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Inland waters are hotspots of greenhouse gas (GHG) cycling, with small water bodies particularly active in the production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, wetland ponds are being replaced rapidly by small constructed reservoirs in agricultural regions, yet it is unclear whether these two water body types exhibit similar physical, chemical, and environmental controls of GHG content and fluxes. Here, we compared the content and regulatory mechanisms of all three major GHGs in 20 pairs of natural wetland ponds and constructed reservoirs in Canada's largest agricultural region. Carbon dioxide content was associated primarily with metabolic indicators in both water body types; however, primary production was paramount in reservoirs, and heterotrophic metabolism a stronger correlate in wetland ponds. Methane concentrations were correlated positively with eutrophication of the reservoirs alone, while competitive inhibition by sulfur-reducing bacteria may have limited CH4 in both waterbody types. Contrary to expectations, N2O was undersaturated in both water body types, with wetlands being a significantly stronger and more widespread N2O sink. Varying regulatory processes are attributed to differences in age, depth, morphology, and water-column circulation between water body types. These results suggest that natural and constructed water bodies should be modeled separately in regional GHG budgets.

Metabolic stoichiometry predicts that dissolved oxygen (O2 ) and carbon dioxide (CO2 ) in aquatic ecosystems should covary inversely; however, field observations often diverge from theoretical expectations. Here, we propose a suite of metrics describing this O 2 and CO2 decoupling and introduce a conceptual framework for interpreting these metrics within aquatic ecosys- tems. Within this framework, we interpret cross-system patterns of high-frequency O 2 and CO2 measurements in 11 northern lakes and extract emergent insights into the metabolic behavior and the simultaneous roles of chemical and physical forcing in shaping ecosystem processes. This approach leverages the power of high-frequency paired O 2–CO2 measurements, and yields a novel, integrative aquatic system typology which can also be applicable more broadly to streams and rivers, wetlands and marine systems. This study evolved from discussions during the “Coupling of O 2 and CO 2 dynamics in Lakes” working group during the Global Lake Ecological Observatory Network GLEON 16 meeting held in Orford, Canada in 2014. We are especially thankful to P. Staehr, M. Head, T. Rasilo, S. Bartlett, R.-M. Couture, D. da Motta Marques, T. Kratz, R. Maranger, D. Tonetta, P. Meison, and T. DelSontro who also participated to this working group and B. Obrador for providing useful suggestions which improved the study. We thank Erik Geibrink and Anders Jonsson for help with data acquisition in the Swedish lakes, which was supported by Formas (Grant 210-2012-1461) and Kempestiftelserna (Grant SMK- 1240). We thank the Ontario Ministry of the Environment, Conservation and Parks (formerly Environment and Climate Change) for funding the Harp Lake initiative and Chris McConnell and Tim Field of the Dorset Environmental Science Centre for help in data collection and curation. Work within Buffalo Pound Lake was supported by instrumentation from the Canada Foundation for Innovation, a National Science and Engineering Council Strategic Project Grant, and support of the Saskatchewan Water Security Agency and Buffalo Pound Water Treat- ment Plant. We thank Jay Bauer, Katy Nugent, and others for technical support in Buffalo Pound Lake. S.S. was supported through a post- doctoral fellowship from the National Science Foundation (EAR- 1249769). Financial support for G.A.W. was received from the Swedish Research Council (Grant 2016-04153) and from the Knut and Alice Wallenberg Foundation (Grant KAW 2013.0091). A.L. was supported by Estonian Research Council Grant PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research. D.V. was supported by postdoctoral fellowships from NSERC and the Knut and Alice Wallenberg Foundation. M.J.B. was supported by postdoctoral fellowships from NSERC and the Delta Stewardship Council Delta Science Program. This study evolved from discussions during the “Coupling of O 2 and CO 2 dynamics in Lakes” working group during the Global Lake Ecological Observatory Network GLEON 16 meeting held in Orford, Canada in 2014. We are especially thankful to P. Staehr, M. Head, T. Rasilo, S. Bartlett, R.-M. Couture, D. da Motta Marques, T. Kratz, R. Maranger, D. Tonetta, P. Meison, and T. DelSontro who also participated to this working group and B. Obrador for providing useful suggestions which improved the study. We thank Erik Geibrink and Anders Jonsson for help with data acquisition in the Swedish lakes, which was supported by Formas (Grant 210-2012-1461) and Kempestiftelserna (Grant SMK- 1240). We thank the Ontario Ministry of the Environment, Conservation and Parks (formerly Environment and Climate Change) for funding the Harp Lake initiative and Chris McConnell and Tim Field of the Dorset Environmental Science Centre for help in data collection and curation. Work within Buffalo Pound Lake was supported by instrumentation from the Canada Foundation for Innovation, a National Science and Engineering Council Strategic Project Grant, and support of the Saskatchewan Water Security Agency and Buffalo Pound Water Treat- ment Plant. We thank Jay Bauer, Katy Nugent, and others for technical support in Buffalo Pound Lake. S.S. was supported through a post- doctoral fellowship from the National Science Foundation (EAR- 1249769). Financial support for G.A.W. was received from the Swedish Research Council (Grant 2016-04153) and from the Knut and Alice Wallenberg Foundation (Grant KAW 2013.0091). A.L. was supported by Estonian Research Council Grant PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research. D.V. was supported by postdoctoral fellowships from NSERC and the Knut and Alice Wallenberg Foundation. M.J.B. was supported by postdoctoral fellowships from NSERC and the Delta Stewardship Council Delta Science Program.

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere.

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass. National Science Foundation (NSF DEB) [1431428, 1136637]; Washington State University; Russian Science Foundation [14-14-00400]; Ministry of education and science of Russia Gos-Zasanie project [1354-2014/51]; Natural Environment Research Council [NE/J00829X/1, 1230750, NE/G019622/1, NE/J010227/1] Funding was provided by the National Science Foundation (NSF DEB #1431428; NSF DEB #1136637) and Washington State University. M. Timofeyev and E. Silow were partially supported by Russian Science Foundation project No 14-14-00400 and Ministry of education and science of Russia Gos-Zasanie project No 1354-2014/51. We are grateful to Marianne Moore, Deniz Ozkundakci, Chris Polashenski and Paula Kankaala for discussions that greatly improved this work. We also gratefully acknowledge the following individuals for contributing to this project: John Anderson, Jill Baron, Rick Bourbonniere, Sandra Brovold, Lluis Camarero, Sudeep Chandra, Jim Cotner, Laura Forsstom, Guillaume Grosbois, Chris Harrod, Klaus D. Joehnk, T.Y. Kim, Daniel Langenhaun, Reet Laugaste, Suzanne McGowan, Virginia Panizzo, Giampaolo Rossetti, R.E.H. Smith, Sarah Spaulding, Helen Tammert, Steve Thackeray, Kyle Zimmer, Priit Zingel and two anonymous reviewers. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.