Inland waters are important sources of greenhouse gases and emissions from polluted subtropical systems may be contributing to the observed global increase in atmospheric methane concentrations. In this study, dissolved concentrations of greenhouse gases methane (CH_4), carbon dioxide (CO_2), and nitrous oxide (N_2O) were measured in two contrasting urban lakes in Bangalore, Karnataka, India from June 2018 to February 2020. Bellandur Lake is a severely polluted system, whilst Jakkur Lake has been subject to partial restoration via treatment of organic matter inputs. Methane concentrations in Bellandur Lake were three orders of magnitude higher than those in Jakkur Lake, with a mean concentration of 3.02 ± 1.57 mg CH_4-C/L compared to 1.72 ± 1.22 μg CH_4-C/L. At Bellandur Lake, dissolved CO_2 concentrations were of the same order of magnitude as for CH_4, whereas at Jakkur Lake dissolved CO_2 concentrations were two orders of magnitude greater than that for CH_4. Concentrations of N_2O were negligible in both lakes. Extrapolating our data to estimate greenhouse gas fluxes, mean daily methane fluxes from Bellandur Lake were consistently in excess of 1000 mg CH_4 m~2/day. We propose that the contribution of severely polluted urban lakes to global CH_4 production warrants further investigation, particularly as our evidence suggests that standard secondary wastewater treatment to support restoration of these systems has the potential to significantly reduce CH_4 emissions.
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