Carbon dioxide, methane, and dissolved carbon dynamics in an urbanized river system

Estimates of greenhouse gas (GHG) evasion from rivers have been refined over the past decades to constrain their role in global carbon cycle processes. However, despite 55% of the human population living in urban areas, urban rivers have had limited attention. We monitored carbon dynamics in an urbanized river (River Kelvin, 331 km2, UK) to explore the drivers of dissolved carbon lateral and vertical export. Over a 2-year sampling period, riverine methane (CH4) and carbon dioxide (CO2) concentrations were consistently oversaturated with respect to atmospheric equilibria, leading to continual degassing to the atmosphere. Carbon stable isotopic compositions (δ13C) indicated that terrestrially derived carbon comprised most of the riverine CH4 and dissolved CO2 (CO2*) load while dissolved inorganic carbon (DIC) from groundwater was the main form of riverine DIC. The dynamics of CH4, CO2*, and DIC in the river were primarily hydrology-controlled, i.e., [CH4] and [CO2*] both increased with elevated discharge, total [DIC] decreased with elevated discharge while the proportion of biologically-derived DIC increased with increasing discharge. The concentration of dissolved organic carbon (DOC) showed a weak relationship with river hydrology in summer and autumn and was likely influenced by the combined sewer overflows. Carbon emission to the atmosphere is estimated to be 3.10 ± 0.61 kg C·m-2·yr-1 normalized to water surface area, with more than 99% emitted as CO2. Annual carbon loss to the coastal estuary is approximately 4.69 ± 0.70 Gg C·yr-1, with annual DIC export approximately double that of DOC. Per unit area, the River Kelvin was a smaller carbon source to the atmosphere than natural rivers / streams but shows elevated fluxes of DIC and DOC under comparable conditions. This research illustrates the role urban systems may have on riverine carbon dynamics and demonstrates the potential tight link between urbanization and riverine carbon export