Urban water bodies are vital for ecological stability yet remain vulnerable to anthropogenic degradation. This study examines the ecological health and greenhouse gas (GHG) dynamics of Hauz Khas Lake (New Delhi, India), a warm polymictic system that has been rejuvenated with treated Sewage effluent. While this intervention preserved the lake's physical volume, it has inadvertently driven severe eutrophication. We assessed water quality using the Water Quality Index (WQI) and Trophic State Index (TSI), quantifying temporal changes over eight years, and linked these states to methane (CH₄) fluxes measured via floating chambers.
Results indicate the lake is critically stressed by elevated nutrient inputs (NO₃-N: 1.9–3.13 mg/L; PO₄³⁻: 4–8 mg/L), manifesting in a hyper-eutrophic TSI of 191.7 ± 10.7. Principal Component Analysis (PCA) reveals intensifying interactions among water quality parameters, with explained variance increasing significantly, signalling stabilizing feedback loops of degradation. Consequently, the lake has become a major GHG hotspot. We recorded a diffusive CH₄ flux of 60 µmol m⁻² s⁻¹, significantly higher than the 36 µmol m⁻² s⁻¹ observed in a reference freshwater lake. This disparity stems from the urban lake’s shallow depth and high organic load, which overwhelm the water column's oxidative capacity, allowing benthic methane to escape unoxidized. The study concludes that the current rejuvenation model has created a "methane factory," affecting conservation gains. We recommend integrating algal biomass harvesting for energy recovery (Anaerobic Digestion) as a remediation strategy to mitigate emissions while generating renewable energy.