Spatial and Temporal Transitions in the Composition and Transport of Carbon under Variable Flow

Abstract

Recent studies have focused on dissolved organic matter (DOM) cycling throughout river corridors or in reservoirs, but few have explored DOM cycling in commonplace but understudied run-of-river (ROR) reservoir systems. Impoundments disrupt river flow patterns, as they increase hydraulic residence time and alter the flow of DOM downstream. During storms when the majority of DOM loading occurs, impoundments become less likely to hold DOM and will increase export of DOM downstream. In this study, we quantified DOM bioavailability and composition, carbon flux, and carbon dioxide (CO2) gas evasion in a ROR reservoir system at baseflow conditions and during a 1.5-year storm event. This study used a combination of high frequency spatial sensor data geotagged to GPS coordinates along the river to reservoir transition, and grab samples of surface water taken at two U.S. Geological Survey stream gauges and three additional sites. The landscape and shallow flow paths to ROR reservoir systems resulted in the export of both aromatic carbon and labile organic matter present within these waters, as water was mixed and exported downstream. Additionally, the reservoir was a net sink of DOC and BDOC flux, while also a source of DIC flux. Finally, CO2 evasion was magnified by high flow, with the reservoir changing from a sink to a source of CO2 to the atmosphere. ROR reservoirs may undergo "short-circuiting" during high flow, which alters DOM transformations and transport of carbon downstream. Our results provide critical insight on carbon dynamics in ROR reservoir systems and highlight the need to incorporate riverine DOM into carbon budgets, especially under variable flow conditions.