Diel variation of CH4 emission fluxes in a small artificial lake: Toward more accurate methods of observation

Methods for greenhouse gas emission accounting in inland aquaculture: a systematic review

This study systematically reviews methodologies for greenhouse gas (GHG) emissions accounting in inland aquaculture, focusing on their applicability to carbon footprint assessments. Aquaculture, as a critical component of global food production, faces growing investigation over its environmental impacts. The review identifies and evaluates two primary GHG measurement methods-floating static chambers and sediment analysis-against four criteria: flexibility, accessibility, precision, and ease of implementation. A  comprehensive feasibility analysis indicates that while floating static chambers offer greater adaptability and simplicity, sediment analysis delivers higher precision, albeit with increased operational complexity. The study highlights the need for standardized functional units, recommending CO₂-equivalent emissions per kilogram of fish or edible meat produced as the most appropriate metric. Although each method has limitations, their integration can enable a more holistic approach to GHG accounting in aquaculture systems. By promoting accurate, scalable, and context-specific tools, this research emphasizes aquaculture's potential to become a low-carbon, sustainable source of protein, capable of meeting global nutritional demands while minimizing environmental impacts.

Utilization patterns strongly dominated the dynamics of CO2 and CH4 emissions from small artificial lakes

Small lakes are significant sources of CO2 and CH4 emissions to atmosphere. The dynamics and controls of CO2 and CH4 emissions from human-dominated small lakes with diverse functions remain poorly understood. We investigated the spatiotemporal dynamics of CO2 and CH4 concentrations and fluxes in 33 small lakes around the urban area with different landscape properties and utilization patterns, to clarify the impact of human-dominated functional shift on their greenhouse gas emissions. Meanwhile, we used microcosm cultivation methods to assess the CO2 and CH4 production rates of sediments in these lakes. The results indicated that the utilization ways significantly influence the CO2 and CH4 emissions in these lakes, with urban landscape lakes and aquaculture lakes showing significantly higher emissions compared to irrigation water-supplying lakes and drinking-water lakes. Extensive urbanization and aquaculture practices could increase the risk of that small lakes turn into hotspots of CO2 and CH4 emissions, and further complicate their spatial heterogeneity. Meanwhile, the production potential of CO2 and CH4 in sediments, as well as gas fluxes in small lakes, exhibited consistent functional differentiation across different utilization patterns. They were mainly driven by changes in sediment organic carbon and microbial carbon. Additionally, the difference of organic carbon and nitrogen loads were another drives for the variability in CO2 and CH4 emissions. We highlighted that the continuous accumulation of nutrient loads in water and sediments in human-dominated small lakes has greatly enhanced the potential for carbon gas emissions. We also found that utilization ways can significantly affect the key controls of CO2 and CH4 emission from small lakes, and also influence the reliability of carbon emission prediction models based on water chemistry parameters. To accurately estimate the contribution of small lakes to the global greenhouse gas inventory, it is essential to establish adaptive predictive models that consider the uncertainties in lake carbon emissions resulting from human utilization patterns.

The non-coevolution of DIC and alkalinity and the CO2 degassing in a karst river affected by acid mine drainage in Southwest China

Dissolved inorganic carbon (DIC) in mine water generated during coal mining is a large and potential source of atmospheric CO2, however its geochemical behaviors under the influence of AMD in relation to CO2 degassing and carbonate buffering are not well known. In this study, water temperature, pH, DO, alkalinity, Ca2+ concentration, and the carbon isotope of DIC were measured monthly from November 2020 to November 2021 and carbonate chemistry and CO2 emission flux were calculated to reveal the processes of DIC evolution and CO2 degassing from the Chetian River draining a karst region, which is materially affected by the input of large quantities of AMD. The results showed that carbonate erosion, the mineralization of terrestrial organic matter, and domestic sewage input are all identified to contribute DIC to different degrees to the river. Throughout the year, the Chetian River undergoes high-intensity CO2 degassing, which is dominated by HCO3--neutralized degassing and proton-enhanced degassing in different reaches. The pCO2 in the river under the influence of AMD is as high as 237,482 μatm, while the F-CO2 approaches 316.9 g C m-2 d-1. Meanwhile, the carbonate system in the downstream karst river buffers an average of 85.2 % of DIC release at the river's outlet. The input of AMD significantly altered the carbon cycle of the surface watershed in the headwaters of tributaries, and greatly enhanced the release of CO2 from surface water to the atmosphere; meanwhile, the buffering of carbonates on acidity in the water of main streams causes pCO2 to rapidly reduce over a short distance. Obviously, the carbon emission effect generated by the interaction between AMD and carbonate mainly occurs in the tributary water system. Considering the huge amount of AMD worldwide, this large potential source of atmospheric CO2 requires a specific and precise quantitative analysis based on actual observations.

