Microbial role in CO₂ fluxes along the river-estuary continuum in a rapidly uplifting catchment of eastern Taiwan

Abstract. The contribution of river metabolisms to carbon cycling is an essential issue, but not well examined in the catchment susceptible to the modulation of active tectonics. This study aims to quantify the rates of autotrophy and heterotrophy, and to identify the community compositions and potential members involved in these microbial processes in the Beinan River in eastern Taiwan. To address this, river water samples were collected in both the wet and dry seasons for incubations amended with ¹³C-labeled dissolved carbon dioxide and amino acids. The analyses revealed a general pattern pointing to the higher rates in the wet season than in the dry season, and for heterotrophy than for autotrophy. The obtained rates were further scaled up, resulting in the catchment-scale CO₂ evasion of ~ 10⁷ mole yr^-1, a range constituting several percent of the CO₂ flux derived from pyrite-induced weathering, oxidation of petrogenic carbon, and the river-air exchange. The community compositions generally varied with season for most upstream sites and with more abundant sulfur or nitrogen metabolizers in the wet season, as opposed to more abundant phototrophs or heterotrophs in the dry season. This study highlights the complex and dynamic nature of river metabolisms that contribute to carbon evasion in oligotrophic mountainous systems prone to the impacts of rapid uplift and erosion.