Inland waters are hotspots of greenhouse gas (GHG) cycling, with small water bodies particularly active in the production and consumption of carbon dioxide (CO 2), 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 CH 4 in both waterbody types. Contrary to expectations, N 2O was undersaturated in both water body types, with wetlands being a significantly stronger and more widespread N 2O 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.