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Inland waters have a significant influence on atmospheric methane (CH 4 ) levels. However, processes determining the strength of CH 4 emissions from these systems are not well defined. Aerobic oxidation is a major sink of CH 4 in freshwater environments and thus an important determinant of aquatic CH 4 emissions, yet strikingly little is known about its drivers. Here we assessed the extent of water column CH 4 oxidation at the whole ecosystem scale using stable carbon isotopic (δ 13 C) mass balance of CH 4 in 14 northern lakes spanning wide range of dissolved organic carbon (DOC) concentrations. We show that the extent of oxidation can vary from near zero to near complete, and for concentrations of 1.9-11 mg/L, DOC is a key modulator of CH 4 oxidation during the summer stratification period. Increasing DOC concentrations enhances oxidation in the upper layers by reducing light inhibition on methanotrophic activity, while reducing oxygen available for oxidation in the deeper layers. The effect of this light inhibition was also observable over the diurnal cycle. We developed simple predictive empirical models (r 2 > 0.82) to estimate the extent of oxidation in the different layers of lakes for the summer period. Applying our surface layer model to a larger data set suggests that about 30% of CH 4 transported to or generated within the epilimnion of Québec lakes is oxidized during summer. Our results imply that DOC concentration, through its effect on the light regime of lakes, has the potential to affect strongly the magnitude and patterns of summer CH 4 emissions.

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Lake methane (CH 4 ) 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 CH 4 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 CH 4 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 CH 4 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 CH 4 oxidation rates in lakes and that information on the MOB community can help predict CH 4 oxidation rates and thus emissions from lakes. Received

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH 4 ) 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 CH 4 production (OMP) have remained ambiguous. Based on the summer CH 4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008−0.44 km 2 ), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L −1 day −1 ) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH 4 (δ 13 C-CH 4 ) indicates direct photoautotrophic release as the most likely source of oxic CH 4 . Furthermore, we show that the oxic CH 4 contribution to the SML CH 4 saturation 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 2 . Given that global lake CH 4 emissions are dominated by small lakes (SA of <1 km 2 ), the large contribution of oxic CH 4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH 4 emissions.

The dynamics of fluorescent dissolved organic matter (FDOM) in the large monomictic freshwater Lake Biwa (surface area 675 km 2 , maximum depth 104 m) was studied from December 2010 to December 2011. The proteinlike FDOM (FDOM T ) and dissolved organic carbon (DOC) showed epilimnetic accumulation (FDOM T from 4.42 6 0.22 quinine sulfate units [QSU] to 6.30 6 0.04 QSU; DOC from 80.8 6 2.7 mmol L 21 to 102.7 6 3.5 mmol L 21 ) between nutrient-replete winter mixing to nutrient-depleted stratified periods. This accumulation is attributed to the reduced heterotrophic activity following severe P-limitation. The positive correlation between accumulated DOC and FDOM T in the epilimnion and their uniform reduction in the hypolimnion (, 9%) suggest FDOM T as a proxy for semi-labile DOM. The humic-like FDOM (FDOM M ) generally increased with depth, a pattern similar to nutrients and total carbon dioxide (TCO 2 ), but adverse to dissolved oxygen. The significant positive correlations of FDOM M with apparent oxygen utilization (r 5 0.86, p , 0.001), TCO 2 (r 5 0.91, p , 0.001), nitrate (r 5 0.83, p , 0.001), and phosphate (r 5 0.76, p , 0.001) in the deeper layers suggest that FDOM M is formed during hypolimnetic mineralization. We estimated that , 8% of the organic carbon degraded in the hypolimnion is transferred into humic substances. The minor contribution of DOC (6.4%) to hypolimnetic mineralization suggests that production of humic substances is mainly fueled by the mineralization of sinking biogenic particles. The production and consumption of FDOM in freshwater lakes may influence the quality and bioavailability of carbon exported from these systems.

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