9:00 AM - 9:30 AM
[ACG39-01] Dissolved Methane Dynamics in the Hydrosphere: From Watersheds to Mangroves
★Invited Papers
Keywords:Methane, River, Mass Balance, Land Use, Eutrophication
Atmospheric methane concentrations, a strong greenhouse gas, continuously increase (IPCC AR6). Methane emissions from freshwater (lakes, reservoirs and rivers) account for 25% of global emissions and 50% from the hydrosphere, making freshwater a prominent methane source.
Much research on methane emissions has concentrated on rice paddies, wetlands and lakes, and there is a lack of knowledge on emission fluxes from rivers. The first step is to clarify the levels, spatial patterns and heterogeneity of dissolved methane concentrations in rivers and to elucidate the factors that increase or reduce methane emissions.
This presentation will provide an overview of methane observations conducted across diverse hydrospheric environments, encompassing (1) rivers in the Tone River system and (2) rivers in the Lake Biwa basin.
(1) Observations in the Tone River system
Dynamics of dissolved methane concentration in rivers are explained by a decrease due to methane emission to the atmosphere and methane oxidation and an increase due to methane loading from the inside (mainly riverbed) and outside the river system. To mitigate methane emissions from rivers, it is necessary to reduce both loadings. However, practical solutions require a precise quantification of these loadings, which entails separately assessing internal and external contributions. However, it is difficult to quantify both loadings, and there are limited studies of direct loading estimation. Therefore, we attempted to estimate the methane loading in a river by mass balance. In this study, we estimated the internal loading of methane in a river by using a stream reach with no tributary inflow and almost no flow change. The sampling was conducted in 2022 on the Kokai River, a tributary of the Tone River, Japan. We measured dissolved methane concentration and oxidation rate and calculated the methane emission to the atmosphere.
The net methane loading rate from the riverbed was estimated to be between -50 and 117 µmol m-2 h-1. This value is comparable to the net methane production (methane production – methane oxidation) of 100 µmol m-2 h-1 in the sediment at eutrophic Lake Kasumigaura, Japan, suggesting that the riverbed may have the same rate of internally loaded methane production as the sediment of eutrophic lakes.
(2) Observations in the Lake Biwa basin
Seasonal and inter-river variations of dissolved methane concentrations in 12 rivers were investigated, and the characteristics of spatio-temporal variations and their relationships with land use and water quality were discussed. Surveys were conducted once a month in FY2022.
Dissolved methane concentrations varied between 4.1 nM and 2.4 × 103 nM, with fluctuations of approximately 103 within the rivers.
A two-way ANOVA was conducted to determine whether seasonal or inter-river variability dominated within the study rivers. Significant differences were found only for inter-river variability. The results suggest that the environmental disparities among rivers, such as land use (agricultural fields, forests and urban areas), contribute most to the variation in dissolved methane concentrations at a broader scale.
Partial least squares (PLS) regression of the relationship with water quality suggested that nutrient dynamics, such as chemical oxygen demand, total organic carbon concentration and total phosphorus concentration, explained 30% of the variation in methane concentration. In conjunction with the previously described relationship with land use, additional factors such as eutrophication, often linked to anthropogenic activities within the catchment, are believed to play a role in shaping the dynamics of dissolved methane concentrations.
Furthermore, time permitting, this presentation will also provide findings of the influence of subsoil flow on methane dynamics within river channels and the dynamics of dissolved methane in mangrove ecosystems.
Much research on methane emissions has concentrated on rice paddies, wetlands and lakes, and there is a lack of knowledge on emission fluxes from rivers. The first step is to clarify the levels, spatial patterns and heterogeneity of dissolved methane concentrations in rivers and to elucidate the factors that increase or reduce methane emissions.
This presentation will provide an overview of methane observations conducted across diverse hydrospheric environments, encompassing (1) rivers in the Tone River system and (2) rivers in the Lake Biwa basin.
(1) Observations in the Tone River system
Dynamics of dissolved methane concentration in rivers are explained by a decrease due to methane emission to the atmosphere and methane oxidation and an increase due to methane loading from the inside (mainly riverbed) and outside the river system. To mitigate methane emissions from rivers, it is necessary to reduce both loadings. However, practical solutions require a precise quantification of these loadings, which entails separately assessing internal and external contributions. However, it is difficult to quantify both loadings, and there are limited studies of direct loading estimation. Therefore, we attempted to estimate the methane loading in a river by mass balance. In this study, we estimated the internal loading of methane in a river by using a stream reach with no tributary inflow and almost no flow change. The sampling was conducted in 2022 on the Kokai River, a tributary of the Tone River, Japan. We measured dissolved methane concentration and oxidation rate and calculated the methane emission to the atmosphere.
The net methane loading rate from the riverbed was estimated to be between -50 and 117 µmol m-2 h-1. This value is comparable to the net methane production (methane production – methane oxidation) of 100 µmol m-2 h-1 in the sediment at eutrophic Lake Kasumigaura, Japan, suggesting that the riverbed may have the same rate of internally loaded methane production as the sediment of eutrophic lakes.
(2) Observations in the Lake Biwa basin
Seasonal and inter-river variations of dissolved methane concentrations in 12 rivers were investigated, and the characteristics of spatio-temporal variations and their relationships with land use and water quality were discussed. Surveys were conducted once a month in FY2022.
Dissolved methane concentrations varied between 4.1 nM and 2.4 × 103 nM, with fluctuations of approximately 103 within the rivers.
A two-way ANOVA was conducted to determine whether seasonal or inter-river variability dominated within the study rivers. Significant differences were found only for inter-river variability. The results suggest that the environmental disparities among rivers, such as land use (agricultural fields, forests and urban areas), contribute most to the variation in dissolved methane concentrations at a broader scale.
Partial least squares (PLS) regression of the relationship with water quality suggested that nutrient dynamics, such as chemical oxygen demand, total organic carbon concentration and total phosphorus concentration, explained 30% of the variation in methane concentration. In conjunction with the previously described relationship with land use, additional factors such as eutrophication, often linked to anthropogenic activities within the catchment, are believed to play a role in shaping the dynamics of dissolved methane concentrations.
Furthermore, time permitting, this presentation will also provide findings of the influence of subsoil flow on methane dynamics within river channels and the dynamics of dissolved methane in mangrove ecosystems.