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Mexico City is a large city, populated by 8.8 million inhabitants. This population density, combined with poor wastewater management, results in aquatic ecosystems receiving a large volume of wastewater which may promote methane (CH) emission. We measured water quality and CH emission from 11 aquatic ecosystems in Mexico City during 1 year, including reservoirs, rivers, lakes, canals and chinampas (system of floating garden on shallow lakes). The total CH emission from aquatic ecosystems was estimated as 3679 Mg CH year, which represents 3.5 % of the annual CH emission of Mexico City. The main contributors are chinampas (33 %), followed by lakes (27 %), reservoirs (19 %), rivers (12 %) and canals (9 %). Water quality indicators were positively correlated with CH emission, therefore a decrease in untreated wastewater discharge may result in a significant reduction of the greenhouse gas footprint of Mexico City, after a transitional period during which the organic content of the sediment would be degraded. [ABSTRACT FROM AUTHOR]

The literature concerning methane (CH 4) emissions from temperate and boreal lakes is extensive, but emissions from tropical and subtropical lakes have been less documented. In particular, methane emissions from Mexican lakes, which are often polluted by anthropogenic carbon and nutrient inputs, have not been reported previously. In this work, methane emissions from six Mexican lakes were measured, covering a broad range of organic inputs, trophic states, and climatic conditions. Methane emissions ranged from 5 to 5,000 mg CH 4 m −2 day −1. Water samples from several depths in each lake were analyzed for correlation between water quality indicators and methane emissions. Trophic state and water quality indexes were most strongly correlated with methane fluxes. The global methane flux from Mexican freshwater lakes was estimated to be approximately 1.3 Tg CH 4 year −1, which is about 20% of methane and 4.4% of total national greenhouse gas emissions. Data for untreated wastewater releases to the environment gave an emission factor of 0.19 kg CH 4 kg −1 of Biochemical Oxygen Demand, which is superior to that previously estimated by the IPCC for lake discharges. Thus, the large volume of untreated wastewater in Mexico implies higher methane emission than previously estimated. [ABSTRACT FROM AUTHOR]