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[BBC02-04] Seasonal variations and its controlling factors of dissolved methane concentrations in lakes and ponds from tropical to temperate region
Keywords:methane, lake, production, oxidation, isotope, tropics
Introduction
Lakes are considered one of the important sources of methane emissions to the atmosphere, and estimates of global lake emissions (159 Tg CH4 yr–1) are highly uncertain due to their large spatio-temporal variability (Saunois et al., 2020). Most of the data used for estimation are from temperate and boreal lakes and observations from the tropical lakes, where methane production is expected to be more active under high temperature conditions, are not sufficient. We present the detailed results of methane and environmental factors observed in a tropical volcanic lake (Philippines; Mendoza et al., 2020, JGR), a subtropical dammed lake (Taiwan; Itoh et al., 2015, JGR) in the Southeast Asian region comparing to the results in temperate lake and ponds (Lake Biwa and an agricultural pond in low rainfall area in Japan).
2. Materials and methods
Three volcanic lakes in the Philippines, Yambo (38 m in depth), Pandin (62 m), and Calibato (135 m (14.1° N, 121.4 °E; April 2018-February 2019); a dam lake in Taiwan (Fei-Tsui Reservour: FTR; 114 m in depth; 24.5° N, 121.3° E; October 2012-March 2014) in Taiwan. Mean annual temperature and annual precipitation were 27.7 °C, 1639 mm (2015-18) and 21.5°C, 4052 mm (2004-13), respectively. Lake water samples by depth were collected using a Go-Flo bottle (General Oceanics, USA). Water samples were collected in glass vials (20-30 ml: dissolved methane concentration) and polyethylene bottles (for water quality analysis, etc.). Vials were sealed with rubber stoppers and aluminum seals, and methane concentrations were measured using GC with an FID detector (headspace method). Major dissolved ion concentrations were analyzed using ion chromatography, and vertical distribution of water temperature and dissolved oxygen concentration (DO) were observed at each site.
3. Results
The mean dissolved methane concentrations in volcanic lakes in the Philippines were Yambo (0.27 ± 0.07, 421 ± 189 μmol L–1), Pandin (0.48 ± 0.37, 1121 ± 125 μmol L–1), and Calibato (0.34 ± 0.23, 943 ± 119 μmol L–1) in surface and deep bottom layers with no clear seasonal variation was observed. On the other hand, dissolved methane concentrations in Taiwan FTR were 0.036 ± 0.026 in the surface layer and 135.6 ± 120 μmol L–1 in the deep bottom layer, both more than one order of magnitude lower than those in similar depth lakes in the Philippines were significantly different from those in 2012 (0.11 ± 0.23 in the deep bottom layer) and 2013(84.0 ± 25.4 μmol L–1).
4. Discussion
In the Philippines, the water temperature in the deep bottom layer was 25°C even at 135 m depth in Calibato, which is a favorable environment for methanogens, and high methane accumulation was observed due to constant stratification in the deeper layers. Surface methane concentrations (0-5 m) were generally below 1 μmol L–1. Dissolved methane concentrations showed a gradual increase below the surface in shallowest lake, Yambo. Methane concentrations increased sharply at depths below 20-30 m depth, suggesting the presence of an accumulation layer of highly concentrated methane. In the surface layers of the shallow and deep lakes, a relationship between meteorological (precipitation, atmospheric pressure, and temperature) variations and methane concentrations was observed, suggesting the influence of partial vertical lake mixing in the surface layers. In the Taiwan FTR, the water temperature in the deep bottom layer (100 m) is lower than that in the tropics (about 16°C year-round), and the vertical mixing that occurs during the low-temperature season supplies DO from the surface layer to the deeper layer. This is thought to suppress methane production in the bottom layer, which result in lower bottom methane concentrations than in the tropics. The intensity of vertical mixing in the previous year was also shown to determine the redox condition of the bottom layer in the following year, which in turn affected methane production and accumulation. These results indicate that the weakening of vertical mixing due to a warmer winter (global warming) may increase methane production and accumulation in the deep bottom of subtropical lakes and increase the opportunity to accumulate high concentrations of methane, as in tropical lakes.
