the water column across multiple tropical aquaculture ponds where one of the most commonly cultivated shrimp species, Litopenaeus vannamei, is farmed. Field measurements showed that the diffusive CH4 emissions, with a mean value of 3.40 ± 1.76 mg m−2 day−1, varied greatly—ranging from 0.68 to 7.12 mg m−2 day−1—and were regulated by a suit of environmental variables. Salinity and total phosphorus (TP) concentration were the key determinants of diffusive CH4 flux: CH4 emission decreased with increasing salinity, while it increased with TP. Accordingly, our results suggest that shifting from freshwater to saline water aquaculture can decrease CH₄ emissions, thereby reducing the carbon footprint of aquaculture production. However, an increase in phosphorus concentration can offset this salinity-driven emission reduction. Therefore, management practices should prioritize reducing phosphorus loads to effectively mitigate CH₄ emissions and enhance the sustainability of aquaculture. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.

Rivers are globally significant sources of atmospheric carbon dioxide (CO2). However, the processes governing supersaturation of CO2 in large tropical fluvial networks are poorly understood. In particular, strikingly little is known about the role of land use in shaping CO2 variability in South Asian river basins, which are undergoing rapid urbanization. Here, we show that the wide variability in the partial pressure of CO2 (pCO2: 246.3-21271.2 μatm) in an agriculture-dominated river basin (Krishna River basin, India) is primarily shaped by the extent of urbanization. Specifically, a strong positive correlation between pCO2 and built-up area (%) was observed when the built-up area exceeded 2%. Furthermore, machine learning analysis showed that pCO2 variability was predicted by built-up area (%), Strahler order, and altitude, together explaining ∼77% of the spatial variability. Additionally, a strong negative correlation between excess CO2 and oxygen relative to atmospheric equilibrium indicates that in-stream metabolism, fueled by organic matter inputs from urbanized areas, is the primary cause of CO2 supersaturation, establishing a mechanistic link between pCO2 and the built-up area. Given that pCO2 increases with urbanization, limiting urban inputs is crucial for reducing fluvial CO2 emissions from South Asian river basins. © 2025 American Chemical Society.

Aquatic sediments represent a key component for understanding CH4 dynamics and emission to the atmosphere. Once produced in the sediments, CH4 is released either by diffusion at the sediment–water interface or by bubbling out to the atmosphere when total gas pressure in the sediment exceeds local ambient pressure due to high CH4 production. Although bubbling is one of the dominant CH4 emission pathways in lakes, direct measurements of this flux are hampered by its high spatiotemporal variability and methodological limitations. Here, we develop a conceptual approach to quantify CH4 production in lake sediments and particularly its release as bubbles based on simple measurements of bubble gas content and depth. Its main assumptions were empirically tested using > 200 long-term bubble trap deployments collected from 4 temperate lakes. We then applied the developed methodology to a suite of 408 Canadian lakes to produce the first standardized large-scale assessment of lakes CH4 ebullitive flux during summer. Our results show that lake sediments produced CH4 at a median rate of 3.3 mmol m−2 d−1 (ranged from 0.2 to 11.8 mmol m−2 d−1), releasing 33% via ebullition to the atmosphere. These rates are remarkably similar in magnitude to other regional estimates in the literature. Moreover, our approach revealed that catchment slope was an important determinant of both the lake-wide ebullitive fluxes and the fraction of sediment CH4 production released as bubbles. © 2024 The Author(s). Limnology and Oceanography published by Wiley Periodicals LLC on behalf of Association for the Sciences of Limnology and Oceanography.

