Riparian vegetation and open water carbon greenhouse gas fluxes of urban stormwater ponds

Abstract

Stormwater ponds (SWPs) are a common stormwater management technology in new urban developments and have been suggested to be significant sources of the carbon greenhouse gases (GHGs); e.g., carbon dioxide (CO2) and methane (CH4). However, they also sequester organic carbon (OC) and reduce the surface runoff of nutrients, hence, altering nutrient limitation patterns, trophic conditions, and GHG exchanges. Although numerous studies have focused on estimating open water GHG emissions in artificial ponds, there are limited studies that evaluate both open water and riparian vegetation fluxes from urban SWP systems comprehensively. This study quantified CO2 and CH4 fluxes from riparian vegetation and open water in two SWPs in the City of Kitchener, Ontario, located in residential (Activa pond) and industrial (Wabanaki pond) catchments. In Chapter 2, the goal was to compare flux pathways and net source-sink status and assess how land use, spatial variability between forebay and main basin zones, and pond design influence the GHG dynamics. Using vegetation and floating chambers, CO2 and CH4 fluxes were measured bi-weekly across all seasons, capturing net ecosystem exchange (NEE), ecosystem respiration (ER), and gross primary production (GPP) from riparian vegetation, plus the diffusive and ebullitive fluxes from the open water surface. Significant differences in the fluxes between the riparian vegetation and open water surfaces were observed. High photosynthetic activity allowed the riparian zone to function as a net carbon sink (-142.3 mol m-2 yr-1 for Activa and -140.5 mol m-2 yr-1 for Wabanaki). However, higher OC inputs from the industrial Wabanaki catchment enhanced sediment CH4 production, particularly in the forebay, resulting in higher vegetation CH4 emissions that weakened its GHG-sink strength relative to the residential Activa pond., resulting in CO2-equivalent fluxes of -141.4 mol CO2-eq m-2 yr-1 for Activa and only -94.8 mol CO2-eq m-2 yr-1 for Wabanaki. Although Activa had greater per-area vegetation CO2-equivalent uptake, its small riparian zone limited whole-pond removal, whereas Wabanaki’s extensive riparian area provided larger total CO2 uptake (-3.6 mol CO2-eq m-2 yr-1 for Activa and -132.1 mol CO2-eq m-2 yr-1 for Wabanaki). Open water fluxes were dominated by ebullitive CH4, which accounted for about 88% of the total CO2-equivalent flux (125.6 mol CO2-eq m-2 yr-1 for Activa and 119.6 mol CO2-eq m-2 yr-1 for Wabanaki), making the open water surface a net GHG source. The forebay of the ponds consistently acted as carbon GHG hotspots due to higher carbon loading, nutrient enrichment, and reducing conditions, while larger, deeper main basins mitigated emissions. Overall, both Activa and Wabanaki ponds ultimately acted as net GHG sources, with annual emissions of 189.6 kmol CO2-eq yr-1 at Activa and 349.9 kmol CO2-eq yr-1 at Wabanaki, driven primarily by CH4 emissions. These findings highlight the combined influence of land-use-driven carbon loading, riparian zone extent, and the contrasting behaviour of forebays and main basins in controlling stormwater pond GHG dynamics. In Chapter 3, the full dataset collected during the study period were presented. Based on field experience, we outline practical recommendations for municipalities, including linking monitoring objectives to decision-making, applying tiered spatial and temporal sampling, quantifying flux pathways, measuring key water and sediment drivers, integrating hydrologic data, and tracking vegetation cover for upscaling. Together, these approaches create a scalable, transparent framework for incorporating SWPs into city-scale GHG monitoring and management.