Diverging Carbon Dynamics in Engineered Ecosystems: Comparing Greenhouse Gases and DOM in Stormwater and Natural Ponds

Urban stormwater ponds (SWPs) are increasingly recognized as critical hotspots for carbon (C) and nitrogen (N) cycling, driven by high external inputs and elevated internal productivity. In developed watersheds, SWPs often replace natural aquatic ecosystems at equal or greater densities, but direct comparisons of the C and N dynamics between these engineered and natural ecosystems remain scarce. During distinct wet and dry seasons (Florida, USA), we compared the diffusive air-water flux and hypolimnetic saturation of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as the composition of dissolved organic matter (DOM) between SWPs (n = 15) and naturally occurring clear (n=3) and dark colored (n=3) ponds in undisturbed watersheds. SWPs had similar CO2 and lower CH4 fluxes compared to natural-dark ponds, and higher fluxes than natural-clear ponds. Both natural pond types (clear and dark) were more stratified than SWPs, resulting in a greater CH4 buildup in the natural ponds. N2O fluxes were negligible across all pond types, with dark ponds being N2O sinks. N2O fluxes were negligible, and negative in natural-dark ponds. SWPs contained 1) identical humic-like, 2) similar microbially derived, and 3) unique humic DOM compared to natural ponds. Our study underscores that SWPs differ from natural ponds in greenhouse gas production and DOM composition, providing evidence that these rapidly emerging ecosystems alter C forms and greenhouse gas fluxes in developed landscapes.