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Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering greenhouse gas reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide direction for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing carbon dioxide (CO 2 ) and methane (CH 4 ) concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO 2 partial pressure resulted in an overall reduction of CO 2 efflux. Ebullition represented between 40 and 99% of the total CH 4 efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO 2 equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Lakes are important sources of greenhouse gases (GHGs) to the atmosphere. Factors controlling CO 2 , CH 4 and N 2 O fluxes include eutrophication and warming, but the integrated influence of climate-warming-driven stratification, oxygen loss and resultant changes in bloom characteristics on GHGs are not well understood. Here we assessed the influence of contrasting meteorological conditions on stratification and phytoplankton bloom composition in a eutrophic lake, and tested for associated changes in GHGs inventories in both the shallow and deep waters, over three seasons (2010-2012). Atmospheric heatwaves had one of the most dramatic effects on GHGs. Indeed, cyanobacterial blooms that developed in response to heatwave events in 2012 enhanced both sedimentary CH 4 concentrations (reaching up to 1mM) and emissions to the atmosphere (up to 8 mmol m -2 d -1 ). That summer, CH 4 contributed 52% of the integrated warming potential of GHGs produced in the lake (in CO 2 equivalents) as compared to between 34 and 39% in years without cyanobacterial blooms. High CH 4 accumulation and subsequent emission in 2012 were preceded by CO 2 and N 2 O consumption and under-saturation at the lake surface (uptakes at -30 mmol m -2 d -1 and -1.6 µmol m -2 d -1 , respectively). Fall overturn presented a large efflux of N 2 O and CH 4 , particularly from the littoral zone after the cyanobacterial bloom. We provide evidence that, despite cooling observed at depth during hot summers, CH 4 emissions increased via stronger stratification and surface warming, resulting in enhanced cyanobacterial biomass deposition and intensified bottom water anoxia. Our results, supported by recent literature reports, suggests a novel interplay between climate change effects on lake hydrodynamics that impacts both bloom characteristics and GHGs production in shallow eutrophic lakes. Given global trends of warming and enrichment, these interactive effects should be considered to more accurately predict the future global role of lakes in GHG emissions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Global warming has accelerated the formation of permafrost thaw ponds in several subarctic and arctic regions. These ponds are net heterotrophic as evidenced by their greenhouse gas (GHG) supersaturation levels (CO2 and CH4), and generally receive large terrestrial carbon inputs from the thawing and eroding permafrost. We measured seasonal and vertical variations in the concentration and type of dissolved organic matter (DOM) in five subarctic thaw (thermokarst) ponds in northern Quebec, and explored how environmental gradients influenced heterotrophic and phototrophic biomass and productivity. Late winter DOM had low aromaticity indicating reduced inputs of terres trial carbon, while the high concentration of dissolved organic carbon (DOC) suggests that some production of non-chromophoric dissolved compounds by the microbial food web took place under the ice cover. Summer DOM had a strong terrestrial signature, but was also characterized with significant inputs of algal-derived carbon, especially at the pond surface. During late winter, bacterial production was low (maximum of 0.8mgCm-3 d-1) and was largely based on free-living bacterioplankton (58%). Bacterial production in summer was high (up to 58mgCm3 d1), dominated by particleattached bacteria (67%), and strongly correlated to the amount of terrestrial carbon. Primary production was restricted to summer surface waters due to strong light limitation deeper in the water column or in winter. The phototrophic biomass was equal to the heterotrophic biomass, but as the algae were mostly composed of mixotrophic species, most probably they used bacteria rather than solar energy in such shaded ponds. According to the δ13C analyses, non-algal carbon supported 51% of winter and 37% of summer biomass of the phantom midge larvae, Chaoborus sp., that are at the top of the trophic chain. Our results point to a strong heterotrophic energy pathway 25 in these thaw pond ecosystems, where bacterioplankton dominates the production of new carbon in both summer and winter. [ABSTRACT FROM AUTHOR]

Ponds and lakes are widespread across the rapidly changing permafrost environments. Aquatic systems play an important role in global biogeochemical cycles, especially in greenhouse gas (GHG) exchanges between terrestrial systems and the atmosphere. The source, speciation and emission of carbon released from permafrost landscapes are strongly influenced by local specific conditions rather than general environmental setting. This study reports on GHG ages and emission rates from aquatic systems on Bylot Island in the eastern Canadian Arctic. Dissolved and ebullition gas samples were collected during the summer season from different types of water bodies located in a highly dynamic periglacial valley: polygonal ponds, collapsed ice-wedge trough ponds, and larger lakes overlying unfrozen soils (talik). The results showed strikingly different ages and fluxes depending on aquatic system types. Polygonal ponds were net sinks of dissolved CO2, but variable sources of dissolved CH4. They presented the highest ebullition fluxes, one or two orders of magnitude higher than from other ponds and lakes. Trough ponds appeared as substantial GHG sources, especially when their edges were actively eroding. Both types of ponds produced modern to hundreds of years old (< 550 yr BP) GHG, even if trough ponds could contain much older carbon (> 2000 yr BP) derived from freshly eroded peat. Lakes had small dissolved and ebullition fluxes, however they released much older GHG, including millennium-old CH4 (up to 3500 yr BP) sampled from lake central areas. Acetoclastic methanogenesis dominated at all study sites and there was minimal, if any, methane oxidation in gas emitted through ebullition. These findings provide new insights on the variable role of permafrost aquatic systems as a positive feedback mechanism on climate. [ABSTRACT FROM AUTHOR]