Controlling Factors of Methane in Tropical Lakes of Different Depths

To understand methane (CH 4 ) dynamics in tropical lakes and describe their importance in overall CH 4 emissions, we observed temporal trends of CH 4 stored in the water columns of Philippine lakes of different depths (shallow, moderately deep, and deep). We detected higher CH 4 stored in profundal anoxic layers of deeper lakes. The average surface and profundal CH 4 concentrations throughout our monthly monitoring were 0.27 ± 0.07 and 421 ± 189 μmol L –1 in the shallow lake, 0.48 ± 0.37 and 1,121 ± 125 μmol L –1 in the moderately deep lake, and 0.34 ± 0.23 and 943 ± 119 μmol L –1 in the deep lake. The fate of CH 4 stored in the water column was related to depth, owing to different stratification and mixing regimes. Analyses of temporal relationships of CH 4 with daily weather and physicochemical parameters revealed their impact on CH 4 dynamics by controlling the frequency and mixing intensity throughout the year. Only shallow lake exhibited increase in CH 4 concentrations with lowering atmospheric pressure. This allows vertical mixing and transport of CH 4 from deeper to shallower layers. In moderately deep, and deep lakes, effects of daily weather parameters were not apparent on CH 4 concentrations in the deepest layers (below 30 m depth). This suggests that lake depth and stratification should be considered for precise estimation of tropical lakes' CH 4 emissions. Comparison with lakes under different climate conditions revealed the importance of tropical lakes for surface CH 4 emission due to higher surface CH 4 concentrations. Changes in water temperature is one of the factors controlling profundal CH 4 production, storage, and its transport to shallower layer.