The Role of Littoral Vegetation and Open Water Greenhouse Gas Fluxes on the Carbon Budget of Urban Stormwater Ponds

Stormwater ponds (SWPs) are a common stormwater management technology in new urban developments and have been suggested to be significant sources of the greenhouse gases (GHGs) carbon dioxide (CO₂) and methane (CH₄). However, they also sequester organic carbon 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 net carbon budgets of urban SWP systems comprehensively. In this study, we assessed the relative contributions of the littoral vegetation and open water GHG fluxes to the carbon budgets in two SWPs located in the City of Kitchener, Ontario, Canada. CO₂ and CH₄ fluxes were measured in the forebay and main basin of two SWPs draining catchments with two different catchment land use (residential versus industrial). 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 Ecosystem Production (GEP) from both bank and submerged vegetation, plus the diffusive and ebullitive fluxes from the open water surface. Additionally, key parameters, including photosynthetically active radiation (PAR), air and soil temperature, water pH, conductivity, and dissolved gas concentrations, were also measured. We observed significant differences in the fluxes between the littoral vegetation and open water surfaces. Carbon gas emissions from the open water surface were dominated by ebullitive CH₄ fluxes, with the open water acting as a net carbon source. Ebullition events occurred more frequently and with greater intensity in the forebay areas of the SWPs, contributing the most to open water carbon emissions. In contrast, carbon gas emissions from the vegetation were largely driven by photosynthesis and soil respiration, with the vegetated littoral zone functioning as a net CO₂ sink. Different vegetation types exhibited varied responses to meteorological conditions, but all showed clear seasonal trends, with higher gas fluxes in summer due to increased biological activity, and minimal fluxes during the frozen season. Unlike vegetation, open water fluxes did not display a distinct seasonal trend; instead, they were primarily influenced by precipitation events and inflow runoff. The forebay of the industrial pond received higher carbon inputs from contaminated stormwater runoff, leading to greater sediment accumulation and elevated GHG fluxes, with frequent and high-intensity CH₄ ebullition events being a notable feature. Our findings highlight the critical influence of land use, hydrological events, and seasonal cycles on the carbon balance of SWPs and their potential role in urban carbon cycling.