Sources of dissolved methane in Lake Biwa

Abstract

The spatial distribution and seasonal variation in the concentration and carbon isotopic composition of dissolved methane in a river-lake ecosystem were studied in Lake Biwa, Japan, and its tributary rivers. Methane concentrations in all subsystems examined were supersaturated with respect to the atmosphere. The epilimnion showed higher concentrations of dissolved methane than the hypolimnion in the pelagic zone. Peak methane concentrations were observed at the thermocline. The largest amount of methane in the pelagic water column was recorded at the end of a stagnant period, at which the bottom water of the sublittoral zone (30 m in depth) exhibited increased methane concentration. Transect observation of dissolved methane revealed three methane peaks at different water depths in the lake, and river water and the sediments in littoral and sublittoral zones were suggested to be the corresponding sources. Water at the river mouth was replete with dissolved oxygen but also contained a high concentration of methane. The present results suggest that river water and littoral sediment are potential sources of dissolved methane in Lake Biwa, and other sources, such as internal waves, are responsible for increased methane in the pelagic zone at the end of stagnant periods. Carbon stable isotope analysis indicated that there were different sources of dissolved methane, although it was difficult to identify the origins due to high variation of the isotopic composition of methane from different sources.

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

“…3b). Consistent with this conclusion, observations of enriched surface d 13 C-CH 4 in numerous lakes 8,10,18,29,32 have been hypothesized to indicate dominance of acetoclastic methanogenesis in oxic freshwater pelagic zones 18 , and both known acetoclastic genera (Methanosaeta and Methanosarcina) have recently been detected in oxic freshwater 19,26 and terrestrial 27,28 environments. Furthermore, acetoclastic dominance was enhanced with the addition of nutrients and increased algal production (Fig.…”

“…Accurately placing freshwaters in the global CH 4 budget requires a better understanding of the controls and contributions of CH 4 from different sources [2][3][4][5] . In this regard, the surface waters of lakes and rivers are systematically supersaturated with CH 4 , and it has been traditionally assumed that this CH 4 is derived from anoxic environments, via vertical and lateral transport from profundal and littoral sediments 3,[6][7][8] . This assumption certainly holds for small, shallow ecosystems where the surface layers are in relatively close contact with sediments.…”

“…This assumption certainly holds for small, shallow ecosystems where the surface layers are in relatively close contact with sediments. Yet CH 4 supersaturation is also prevalent in large, deep lakes 3,5,[7][8][9][10][11][12] and oceanic surface waters [13][14][15][16][17] , where deeper water columns result in significantly reduced surface water exchange with anoxic sediments. Studies of CH 4 dynamics in surface waters of oceans and large lakes have indeed concluded that pelagic CH 4 supersaturation cannot be sustained either by lateral inputs from the littoral or from benthic inputs alone 9,10,[13][14][15][17][18][19] .…”

“…At the same time, marine studies suggest that the microbial decomposition of methylated compounds such as methanethiol 15 and methylphosphonate 16,[23][24][25] could be an important source of CH 4 in surface waters of the open ocean. Further, metalimnetic peaks of CH 4 are a recurrent feature in many ecosystems, which correlate positively with dissolved O 2 (DO), algal biomass and production [8][9][10]14,15,18,19 . However, despite past efforts, the ecological and biogeochemical significance of oxic water column methanogenesis remain speculative, and we have yet to determine the dominant biochemical pathway, its controls, and its contribution to diffusive CH 4 emissions from aquatic ecosystems.…”

Carbon gas fluxes from a brown‐water and a clear‐water lake in the boreal zone during a summer with extreme rain events

“…Thus, the ''excess'' carbon gases discharged by Lakes Pääjä rvi and Ormajä rvi in late summer could have entered the lakes either in groundwater or surface-water inputs. The anomalous summer peaks of CH 4 , besides originating possibly from the inflowing streams and sediments of littoral zones, could have entered the lakes also through groundwater inputs (Murase et al 2003). Besides the deep-flowing groundwater, inputs of carbon gases to the lakes could have taken place via shallow throughflow transporting soil-and plantderived gases.…”