Delineating the Continuum of Dissolved Organic Matter in Temperate River Networks

The origin and reactivity of dissolved organic matter (DOM) have received attention for decades due to the key role DOM plays in global carbon cycling and the ecology of aquatic systems. However, DOM dynamics in river networks remain unresolved, hampered by the lack of data integrating the spatial and temporal dimensions inherent to riverine ecosystems. Here we examine the longitudinal patterns of dissolved organic carbon (DOC) concentration and DOM chemical diversity along a temperate river network under different hydrological conditions, encompassing small headwater streams to the river mouth and base flow to storm events. We show that, during nonstorm conditions, the concentration of DOC and the chemical diversity of DOM exhibit their maxima in the middle section of the network, depicting a bell-shaped pattern along the river continuum. In contrast, DOM shows a homogeneous longitudinal pattern during storm events, with highly concentrated and diverse DOM along the river network. We posit that these emerging patterns result from changes in the relative influence of catchment versus in-stream biogeochemical processes along the river continuum and that the degree of influence is modulated by river network hydrology. Based on these findings we put forward the "Bending DOM Concept," a new conceptual framework around which testable hypotheses on the spatiotemporal dynamics of DOM and the functioning of temperate river networks may be formulated.