Agricultural ditches are hotspots of greenhouse gas emissions controlled by nutrient input

Agricultural ditches are pervasive in agricultural areas and are potential greenhouse gas (GHG) hotspots, since they directly receive abundant nutrients from neighboring farmlands. However, few studies measure GHG concentrations or fluxes in this particular water course, likely resulting in underestimations of GHG emissions from agricultural regions. Here we conducted a one-year field study to investigate the GHG concentrations and fluxes from typical agricultural ditch systems, which included four different types of ditches in an irrigation district located in the North China Plain. The results showed that almost all the ditches were large GHG sources. The mean fluxes were 333 μmol m-2 h-1 for CH4, 7.1 mmol m-2 h-1 for CO2, and 2.4 μmol m-2 h-1 for N2O, which were approximately 12, 5, and 2 times higher, respectively, than that in the river connecting to the ditch systems. Nutrient input was the primary driver stimulating GHG production and emissions, resulting in GHG concentrations and fluxes increasing from the river to ditches adjacent to farmlands, which potentially received more nutrients. Nevertheless, the ditches directly connected to farmlands showed lower GHG concentrations and fluxes compared to the ditches adjacent to farmlands, possibly due to seasonal dryness and occasional drainage. All the ditches covered approximately 3.3% of the 312 km2 farmland area in the study district, and the total GHG emission from the ditches in this area was estimated to be 26.6 Gg CO2-eq yr-1, with 17.5 Gg CO2, 0.27 Gg CH4, and 0.006 Gg N2O emitted annually. Overall, this study demonstrated that agricultural ditches were hotspots of GHG emissions, and future GHG estimations should incorporate this ubiquitous but underrepresented water course.

Evaluation of the methane paradox in four adjacent pre-alpine lakes across a trophic gradient

Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP, aka the methane paradox) occurs in oxygenated surface waters worldwide. OMP drivers and their contribution to global methane emissions, however, are not well constrained. In four adjacent pre-alpine lakes, we determine the net methane production rates in oxic surface waters using two mass balance approaches, accounting for methane sources and sinks. We find that OMP occurs in three out of four studied lakes, often as the dominant source of diffusive methane emissions. Correlations of net methane production versus chlorophyll-a, Secchi and surface mixed layer depths suggest a link with photosynthesis and provides an empirical upscaling approach. As OMP is a methane source in direct contact with the atmosphere, a better understanding of its extent and drivers is necessary to constrain the atmospheric methane contribution by inland waters.

Lake metabolic processes and their effects on the carbonate weathering CO2 sink: Insights from diel variations in the hydrochemistry of a typical karst lake in SW China

The precipitation of carbonate minerals does not invariably result in CO₂ emission to the atmosphere, because dissolved inorganic carbon (DIC) can be partially utilized by terrestrial aquatic phototrophs, thus generating an autochthonous organic carbon (AOC) sink. However, little is known about the potential effects of this mechanism on carbon cycles in DIC-rich lakes, mainly due to the lack of detailed documentation of the related processes, which limits our ability to accurately evaluate and predict the magnitude of this carbon sink. We conducted field observations in Fuxian Lake, a large and representative karst lake in the Yunnan-Guizhou Plateau, SW China. Continuous diel monitoring was conducted to quantitatively assess the coupled relationship between lake metabolism and DIC cycling and its influence on the carbonate weathering-related CO₂ sink. We found that the diel physicochemical variations and isotopic characteristics were mainly controlled by the metabolism of aquatic phototrophs, evidenced by a significant relationship between net ecosystem production and diel DIC cycling, and demonstrating the significance of DIC fertilization in supporting high primary production in karst lakes. The data showed that a reduction in photosynthesis occurred in the afternoon of almost every day, which can be explained by the lower CO₂/O₂ ratio that increased the potential for the photorespiration of aquatic plants, thus reducing photosynthesis. We found that a net autotrophic ecosystem prevailed in Fuxian Lake, suggesting that the lake functions more as a sink than a source of atmospheric CO₂. Considering carbonate weathering, the estimated AOC sink amounted to 650-704 t C km^-2 yr^-1, demonstrating both the potentially significant role of metabolism in lacustrine carbon cycling and the potential of the combination of photosynthesis and carbonate weathering for carbon sequestration. Our findings may help to quantitatively estimate the future impact of lake metabolism on carbon cycling, with implications for formulating management policies needed to regulate the magnitude of this carbon sink.