Unexpected low CO₂ emission from highly disturbed urban inland waters

Constituents and functionality of urban inland waters are significantly perturbed by municipal sewage inputs and tailwater discharge from wastewater treatment plants. However, large knowledge gaps persist in understanding greenhouse gas dynamics in urban inland waters due to a lack of in situ measurements. Herein, via a 3-year field campaign (2018-2020), we report river and lake CO₂ emission and related aquatic factors regulating the emission in the municipality of Beijing. Mean pCO₂ (546 ± 481 μatm) in the two urban lakes was lower than global non-tropical freshwater lakes and CO₂ flux in 47% of the lake observations was negative. Though average pCO₂ in urban rivers (3124 ± 3846 μatm) was among the higher range of global rivers (1300-4300 μatm), average CO₂ flux was much lower than the global river average (99.7 ± 147.5 versus 358.4 mmol m^-2 d^-1). The high pCO₂ cannot release to the atmosphere due to the low gas exchange rate in urban rivers (average k₆₀₀ of 1.3 ± 1.3 m d^-1), resulting in low CO₂ flux in urban rivers. Additionally, eutrophication promotes photosynthetic uptake and aquatic organic substrate production, leading to no clear relationships observed between pCO₂ and phytoplankton photosynthesis or dissolved organic carbon. In consistence with the findings, CO₂ emission accounted for only 32% of the total greenhouse gas (GHG) emission equivalence (CO₂, CH₄ and N₂O) in Beijing waters, in contrast to a major role of anthropogenic CO₂ to anthropogenic GHG in the atmosphere in terms of radiative forcing (66%). These results pointed to unique GHG emission profiles and the need for a special account of urban inland waters in terms of aquatic GHG emissions.