[AHW32-06] 熱帯湖沼のメタン栄養食物網
キーワード：メタン、メタン酸化細菌、熱帯湖沼、部分循環湖、メタン栄養食物網
1. Introduction
Freshwater ecosystems are regarded as the primary source of atmospheric CH4 which is 25-times more effective than CO2 as a greenhouse gas. This estimation, however, could be biased because of limited knowledge on limno-physical and ecological processes of CH4 cycling in tropical lakes. In tropical lakes which are meromictic, i.e., there is no mixing of surface and deep waters, CH4 is stored in the deep anoxic waters, whereas its upward advection is hindered due to strong thermal stratification. To date, how much the CH4 storage can contribute to the overall emission from the tropical lakes have been poorly understood for lack of intensive monitoring.
Since methane oxidizing bacteria (MOB) which assimilate dissolved CH4 aerobically or anaerobically embed CH4-derived carbon in consumer biomass through methanotrophic food webs (MFWs), they have great impacts on CH4 cycling in lake ecosystems. Therefore, understanding of controlling mechanisms for the MFWs can also improve our estimation of CH4 flux. Here we conduct monitoring survey for dynamics of CH4, MOB and MFWs in tropical maar lakes of the Philippines.
2. Materials and Methods
We monthly monitored vertical profiles of physico-chemical environments in the Seven Lakes, which is a cluster of 7 volcanic crater lakes called ‘maar lakes’, located on the Luzon Island in the Philippines, from October 2016 to February 2019, using a CTD profiler. For the deepest three lakes (38, 62 and 156 m at depth for Yambo, Pandin and Calibato, respectively), we further measured dissolved CH4 in 6-7 layers from surface to near-the-bottom waters. We also monitored vertical profiles of MOB and MFWs during the mixing season (February 2019), using NGS (Illumina Miseq), CARD-FISH and fatty acid analysis.
3. Results and Discussion
During the most of time, lakes experienced strong thermal stratification, except for the shallowest lake Palakpakin (7.7 m at depth) regarded as polymictic. During Amihan, which is Northeast Monsoon characterized by cool temperature and strong trade wind during months (December-March) shifting from wet to dry seasons, however, complete vertical mixing occurred in some lakes whose depth is 38 m or less. The correlation analysis for monthly data revealed that the thermal stratification was destructed with the decreasing air temperature rather than with the increasing wind speed, suggesting that temperature-driven advection is the primary limno-physical mechanism to cause vertical mixing in the tropical lakes.
Three deepest lakes had high CH4 concentrations in hypolimnion, ranging from 670 to 1172 μmol/L near the bottom at the end of the stratification period, which shows high potential for CH4 storage in the deep tropical lakes. During the Amihan, however, the CH4 storage almost disappeared in Yambo due to the vertical mixing.
In these three lakes, MOB communities were composed mainly of three types, Type I and II, and NC10, while the NGS analysis revealed that a novel taxon, Methylacidiphilaceae, dominated in an epilimnion. Despite the high CH4 concentrations and the presence of MOB, contribution of CH4-derived carbon to zooplankton was low in these lakes, compared to that in non-tropical lakes, suggesting that MFWs do not function as carbon recycling so much in the tropical lake ecosystems.
To quantitatively estimate atmospheric CH4 emission during the mixing event, more intensive monitoring and incubation experiment are needed to measure advection flux of stored CH4 and CH4 oxidation rate for the MOB.
Freshwater ecosystems are regarded as the primary source of atmospheric CH4 which is 25-times more effective than CO2 as a greenhouse gas. This estimation, however, could be biased because of limited knowledge on limno-physical and ecological processes of CH4 cycling in tropical lakes. In tropical lakes which are meromictic, i.e., there is no mixing of surface and deep waters, CH4 is stored in the deep anoxic waters, whereas its upward advection is hindered due to strong thermal stratification. To date, how much the CH4 storage can contribute to the overall emission from the tropical lakes have been poorly understood for lack of intensive monitoring.
Since methane oxidizing bacteria (MOB) which assimilate dissolved CH4 aerobically or anaerobically embed CH4-derived carbon in consumer biomass through methanotrophic food webs (MFWs), they have great impacts on CH4 cycling in lake ecosystems. Therefore, understanding of controlling mechanisms for the MFWs can also improve our estimation of CH4 flux. Here we conduct monitoring survey for dynamics of CH4, MOB and MFWs in tropical maar lakes of the Philippines.
2. Materials and Methods
We monthly monitored vertical profiles of physico-chemical environments in the Seven Lakes, which is a cluster of 7 volcanic crater lakes called ‘maar lakes’, located on the Luzon Island in the Philippines, from October 2016 to February 2019, using a CTD profiler. For the deepest three lakes (38, 62 and 156 m at depth for Yambo, Pandin and Calibato, respectively), we further measured dissolved CH4 in 6-7 layers from surface to near-the-bottom waters. We also monitored vertical profiles of MOB and MFWs during the mixing season (February 2019), using NGS (Illumina Miseq), CARD-FISH and fatty acid analysis.
3. Results and Discussion
During the most of time, lakes experienced strong thermal stratification, except for the shallowest lake Palakpakin (7.7 m at depth) regarded as polymictic. During Amihan, which is Northeast Monsoon characterized by cool temperature and strong trade wind during months (December-March) shifting from wet to dry seasons, however, complete vertical mixing occurred in some lakes whose depth is 38 m or less. The correlation analysis for monthly data revealed that the thermal stratification was destructed with the decreasing air temperature rather than with the increasing wind speed, suggesting that temperature-driven advection is the primary limno-physical mechanism to cause vertical mixing in the tropical lakes.
Three deepest lakes had high CH4 concentrations in hypolimnion, ranging from 670 to 1172 μmol/L near the bottom at the end of the stratification period, which shows high potential for CH4 storage in the deep tropical lakes. During the Amihan, however, the CH4 storage almost disappeared in Yambo due to the vertical mixing.
In these three lakes, MOB communities were composed mainly of three types, Type I and II, and NC10, while the NGS analysis revealed that a novel taxon, Methylacidiphilaceae, dominated in an epilimnion. Despite the high CH4 concentrations and the presence of MOB, contribution of CH4-derived carbon to zooplankton was low in these lakes, compared to that in non-tropical lakes, suggesting that MFWs do not function as carbon recycling so much in the tropical lake ecosystems.
To quantitatively estimate atmospheric CH4 emission during the mixing event, more intensive monitoring and incubation experiment are needed to measure advection flux of stored CH4 and CH4 oxidation rate for the MOB.