Lakes are considered one of the important sources of methane emissions to the atmosphere, and estimates of global lake emissions (159 Tg CH4 yr–1) are highly uncertain due to their large spatio-temporal variability (Saunois et al., 2020). Most of the data used for estimation are from temperate and boreal lakes and observations from the tropical lakes, where methane production is expected to be more active under high temperature conditions, are not sufficient. We present the detailed results of methane and environmental factors observed in a tropical volcanic lake (Philippines; Mendoza et al., 2020, JGR), a subtropical dammed lake (Taiwan; Itoh et al., 2015, JGR) in the Southeast Asian region comparing to the results in temperate lake and ponds (Lake Biwa and an agricultural pond in low rainfall area in Japan).
2. Materials and methods
Three volcanic lakes in the Philippines, Yambo (38 m in depth), Pandin (62 m), and Calibato (135 m (14.1° N, 121.4 °E; April 2018-February 2019); a dam lake in Taiwan (Fei-Tsui Reservour: FTR; 114 m in depth; 24.5° N, 121.3° E; October 2012-March 2014) in Taiwan. Mean annual temperature and annual precipitation were 27.7 °C, 1639 mm (2015-18) and 21.5°C, 4052 mm (2004-13), respectively. Lake water samples by depth were collected using a Go-Flo bottle (General Oceanics, USA). Water samples were collected in glass vials (20-30 ml: dissolved methane concentration) and polyethylene bottles (for water quality analysis, etc.). Vials were sealed with rubber stoppers and aluminum seals, and methane concentrations were measured using GC with an FID detector (headspace method). Major dissolved ion concentrations were analyzed using ion chromatography, and vertical distribution of water temperature and dissolved oxygen concentration (DO) were observed at each site.
3. Results
The mean dissolved methane concentrations in volcanic lakes in the Philippines were Yambo (0.27 ± 0.07, 421 ± 189 μmol L–1), Pandin (0.48 ± 0.37, 1121 ± 125 μmol L–1), and Calibato (0.34 ± 0.23, 943 ± 119 μmol L–1) in surface and deep bottom layers with no clear seasonal variation was observed. On the other hand, dissolved methane concentrations in Taiwan FTR were 0.036 ± 0.026 in the surface layer and 135.6 ± 120 μmol L–1 in the deep bottom layer, both more than one order of magnitude lower than those in similar depth lakes in the Philippines were significantly different from those in 2012 (0.11 ± 0.23 in the deep bottom layer) and 2013(84.0 ± 25.4 μmol L–1).
4. Discussion
In the Philippines, the water temperature in the deep bottom layer was 25°C even at 135 m depth in Calibato, which is a favorable environment for methanogens, and high methane accumulation was observed due to constant stratification in the deeper layers. Surface methane concentrations (0-5 m) were generally below 1 μmol L–1. Dissolved methane concentrations showed a gradual increase below the surface in shallowest lake, Yambo. Methane concentrations increased sharply at depths below 20-30 m depth, suggesting the presence of an accumulation layer of highly concentrated methane. In the surface layers of the shallow and deep lakes, a relationship between meteorological (precipitation, atmospheric pressure, and temperature) variations and methane concentrations was observed, suggesting the influence of partial vertical lake mixing in the surface layers. In the Taiwan FTR, the water temperature in the deep bottom layer (100 m) is lower than that in the tropics (about 16°C year-round), and the vertical mixing that occurs during the low-temperature season supplies DO from the surface layer to the deeper layer. This is thought to suppress methane production in the bottom layer, which result in lower bottom methane concentrations than in the tropics. The intensity of vertical mixing in the previous year was also shown to determine the redox condition of the bottom layer in the following year, which in turn affected methane production and accumulation. These results indicate that the weakening of vertical mixing due to a warmer winter (global warming) may increase methane production and accumulation in the deep bottom of subtropical lakes and increase the opportunity to accumulate high concentrations of methane, as in tropical lakes.