Methane (CH4), a potent greenhouse gas, is produced in and emitted from lakes at globally significant rates. The drivers controlling the proportion of produced CH4 that will reach the atmosphere, however, are still not well understood. We sampled a small eutrophic lake (Soppensee, Switzerland) in 2016–2017 for CH4 concentrations profiles and emissions, combined with water column hydrodynamics to investigate the fate of CH4 produced in hypolimnetic sediments. Using a mass balance approach for the periods between April and October of both years, net CH4 production rates in hypolimnetic sediments ranged between 11.4 and 17.7 mmol m−2 d−1, of which 66–88% was stored in the hypolimnion, 13–27% was diffused to the epilimnion, and 6–7% left the sediments via ebullition. Combining these results with a process‐based model we show that water column turbulent diffusivity (K z) had a major influence on the fate of produced CH4 in the sediments, where higher K z values potentially lead to greater proportion being oxidized and lower K z lead to a greater proportion being stored. During fall when the water column mixes, we found that a greater proportion of stored CH4 is emitted if the lake mixes rapidly, whereas a greater proportion will be oxidized if the water column mixes more gradually. This work highlights the central role of lake hydrodynamics in regulating CH4 dynamics and further suggests the potential for CH4 production and emissions to be sensitive to climate‐driven alterations in lake mixing regimes and stratification.
Here we used flow cytometry (FCM) and filtration paired with amplicon sequencing to determine the abundance and composition of small low nucleic acid (LNA)-content bacteria in a variety of freshwater ecosystems. We found that FCM clusters associated with LNA-content bacteria were ubiquitous across several ecosystems, varying from 50 to 90% of aquatic bacteria. Using filter-size separation, we separated small LNA-content bacteria (passing 0.4 µm filter) from large bacteria (captured on 0.4 µm filter) and characterized communities with 16S amplicon sequencing. Small and large bacteria each represented different sub-communities within the ecosystems’ community. Moreover, we were able to identify individual operational taxonomical units (OTUs) that appeared exclusively with small bacteria (434 OTUs) or exclusively with large bacteria (441 OTUs). Surprisingly, these exclusive OTUs clustered at the phylum level, with many OTUs appearing exclusively with small bacteria identified as candidate phyla (i.e. lacking cultured representatives) and symbionts. We propose that LNA-content bacteria observed with FCM encompass several previously characterized categories of bacteria (ultramicrobacteria, ultra-small bacteria, candidate phyla radiation) that share many traits including small size and metabolic dependencies on other microorganisms.
Atmospheric methane (CH 4 ) concentrations have more than doubled in the past~250 yr, although the sources of this potent greenhouse gas remain poorly constrained. Freshwaters contribute~20% of natural CH 4 emissions, about half attributed to ebullition. Estimates remain uncertain as ebullition is stochastic, making measurements difficult, time consuming, and costly with current methods (e.g., floating chambers, funnel gas traps, and hydroacoustics). We present a novel approach to quantify basin-wide hypolimnetic CH 4 fluxes at the sediment level based on measurements of bubble gas content and modeling of dissolved pore-water gases. We show that the relative ebullition flux pathway can be resolved by knowledge of only bubble gas content. As sediment CH 4 production, diffusion, and ebullition are interrelated, the addition of a second observation allows closing the entire sediment CH 4 balance. Such measurements could include bubble formation depth, sediment diffusive fluxes, ebullition, sediment CH 4 production, or the hypolimnetic CH 4 mass balance. The measurement of bubble gas content is particularly useful for identifying local ebullitive hotspots and integrating spatial heterogeneity of CH 4 fluxes. Our results further revealed the crucial effect of water column depth, production rates, and hypolimnetic dissolved CH 4 concentrations on sediment CH 4 dynamics. Although we apply the model to cohesive sediments in an anoxic hypolimnion, the model can be applied to shallow, oxic settings by altering the CH 4 production rate curve to account for oxidation. Utilizing our approach will provide a deeper understanding of in-lake CH 4 budgets, and thus improve CH 4 emission estimates from inland freshwaters at the regional and global scales.
Carbon budgets of natural and impacted ecosystems, including lakes, are critical to quantify their role in regulating Earth's climate. Excessive nutrient loading to lakes both increases their algal production resulting in atmospheric CO 2 uptake and increases CH 4 production due to anaerobic decomposition of organic matter. The net balance between CO 2 uptake and CH 4 emissions from lakes, however, has not been extensively addressed. Our work reveals that a substantial proportion of the organic carbon supplied by the net ecosystem production is used for methanogenesis and emitted back to the atmosphere as CH 4. From a climate change perspective, exchanging CO 2 uptake with CH 4 release is an "unfair trade" for the atmosphere, as CH 4 has a much greater global warming potential than CO 2 .
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.
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