Quality and reactivity of dissolved organic matter in a Mediterranean river across hydrological and spatial gradients

Understanding DOM transport and reactivity in rivers is essential to having a complete picture of the global carbon cycle. In this study,we explore the effects of hydrological variability and downstreamtransport on dissolved organic matter (DOM) dynamics in a Mediterranean river. We sampled the main stem of the river Tordera from the source to the sea, over a range of fifteen hydrological conditions including extreme events (flood and drought). By exploring spatial and temporal gradients of DOM fluorescence properties, river hydrology was found to be a significant predictor of DOMspatial heterogeneity. An additional space-resolved mass balance analysis performed on four contrasting hydrological conditions revealed that this was due to a shift in the biogeochemical function of the river. Flood conditions caused a conservative transport of DOM, generating a homogeneous, humic-like spatial profile ofDOMquality. Lower flows induced a non-conservative, reactive transport ofDOM,which enhanced the spatial heterogeneity of DOMproperties.Moreover, the downstreamevolution of DOMchemostatic behaviour revealed that the role of hydrology in regulating DOMproperties increased gradually downstream, indicating an organised inter-dependency between the spatial and the temporal dimensions. Overall, our findings reveal that riverine DOMdynamics is in constant change owing to varying hydrological conditions, and emphasize that in order to fully understand the role of rivers in the global carbon cycle, it is necessary to take into account the full range of hydrological variability, from floods to droughts.