Resumen de Sources, transformations and controls of dissolved organic matter (DOM) in a Mediterranean catchment = Fonts, transformacions i controls de la matèria orgànica dissolta (DOM) a una conca Mediterrània
Dissolved organic matter (DOM) is a primary source of organic carbon in aquatic ecosystems. Inland waters actively transform DOM, having a relevant effect on the regional and global carbon budgets. DOM quantity and quality determine the efficiency of any transformation pathway and are intensely defined by the landscape, as it controls both the DOM origin and the transformations processes acting over it. This thesis aims to unravel the sources, controls and transformations of DOM in a Mediteranean catchment using both laboratory experimental designs to address the effects of degradation processes, and field monitoring approaches to study the natural DOM variability. The Mediterranean region presents a marked hydrological seasonality characterized by intermittent surface water flows like the ephemeral washes of the studied catchment. The receiving water body is, as most natural lentic waters in the Mediterranean region, a shallow system dominated by submerged vegetation. The characteristics of the study catchment allow assessing the interplay between the landscape properties, contrasted DOM sources and different mechanisms of control over DOM quality and quantity. The quality of the DOM present in ephemeral washes is influenced by landscape factors, and this influence varies in a seasonal basis. Two seasonal periods are distinguished in terms of DOM quality: autumn, showing an aromatic character related with hydromorphological variables, and winter-spring, with a microbial-like DOM derived from in-situ processes and linked to specific soil types and land uses. The effect of the biodegradation and the combined effect of photo- and biodegradation processes is stronger on allochthonous than on autochthonous DOM sources. These differences are related to distinct previous exposure to degradation pathways and to specific composition of each source. These results are in agreement with an increasing number of works questioning the classical paradigm linking allochthonous DOM with unreactive and autochthonous DOM with labile materials. The instantaneous rates of change in DOM properties showed that qualitative changes during degradation cannot be assumed to follow a regular decay pattern. The addition of small inputs of labile carbon to lake water does not enhance the decomposition rate of the DOM present in the receiving lake water, and thus no evidences of priming effect are found. Neither nutrient availability nor the available surface for cell attachment have any significant effect on the occurrence of priming effect. The fingerprint of different sources and processing mechanisms are reflected in the variability of the DOM in the studied lagoon. The phenological cycle of submerged vegetation and the pulses of torrential water inputs from the ephemeral washes are the main drivers of DOM quality in the lagoon.
