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 & PLUSMN; 481 & mu;atm) 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 & PLUSMN; 3846 & mu;atm) was among the higher range of global rivers (1300-4300 & mu;atm), average CO2 flux was much lower than the global river average (99.7 & PLUSMN; 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 & PLUSMN; 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.

Automated summary

The constituents and functionality of urban inland waters are disturbed by municipal sewage inputs and tailwater discharge from wastewater treatment plants, leading to significant perturbations in greenhouse gas dynamics. A lack of in situ measurements has resulted in large knowledge gaps in this understanding. Through a 3-year field campaign, the CO2 emission and related factors in the municipality of Beijing were investigated, revealing unique greenhouse gas emission profiles and emphasizing the importance of accounting for urban inland waters in terms of aquatic greenhouse gas emissions.