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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. [ABSTRACT FROM AUTHOR]

Summary: Lake methane (CH4) emissions are largely controlled by aerobic methane‐oxidizing bacteria (MOB) which mostly belong to the classes Alpha‐ and Gammaproteobacteria (Alpha‐ and Gamma‐MOB). Despite the known metabolic and ecological differences between the two MOB groups, their main environmental drivers and their relative contribution to CH4 oxidation rates across lakes remain unknown. Here, we quantified the two MOB groups through CARD‐FISH along the water column of six temperate lakes and during incubations in which we measured ambient CH4 oxidation rates. We found a clear niche separation of Alpha‐ and Gamma‐MOB across lake water columns, which is mostly driven by oxygen concentration. Gamma‐MOB appears to dominate methanotrophy throughout the water column, but Alpha‐MOB may also be an important player particularly in well‐oxygenated bottom waters. The inclusion of Gamma‐MOB cell abundance improved environmental models of CH4 oxidation rate, explaining part of the variation that could not be explained by environmental factors alone. Altogether, our results show that MOB composition is linked to CH4 oxidation rates in lakes and that information on the MOB community can help predict CH4 oxidation rates and thus emissions from lakes. [ABSTRACT FROM AUTHOR]

Carbon biogeochemistry of a tropical ecosystem (The Cochin Estuary, India) undergoing increased human intervention was studied during February (premonsoon), April (early monsoon) and September (monsoon) 2005. The Cochin estuary sustains high levels of pCO2 (up to 6000 μatm) and CO2 effluxes (up to 274 mmolC m−2 d−1) especially during monsoon. A first-order estimate of the carbon mass balance shows that net production of dissolved inorganic carbon is an order of magnitude higher than the net loss of dissolved and particulate organic carbon from the estuary. This imbalance is attributed to the organic inputs to the estuary through anthropogenic supplies. The bacteria-mediated mineralization of organic matter is mainly responsible for the build-up of pCO2 and increased CO2 emission to the atmosphere indicating heterotrophy. The linear correlation between excess CO2 and apparent oxygen utilization indicates respiration as the chief mechanism for CO2 supersaturation. An increase in the net negative ecosystem production (–ve NEP) between premonsoon (−136 mmolC m−2 d−1 or −376 MgC d−1) and monsoon (−541 mmolC m−2 d−1 or −1500 MgC d−1) is supported by a corresponding increase in O2 influxes from 17 mmol O2 m−2 d−1 (126 MgC d−1) to −128 mmol O2 m−2 d−1 (−946 MgC d−1) and CO2 emissions from 65 mmolC m−2 d−1 (180 MgC d−1) to 267 mmolC m−2 d−1 (740 MgC d−1). There is a significant north-south gradient in metabolic rates and CO2 fluxes attributable to the varying flow patterns and anthropogenic inputs into the estuary. The study reveals that the Cochin estuary, a previously autotrophic (CO2 sink) system, has been transformed to a heterotrophic (CO2 source) system following rapid urbanization and industrialization. Moreover, the export fluxes from the Cochin estuary appear to be quite important in sustaining net heterotrophy in the southeastern Arabian Sea. [ABSTRACT FROM AUTHOR]