Patterns and Regulation of Hypolimnetic CO<sub>2</sub> and CH<sub>4</sub> in a Tropical Reservoir Using a Process‐Based Modeling Approach
Cynthia Soued, Yves T. Prairie
Abstract
Abstract Hypolimnetic waters and sediments of lakes and reservoirs are active sites of carbon dioxide (CO 2 ) and methane (CH 4 ) production and accumulation. These potent carbon greenhouse gases (C‐GHG) can be emitted to the atmosphere during water column mixing and, for hydropower reservoirs, through deep water discharge from the outflow. Deep CO 2 and CH 4 concentrations are regulated by sediment‐water flux, aerobic respiration, methanogenesis, CH 4 oxidation, and vertical diffusion. These processes are interlinked but have mostly been studied individually, making their relative importance and interactions difficult to disentangle. In this study, we combined empirical observations with an integrative process‐based modeling to examine ecosystem‐scale dynamics of CO 2 and CH 4 in the deep layer of a permanently stratified reservoir located on the tropical Borneo Island (Malaysia). Oxygen (O 2 ) was a key modulator of deep CO 2 and CH 4 metabolism, and shaped their spatial and temporal patterns within the reservoir. Our model reproduced the shape of the vertical profiles of gas concentration and isotopic signature, and suggested that deep C‐GHG stocks reached a steady‐state about 10 years after impoundment. Vertical gas diffusivity was the most determinant parameter controlling reservoir deep C‐GHG accumulation through its direct effect on gas transport and its interaction with water column metabolism through O 2 regulation. Model results highlighted the interactions, relative importance, and uncertainties of processes controlling deep C‐GHG fluxes. This case study offers an in‐depth comprehension of C‐GHG storage and emissions in a tropical system, as well as a mechanistic framework transferable to other lakes and reservoirs.