A tale of pipes and reactors: Controls on the in‐stream dynamics of dissolved organic matter in rivers

Abstract

The potential for rivers to alter the flux of dissolved organic matter (DOM) from land to ocean is widely accepted. Yet anticipating when and where rivers behave as active reactors vs. passive pipes of DOM stands as a major knowledge gap in river biogeochemistry, resulting in uncertainties for global carbon models. Here, we investigate the controls on in‐stream DOM dynamics by evaluating changes in DOM concentration and composition along several reaches of a medium‐sized river network over one full hydrological year. Roughly half of the observations over time and space showed active reactor conditions and, among these, similar proportion of gains and losses was measured. High water residence times promoted the active over passive behavior of the reaches, while DOM properties and nitrate availability determined whether they supplied or removed DOM from the river. Among different DOM fractions, protein‐like DOM both of terrestrial and aquatic origin seemed to drive bulk DOM patterns. Our study emphasizes the role of water residence time as a physical constraint for in‐stream processes, and provides new insights into the key factors governing the net balance between in‐stream gains and losses of DOM in rivers

This research was funded by the Spanish Ministry of Economy and Competitiveness through the projects REMEDIATION (CGL2014-57215-C4-2-R) and FUNSTREAM (CGL2014-58760-C3-3-R). J. P. Casas-Ruiz and Ll. Gómez-Gener were supported by FPI predoctoral grants (BES-2012-059655 and BES-2012-059743), and N. Catal an held a Wenner-Gren foundation stipend (2014–2016; Sweden). This work profited from the research network NETLAKE COST Action (ES1201), supported by COST (European Cooperation in Science and Technology). Authors also acknowledge the support from the Economy and Knowledge Department of the Catalan Government through Consolidated Research Group (2014 SGR 291)— Catalan Institute for Water Research