Hydrologic connectivity with peatland soils drives very high carbon fluxes in a tropical, mountain stream

Inland waters receive large quantities of carbon from the surrounding landscape and are active sites of carbon transport, transform, and emission. Global carbon emission estimates are limited by sparse and unevenly distributed carbon flux observations, particularly in the tropics. We evaluated hydrological and metabolic controls on carbon export variability from a large peatland in a tropical ecosystem typical of the Northern Andes mountains. We recorded dissolved CO2 (pCO2), dissolved oxygen (DO), and discharge continuously at 15-minute intervals 5 m downstream of a peatland outlet (Station 1) and at 3 additional locations downstream (Stations 2, 3 and 4) from July 2019 until Jan 2020 and from June 2021 until March 2023. Continuous measurements of DO and discharge were also measured 2 km away in a stream draining an adjoining catchment (Station 5). Discrete measurements of dissolved organic carbon (DOC) and dissolved methane (pCH4) were collected June-July of 2021 and 2022. Stream discharge was a primary control on pCO2 and DOC in the stream network at both seasonal and event scales. DOC concentration increased with discharge and while pCO2 decreased during higher flows, CO2 loading increased. Pronounced seasonal changes were observed with lowest pCO2 recorded at the peatland outlet in wet months (June-August: 5,845±2,325 ppm, mean±standard deviation), and the highest in dry months (Nov-Feb, 16,677±3,685). Anoxic or hypoxic conditions persisted for over half of our study and measurements of pCH4 at the peatland outlet were very high (982±797 ppm), underscoring the importance of anaerobic activity in this system. Aerobic processes also influenced pCO2 dynamics. Aquatic metabolism at Station 5 (29 July–19 Oct 2021) was net heterotrophic, with ER exceeding GPP and net pCO2 production (mean ER: -6.5 g O2 m-2 d-1, GPP: 0.44 g O m-2 d-1). Our study highlights the role of hydrologic connectivity and diverse biogeochemical processes in shaping carbon export and cycling in páramo streams, which results in pCO2 and pCH4 levels among the highest reported in streams and rivers worldwide.