Roles of planktonic metabolism in CO 2 fluxes along the river-estuary continuum in a rapidly uplifting catchment of eastern Taiwan

The contribution of river metabolism to carbon cycling remains poorly examined in catchments susceptible to the modulation of active tectonics. In these high-energy mountainous systems, chronically high turbidity and frequent sediment mobility create an unstable fluid regime that may impact planktonic metabolism and its contribution to aquatic carbon transformation. Using the Beinan River in southeastern Taiwan as a model system, this study aimed to quantify the potential rates of planktonic autotrophy and heterotrophy, and to identify the potential community members involved in these processes. To address this, river water samples were collected across wet and dry seasons for incubations with 13 C-labeled dissolved inorganic carbon and amino acids. Our results revealed that metabolic rates were consistently higher in the wet season than in the dry season, with heterotrophic catabolic rates exceeding autotrophic carbon fixation by one to two orders of magnitude, indicating strong net heterotrophy. The obtained rates were further scaled up, resulting in a catchment-scale CO 2 evasion of 7.89×10 7 mol yr −1 , which constitutes about 3.8 % of the CO 2 flux derived from pyrite-induced carbonate weathering, oxidation of petrogenic carbon, and the river-air exchange. The microbial community structures varied seasonally at upstream sites, with more abundant lithoautotrophic sulfur- and ammonia/nitrite-oxidizing taxa in the wet season, as opposed to more abundant phototrophs or heterotrophs in the dry season. This study highlights that even in oligotrophic, physically dynamic mountainous systems prone to rapid uplift and erosion, planktonic metabolisms constitute a measurable and vital component of the riverine carbon cycle.