Methane emissions in two small shallow neotropical lakes: The role of temperature and trophic level

It is estimated that lakes are responsible for up to 16% of global methane (CH4) emissions. Studying the CH4 concentrations and emissions in these environments is important to estimate the total regional production of CH4 and to understand the main factors related to these emissions. The aim of this study was to measure the CH4 emissions from two shallow polymictic lakes in southern Brazil and to evaluate the roles of temperature and trophic level in increasing these emissions. Temperature was positively correlated with CH4 concentration in the water column, bubble emissions and diffusion. Both lakes exhibited significant seasonal differences in water-column and sediment methane concentrations and diffusion. The eutrophic lake produced more bubble emissions [6868.95 (±7645.97) μmol m−2 d−1 in winter and 11,251.10 (±10,160.92) μmol m−2 d−1 in summer]. Water-column and sediment concentrations [19.92 (±11.74) μmol L−1 and 1727.85 (±1581.19) μmol g−1, respectively)] and diffusion [27,549.94 (±18,258.15) μmol m−2 d−1] were also higher in summer than in winter in both lakes. All measured parameters were higher in the eutrophic lake, highlighting the maintenance of aquatic ecosystems in a low trophic state. Based on bubble emissions, the oligo-mesotrophic lake released an estimated 3142.8 g ha−1 yr−1 of CH4, while the eutrophic lake contributed 287,868.6 g ha−1 yr−1. Estimates of diffusive flux were higher: 41,832 g ha−1 yr−1 in summer for the oligo-mesotrophic lake and 1388.52 kg ha−1 yr−1 for the eutrophic lake. Our results show that shallow aquatic subtropical ecosystems are potential sources of atmospheric methane, and their contribution to global warming must be taken into account.