Limnetica 38

Hydrological variability is the key factor that modulates allochthonous inputs and in-river biotic processes and, thereby, the fate of dissolved organic matter (DOM) in rivers. However, little is known about how these factors, combined, change DOM quantity and quality along river courses.

This study explored how DOM quantity (in terms of dissolved organic carbon – DOC) and quality (in terms of optical properties) varied along a Mediterranean river, under contrasting hydrological conditions: drought and high flow. The study was performed in the Matarranya river, a system severely affected by water abstraction for agricultural irrigation and inputs of untreated residual waters.

DOM properties changed according to hydrology. DOM under high flow conditions was more terrigenous, humified, aromatic, and degraded and the concentration gradually increased downriver. In contrast, DOM was less degraded and more aliphatic under drought conditions. DOM spatial variability under drought and high flow conditions revealed that hydrology has a greater impact on DOM quality in headwaters than at downriver sites. Longitudinal changes in DOM were more evident under drought conditions. For instance, a longitudinal depletion of DOC, together with a decrease of the fresh DOM pool, was observed in a large fluvial segment (35 km long) that does not receive any notable anthropogenic inputs. In contrast, the contribution of the most aromatic and humified DOM pool was significantly higher downriver. This study confirms the role of hydrology as a main driver of DOM dynamics. Additionally, it shows that hydrological variability does not impact DOM properties uniformly along the river continuum. On the contrary, DOM properties are more sensitive to hydrological changes in headwaters than in downriver reaches.