Abstract

Although most studies have being focused on tropical systems, urban lakes in temperate zones are also emerging as important yet understudied sources of dissolved methane (CH₄). The drivers of CH4 dynamics in heavily impacted lakes remain poorly understood, particularly in the Southern Hemisphere. This study investigates seasonal variations in dissolved CH4 concentrations and diffusive fluxes across three urban lakes: Laguna Tres Pascualas, Laguna Lo Mendez, and Laguna Lo Galindo, located in Concepcion, central Chile (36°46'S; 73°03'W) during 2023-2024. All lakes exhibited extreme CH4 oversaturation (saturation ratios, 66-14,949), especially during the dry and early autumn seasons, representing the highest values reported for Chilean freshwater systems. CH4 fluxes varied significantly across lakes and seasons, with peak emissions in Laguna Lo Galindo (up to 26mmolm⁻2 day⁻1) and the lowest in Laguna Tres Pascualas (1.0±1.08mmolm⁻2 day⁻1). Strong seasonal contrasts suggest environmental regulation of CH4 fluxes. CH4 concentrations were positively correlated mainly with turbidity and temperature and were negatively correlated with other environmental variables. Although classic anaerobic methanogenesis in surface sediments likely dominates CH4 production, shallow depths and weak stratification may enhance vertical mixing and surface transport. These findings highlight a complex balance between CH4 production and consumption, even in oxygenated lakes, with emissions linked to eutrophication and potentially amplified by unmanaged urban sewage. The comparison across eutrophic systems allows identification of nutrient thresholds linked to sharp CH4 increases, offering valuable insights for sustainable urban planning and the need for integrated nutrient and wastewater management to mitigate CH4 emissions.