Aerobic methane oxidation (MOX) significantly reduces methane (CH4) emissions from inland water bodies and is, therefore, an important determinant of global CH4 budget. Yet, the magnitude and controls of MOX rates in rivers – a quantitatively significant natural source of atmospheric CH4 – are poorly constrained. Here, we conducted a series of incubation experiments to understand the magnitude and environmental controls of MOX rates in tropical fluvial systems. We observed a large variability in MOX rate (0.03 - 3.45 μmol l-1d-1) shaped by a suit of environmental variables. Accordingly, we developed an empirical model for MOX that incorporate key environmental drivers, including temperature, CH4, total phosphorus, and dissolved oxygen (O2) concentrations, based on the results of our incubation experiments. We show that temperature dependency of MOX (activation energy: 0.66 ± 0.18 eV) is lower than that of sediment methanogenesis (0.71 ± 0.21 eV) in the studied tropical fluvial network. Furthermore, we observed a non-linear relationship between O2 concentration and MOX, with the highest MOX rate occuring ∼135 μmol O2l-1, above or below this “optimal O2” concentration, MOX rate shows a gradual decline. Together, our results suggest that the relatively lower temperature response of MOX compared to methanogenesis along with the projected decrease of O2 concentration due to organic pollution may cause elevated CH4 emission from tropical southeast Asian rivers. Since estimation of CH4 oxidation is often neglected in routine CH4 monitoring programs, the model developed here may help to integrate MOX rate into process-based models for fluvial CH4 budget. © 2024 Elsevier Ltd

Oxygenated surface layers of aquatic systems are ubiquitously oversaturated with methane (CH4). A growing number of studies suggest that CH4 oversaturation in surface waters can be sustained, at least partly, by methanogenesis occurring under oxic conditions. Although we are gaining a better understanding of the extent and drivers of oxic CH4 production (OMP) in oceanic and lake environments, the existence and variability of OMP in rivers and streams remain unknown. Here, we present experimental evidence for the occurrence and a large variability of OMP rates in a tropical river network. The positive correlation between chlorophyll a concentration and OMP rates and reduction of OMP during the experimental inhibition of photosynthesis establishes a clear link between OMP and photosynthesis. At the same time, a general decrease of the OMP rates with increasing total phosphorus (TP) concentration and the correlation between stable carbon isotopic (δ13C-CH4) values of the OMP-derived CH4 and TP suggest the likely involvement of P-availability as well in regulating the OMP rates. While our estimation suggested a minor contribution of the OMP in the CH4 cycling of the studied tropical system, we show that the OMP in the fluvial environment may be highly sensitive to the current and future changes in algal and nutrient dynamics. © 2024 American Chemical Society.

Rivers are globally significant natural sources of atmospheric methane (CH 4 ). However, the effect of land use changes on riverine CH 4 dynamics, particularly in tropical zones, remain ambiguous, yet important to predict and anticipate the present and future contribution of rivers to the global CH 4 budget. The present study examines the magnitude and drivers of riverine CH 4 concentration and emission in the tropical Krishna River (KR) basin, India. The large spatial variability of CH 4 concentration (0.03 to 185.34 ?mol L ?1 ) and emissions (0.04 mmol m ?2 d ?1 to 1666.24 mmol m ?2 d ?1 ) in the KR basin was linked to the site-specific features of the catchments through which rivers are draining. Several fold higher CH 4 concentration and emission was observed for the urban river sites (64.63 ± 53.17 µmol L ?1 and 294.15 ± 371.52 mmol m 2 d ?1, respectively) than the agricultural (1.05 ± 2.22 µmol L ?1 and 3.45 ± 9.72 mmol m 2 d ?1, respectively) and forested (0.49 ± 0.23 µmol L ?1 and 1.26 ± 0.73 mmol m 2 d ?1, respectively) sites. The concentrations of dissolved oxygen, total phosphorus, and Chlorophyll- a were significant hydrochemical variables strongly coupled with the dissolved CH 4 concentrations. On the other hand, percentage of built-up area emerged as the most important landscape-level driver indicating that urbanization has an overriding effect on riverine CH 4 concentration in the agriculture dominated KR basin. Our study supports the growing notion that tropical urban rivers are hotspot of CH 4 emission. Furthermore, we show that the pattern of increasing in riverine CH 4 concentration with built-up area (%) is a general feature of Asian river basins. As the urban land cover and population following an exponential increase, Asian rivers might contribute substantially to the regional and global CH 4 budget.

