Processing of organic matter in a Mediterranean river network : heterogeneity under water flow variability = Processament de la matèria orgànica en una xarxa fluvial mediterrània : heterogeneïtat sota la variabilitat hidrològica

Abstract

Nowadays, it is recognized that freshwater ecosystems can process a substantial part of the terrestrial organic matter they transport, thereby contributing significantly to global C fluxes. However, the mechanisms behind this processing are still poorly defined. The scant consideration of the environmental heterogeneity commonly found in river networks in ecological studies could be limiting our comprehension of the real capacity of freshwater ecosystems to process organic matter and the range of existing pathways to do so. Mediterranean river networks are ideal settings to explore this knowledge gap because their large variability in flow conditions, both climate- or human-induced, lead to high spatial and temporal environmental heterogeneity. This flow variability results in complex river networks that commonly include temporary streams that cease to flow at some point in space and time, and a notable presence of lentic waterbodies generated by flow regulation. These are features that are also expected to expand to freshwater ecosystems worldwide due to the ongoing global change. Despite their current and predicted widespread occurrence, the processing of organic matter under these conditions is far from fully understood.

This thesis contributes to filling this shortage of knowledge by exploring different mechanisms involved in the processing of terrestrial organic matter along a Mediterranean river network. Specifically, it aims to increase current knowledge on POM decomposition across the mosaic of habitats generated by flow variability and the DOM uptake by bacteria in a forested stream.

The findings of this thesis showed that flow variability induced by both natural and human disturbances generates a complex mosaic of habitats with distinct capacity to transport, retain and process POM. On the one hand, our results indicated that flow regulation by small dams and weirs triggers changes in the ecosystem functioning along the river network, as revealed by differences in wood decomposition rates between lotic and lentic reaches. However, these changes occurred only in high-order streams, where decomposition rates were faster in lotic than in lentic reaches, probably due to more physical abrasion and differences in microbial decomposer assemblages. On the other hand, our results pointed out that the recurrent flow cessation in temporary streams generates discontinuities in leaf litter decomposition along it. Dry and emerged conditions slowed down the decomposition in intermittent reaches compared to perennial ones. However, the

specific moment of flow fragmentation entails a mosaic of aquatic and terrestrial habitats that encompasses a highly heterogeneous decomposition process. In isolated pools, decomposition rates were as faster as in a perennial reach, with an intense leaching and an important contribution of microbial decomposers. In emerged streambed sediments, leaf litter decomposed slower than in aquatic habitats and the decomposition was driven by abiotic factors. In addition, results from this thesis also indicated that the labile forms of DOC are as highly immobilized from the water column as DIN even when large amounts of detritus are available in a forested stream. Indeed, bacteria established on leaf litter had the highest contribution to this immobilization, indicating the influence of autumnal inputs on the cycling of C and N in forested streams.

Overall, this doctoral thesis contributes to a better understanding of how Mediterranean river networks process organic matter and highlights the need to study it taking into account the mosaic of habitats generated by flow variability.