Greenhouse gas fluxes from stormwater ponds and urban wetlands in southern Ontario

Abstract

Urban wetlands and stormwater ponds are an established component of stormwater management integrated with green infrastructure elements. The main aim of these developments is to serve as receptacles for stormwater runoff and mitigate the adverse impact of urbanization on water quality and quantity. While created for stormwater management, urban wetlands and stormwater ponds also provide essential ecosystem services, including sequestration of carbon and preservation of biodiversity while enhancing the aesthetic value of the surrounding environment. Despite these services, these small water bodies accumulate loads of organic matter and nutrients, which undergo different conditions (aerobic/anaerobic) and microbial processes, releasing significant amounts of greenhouse gases (GHGs) such as methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). Design specification and exposure to aspects of the urban environment may affect the amount of GHGs released. Hence, this study aimed to quantify GHG emissions from 24 distinct stormwater ponds and urban wetlands in the Kitchener- Waterloo region, Southern Ontario, over a seven-month period from May to November and assess the impact of selected environmental, physical and chemical parameters on these GHG emissions. The average daily fluxes across the sampling period were 760 mg CO2 m-2 d-1, 417 mg CH4 m-2 d-1, and 0.23 mg N2O m-2 d-1. Seasonal variation was evident for CO2 and CH4 fluxes, whereas N2O fluxes showed minimal seasonal variation. For CO2, significant predictors included water temperature, pH, dissolved oxygen (DO), dissolved organic carbon and NO3- concentrations, and physical features, including sediment depth, pond depth, catchment area and dredging. Methane emissions were primarily driven by in-situ environmental variables, including water temperature, DO concentration and pH, while chemical and physical factors did not significantly influence CH4 fluxes. For N2O, key drivers included NO3- concentration, DO concentration, electrical conductivity and sediment depth. These findings highlight the complex and dynamic role of stormwater management facilities in GHG emissions and the importance of seasonal changes and abiotic factors in shaping emission patterns.