Changing sources and processes sustaining surface CO 2 and CH 4 fluxes along a tropical river to reservoir system

[摘要] Freshwaters are important emitters of carbon dioxide ( CO 2 ) and methane ( CH 4 ), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO 2 and CH 4 surface fluxes. In this study, we quantified sources and sinks of CO 2 and CH 4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO 2 and CH 4 ( >90  % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO 2 and CH 4 are uncoupled and driven by different processes. In the main basin, vertical CO 2 inputs and sediment CH 4 inputs contributed to on average 60  % and 23  % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO 2 and CH 4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scale CO 2 and CH 4 metabolism, which hinder aquatic GHG flux predictions.

[学科分类] 大气科学