09:30 〜 09:45
[AHW24-03] 温暖化における琵琶湖のメタン栄養食物網の空間パターン
キーワード：食物網、脂肪酸、安定同位体比、底生動物
Introduction
Increasing lake water temperature induced by global warming would strengthen stratification and leads to hypoxia in deep lakes in temperate zone. While methane (CH4), which is a powerful greenhouse gas is produced during the decomposition of organic carbon under anoxic condition, global warming may lead to a positive feedback to lake CH4 production. For lake managements, it is important to predict the response of lake ecosystems to the such environmental changes.
Methane oxidizing bacteria (MOB) are one of the key players of CH4 dynamics in lake ecosystems. Methane oxidizing bacteria utilize CH4 as energy and carbon source under aerobic condition. In turn, assimilated CH4 by MOB is incorporated as basal dietary sources to lake food webs. This methanotrophic food web would associate several environmental variables such as CH4 supply, DO concentration, and water temperature. Therefore, methanotrophic food web can be a useful indicator to assess the influence of climate change to lake ecosystems.
Here, we investigated methanotrophic food web at 15 sampling sites along wide ranges of environmental variables (e.g. CH4 and DO concentration, water temperature, etc.) in Lake Biwa to evaluate the relationship between the development of methanotrophic food web and environmental variables.
Materials and Methods
Benthic animals, bottom water, and sediment cores were collected from 15 offshore sampling sites in Lake Biwa during October, 2021. Water temperature and DO concentration at the lake bottom were obtained by a CTD profiler at each sampling site. Methane concentration in bottom water and several layers of surface sediment (0~1, 2~3, 4~5 cm) were analyzed at a laboratory.
Methanotrophic food web was evaluated by fatty acid biomarker and stable isotope analysis. Methane-oxidizing bacteria are classified into three groups, type I, type II, and NC 10 according to the differences of their physiology. Each group of MOB contains specific fatty acids, which are transferred to higher trophic level animals through food chain. We used 16:1n5+16:1n8, 18:1n8, and Me16:0 as fatty acid biomarkers of type I, type II, and NC 10, respectively.
It is known that stable isotope ratios of carbon and nitrogen of MOB are significantly depleted comparing with those of other dietary sources (i.e. phytoplankton). Therefore, we calculate the dietary contribution of MOB and other sources by two source mixing model using stable isotope ratios.
Results & Discussion
Contents of fatty acid biomarkers (% of total detected fatty acids) in the benthic animals ranged from 0.2 to 9.9%, which indicate dietary contribution of MOB largely varied among the sampling sites. Stable isotope mixing models also showed that dietary contribution of MOB in the benthic animals had large variation ranging from 0 to 32.2%. A significant positive relationship between the MOB fatty acids contents and MOB dietary contribution estimated by stable isotope mixing model in benthic animals was observed (Pearson correlation analysis, r = 0.739, p < 0.01, n = 26).
The effects of the environmental variables on the methanotrophic food webs will be evaluated and discussed.
Increasing lake water temperature induced by global warming would strengthen stratification and leads to hypoxia in deep lakes in temperate zone. While methane (CH4), which is a powerful greenhouse gas is produced during the decomposition of organic carbon under anoxic condition, global warming may lead to a positive feedback to lake CH4 production. For lake managements, it is important to predict the response of lake ecosystems to the such environmental changes.
Methane oxidizing bacteria (MOB) are one of the key players of CH4 dynamics in lake ecosystems. Methane oxidizing bacteria utilize CH4 as energy and carbon source under aerobic condition. In turn, assimilated CH4 by MOB is incorporated as basal dietary sources to lake food webs. This methanotrophic food web would associate several environmental variables such as CH4 supply, DO concentration, and water temperature. Therefore, methanotrophic food web can be a useful indicator to assess the influence of climate change to lake ecosystems.
Here, we investigated methanotrophic food web at 15 sampling sites along wide ranges of environmental variables (e.g. CH4 and DO concentration, water temperature, etc.) in Lake Biwa to evaluate the relationship between the development of methanotrophic food web and environmental variables.
Materials and Methods
Benthic animals, bottom water, and sediment cores were collected from 15 offshore sampling sites in Lake Biwa during October, 2021. Water temperature and DO concentration at the lake bottom were obtained by a CTD profiler at each sampling site. Methane concentration in bottom water and several layers of surface sediment (0~1, 2~3, 4~5 cm) were analyzed at a laboratory.
Methanotrophic food web was evaluated by fatty acid biomarker and stable isotope analysis. Methane-oxidizing bacteria are classified into three groups, type I, type II, and NC 10 according to the differences of their physiology. Each group of MOB contains specific fatty acids, which are transferred to higher trophic level animals through food chain. We used 16:1n5+16:1n8, 18:1n8, and Me16:0 as fatty acid biomarkers of type I, type II, and NC 10, respectively.
It is known that stable isotope ratios of carbon and nitrogen of MOB are significantly depleted comparing with those of other dietary sources (i.e. phytoplankton). Therefore, we calculate the dietary contribution of MOB and other sources by two source mixing model using stable isotope ratios.
Results & Discussion
Contents of fatty acid biomarkers (% of total detected fatty acids) in the benthic animals ranged from 0.2 to 9.9%, which indicate dietary contribution of MOB largely varied among the sampling sites. Stable isotope mixing models also showed that dietary contribution of MOB in the benthic animals had large variation ranging from 0 to 32.2%. A significant positive relationship between the MOB fatty acids contents and MOB dietary contribution estimated by stable isotope mixing model in benthic animals was observed (Pearson correlation analysis, r = 0.739, p < 0.01, n = 26).
The effects of the environmental variables on the methanotrophic food webs will be evaluated and discussed.