Previous stable isotope and biomarker evidence has indicated that methanotrophy is an important pathway in the microbial loop of freshwater ecosystems, despite the low cell abundance of methane-oxidizing bacteria (MOB) and the low methane concentrations relative to the more abundant dissolved organic carbon (DOC). However, quantitative estimations of the relative contribution of methanotrophy to the microbial carbon metabolism of lakes are scarce, and the mechanism allowing methanotrophy to be of comparable importance to DOC-consuming heterotrophy remained elusive. Using incubation experiments, microscopy, and multiple water column profiles in six temperate lakes, we show that MOB play a much larger role than their abundances alone suggest because of their larger cell size and higher specific activity. MOB activity is tightly constrained by the local methane:oxygen ratio, with DOC-rich lakes with large hypolimnetic volume fraction showing a higher carbon consumption through methanotrophy than heterotrophy at the whole water column level. Our findings suggest that methanotrophy could be a critical microbial carbon consumption pathway in many temperate lakes, challenging the prevailing view of a DOC-centric microbial metabolism in these ecosystems.

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CHproduction (OMP) have remained ambiguous. Based on the summer CHmass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 ?mol Lday) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH(?C-CH) indicates direct photoautotrophic release as the most likely source of oxic CH. Furthermore, we show that the oxic CHcontribution to the SML CHsaturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km. Given that global lake CHemissions are dominated by small lakes (SA of <1 km), the large contribution of oxic CHproduction (up to 76%) observed in this study suggests that OMP can contribute significantly to global CHemissions.

The aerobic oxidation of methane (CH) by methanotrophic bacteria (MOB) is the major sink of this highly potent greenhouse gas in freshwater environments. Yet, CH oxidation is one of the largest uncertain components in predicting the current and future CH emissions from these systems. While stable carbon isotopic mass balance is a powerful approach to estimate the extent of CH oxidation in situ, its applicability is constrained by the need of a reliable isotopic fractionation factor (?), which depicts the slower reaction of the heavier stable isotope (C) during CH oxidation. Here we explored the natural variability and the controls of ? across the water column of six temperate lakes using experimental incubation of unamended water samples at different temperatures. We found a large variability of ? (1.004–1.038) with a systematic increase from the surface to the deep layers of lake water columns. Moreover, ? was strongly positively coupled to the abundance of MOB in the ?-proteobacteria class (?-MOB), which in turn correlated to the concentrations of oxygen and CH, and to the rates of CH oxidation. To enable the applicability in future isotopic mass balance studies, we further developed a general model to predict ? using routinely measured limnological variables. By applying this model to ?C-CH profiles obtained from the study lakes, we show that using a constant ? value in isotopic mass balances can largely misrepresent and undermine patterns of the extent of CH oxidation in lakes. Our ? model thus contributes towards more reliable estimations of stable carbon isotope-based quantification of CH oxidation and may help to elucidate large scale patterns and drivers of the oxidation-driven mitigation of CH emission from lakes.

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH 4 ) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH 4 emission and isotopic signature. Here, we examine the magnitude of CH 4 ebullition (bubbling) and stable carbon isotopic signature (? 13 C-CH 4 ) of bubble CH 4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH 4 flux and bubble ? 13 C-CH 4 varied from 0.01 to 37.0 mmol m ?2  d ?1 and between ?71.0‰ and ?50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched ? 13 C-CH 4 signature. Subsequently, in addition to the temperature dependence (1.4 ± 0.1 eV), the rates of ebullition strongly correlated with the variability of ? 13 C-CH 4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted ? 13 C-CH 4 values (?64.6 ± 0.6‰ to ?60.1 ± 3.2‰), yet the signature of the total CH 4 emission (ebullition + diffusion) was relatively enriched (?60.7‰ to ?52.6‰) due to high methanotrophic activity in the water column. We show that ? 13 C-CH 4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH 4 source signature to northern lakes.