Seasonal Variation in Carbon Dynamics in the Lower Tocantins River: Influence of Discharge and Environmental Drivers

ABSTRACT Tropical rivers play a key role in regional carbon budgets, yet greenhouse gas (GHG) dynamics in regulated systems remain poorly understood. This study investigates the seasonal variability of carbon dioxide (CO 2 ) and methane (CH 4 ) concentrations, isotopic composition (δ 13 C), and river‐atmosphere fluxes in the lower Tocantins River, a clearwater system affected by flow regulation and land‐use change. Over a three‐year period (2014–2016), monthly biogeochemical measurements were conducted along with hydrological monitoring. CO 2 concentrations and fluxes peaked during the high‐water season and were positively correlated with discharge, dissolved oxygen, pH, suspended sediments, and dissolved organic carbon. In contrast, CH 4 fluxes were significantly higher during the low‐water season and associated with reduced river discharge, wind speed, and fine suspended sediments. Isotopic analysis of δ 13 CO 2 suggested shifts in carbon sources across seasons, with respired terrestrial inputs dominating contributions to river p CO 2 during high‐water. These findings highlight the influence of seasonal hydrology on GHG emissions, even though discharge regulation by upstream dams dampens flood pulse dynamics and reduces lateral connectivity with floodplains. Given the increasing regulation and land conversion in Amazonian basins, understanding these processes is essential to guide sustainable river management and improve regional GHG inventories.