Variations of dissolved methane in mountainous river networks and its estuaries in Taiwan

Methane (CH₄), the second most important greenhouse gases (GHGs) in the atmosphere, has a global warming potential in a 20-year time frame (GWP₂₀) 86 times than carbon dioxide (CO₂). Rivers and streams are one of the most important natural sources of atmospheric CH₄, which is greater than half of the entire efflux from freshwaters as a whole. Riverine CH₄ investigation is pivotal in terms of scientific research and national development (e.g. GHGs mitigation). However, a comprehensive riverine data collection is often hindered by national boundaries. In addition, the variations of dissolved CH₄ in river networks may change with geographical distribution. Taiwan, a tropical/subtropical mountainous island, is located in the northwest Pacific Ocean which possesses wide range of geographical features. The geographically high diversity within relatively short distance in sole regime, makes Taiwan one if not only ideal place to study the variation of dissolved CH₄ in different geographical distribution. As the high mountains cover around 70% of the area in Taiwan, more than 80% of the population (> 18.4 million people) is dwelling in the flat plain or estuaries. This results in high population density, especially in the urban area, which can lead to sever anthropogenic influence in the water system. In order to understand the distribution of CH₄ in the mountainous river networks, we have conducted 10 sampling campaigns from 2003 to 2016 and taken samples from an elevation of about 3300 to 0 m. At the altitude greater than 500 m, the CH₄ concentrations have not varied much spatially, however, seasonal variations were revealed. On the other hand, at the altitude less than 500 m, the CH₄ concentrations have varied seasonally and spatially due to natural variations and anthropogenic influence. In the wet season (from May to October), the average riverine CH₄ concentration was 23.8±13.2 nM at an altitude greater than 500 m while it was 843.6±1739.1 nM at an altitude less than 500 m. In the dry season (from November to April), the average riverine CH₄ concentration was 9.4±17.6 nM at an altitude greater than 500 m while it was 2828.6±9296.8 nM at an altitude less than 500 m. This result indicates that the anthropogenic influence is more prominent than natural change so the spatial variations are more significant than seasonal differences.