Greenhouse gas emissions from a subtropical cultural-eutrophic lake

During the past decade the global warming has taken a central role in scientific community, either as a growing research topic or as debate node. Recent studies have shown that freshwater environments contribute significantly to the increase of atmospheric CO₂ and CH₄ concentrations; the N₂O emission contribution still unknown. Most of the research focuses on boreal ecosystems but little is known about tropical or subtropical ones. The aim of our research was to evaluate greenhouse gas emissions from a subtropical eutrophicated lake. The model system chosen was a lake located in an urban area with clear pollution evidences (Lake of Guadalupe, State of Mexico, Mexico). CH₄ and N₂O fluxes were quantified through the static chamber technique. Additionally, trophic state and pollution indicators were monitored for two years. The lake, of 16 m maximum depth, was characterized by yearly average temperature of 16.5 ± 1.43 ºC and by anoxic conditions below 2.5 m. Total organic carbon was: 42.73 ± 5.7 mg/l; total nitrogen: 6.6 ± 0.8 mg/l; sulphate: 23.0 ± 0.15 mg/l and total coliforms 239 10³ MPN/100 ml. The trophic state index (TSI) estimated from 3 parameters was 80 and 91 during the rainy and the dry season respectively; this values classify the lake as hypereutrophic. The average CH₄ and N₂O emissions were 124.0 and 3.1 mg/m².d, respectively. These results are discussed with emphasis on the biological process involved. A global carbon and nitrogen mass balance of the lake is also presented.