The role of leaf litter quality and streambed hydro-morphology on in-stream leaf litter dynamics = El rol de la qualitat de la fullaraca i de la hidro-morfologia del tram fluvial en les dinàmiques de la fullaraca als rierols

Abstract

Understanding the dynamics of leaf litter inputs in streams involves assessment of retention, spatial distribution and processing of leaf litter inputs. However, leaf litter dynamics in streams has been mostly studied by determining the controlling factors of leaf litter processing, ignoring if leaf litter inputs are retained within the streams, and how these inputs are spatially distributed within the streambed. Furthermore, leaf litter processing in streams is not an isolated process because it can be linked with the dynamics of solutes of these streams. In fact, leachates from leaf litter provide to the streams with dissolved organic matter (DOM), as well as, dissolved nutrients (i.e., nitrogen [N] and phosphorous [P]). Also, the mineralization of leaf litter inputs implies the releasing of dissolved forms of C and nutrients to the stream water column. In addition, microbial communities inhabiting leaf litter uptake dissolved forms from water column during leaf litter decomposition. Despite the obvious linkage between leaf litter processing and in-stream solute dynamics, few studies provided information about the main drivers controlling this connection.

In order to understand how leaf litter dynamics operates in stream ecosystems, the present thesis aims to examine (a) how the hydro-morphological characteristics of the recipient stream can influence the retention, spatial distribution, physical fragmentation and decomposition of leaf litter inputs. Moreover, we also examine (b) the interaction between leaf litter processing and in-stream dynamics of dissolved organic carbon and dissolved inorganic nitrogen and how it varies depending on the leaf litter species considered. To

approach this aim, we specifically examine how the leaf litter quality of different riparian tree species can influence: (b1) the chemical-composition and bioavailability of leaf litter leachates to streams (b2) the uptake of DIN and DOC from water column by microbial decomposers during decomposition process and how it is related to the activity of microbial decomposers and, (b3) how microbially-driven leaf litter decomposition vary among streams which covered a wide gradient of inorganic nutrient concentrations.

The present thesis reveals that heterogeneity of water velocity within a reach is a factor controlling different aspects of leaf litter dynamics in streams because not only influences the fraction of leaf litter retained but also how these inputs are spatially distributed and further processed within the streams. Therefore, we indicate that the relevance of leaf litter inputs as organic matter source to in-stream communities can be subjected to the hydro- morphological characteristics of the receiving stream reaches, since they determine the spatial heterogeneity of water velocity within the reach. In addition, we suggest that leaf litter quality influence on the chemical and structural properties of the leaf litter-leachates and also how these leaches are used by in-stream microbial assemblages. The quality of leaf litter can also dictate the microbial demands of dissolved C and N from the water column. Finally, we suggested that the influence of in-stream nutrient concentrations on leaf litter decomposition can be subjected to the leaf litter quality. Considering all our results together we suggest that riparian composition, which dictates the quality of leaf litter inputs, may control the linkage between leaf litter processing and in-stream C and N solute dynamics. However, the heterogeneity of water velocity may be the most relevant factor determining the importance of leaf litter to streams because it can control the probability that leaves contribute to the stream metabolism or to be exported downstream.

Para comprender cómo funciona la dinámica de la hojarasca en arroyos, la presente tesis pretende examinar (a) cómo las características hidro-morfológicas del lecho del arroyo pueden influir en la retención, distribución espacial, fragmentación física y descomposición de la hojarasca. Además, también examinamos (b) la interacción entre el procesamiento de hojarasca y la dinámica del carbono orgánico disuelto y nitrógeno inorgánico disuelto del arroyo y cómo varía dependiendo de las especies de hojarasca consideradas. La presente tesis revela que la heterogeneidad de la velocidad del agua dentro de un tramo es un factor que controla diferentes aspectos de la dinámica de hojarasca en arroyos, ya que no solo influye en la fracción de hojarasca retenida, sino también en cómo estas entradas se distribuyen espacialmente y se procesan en los arroyos. Por lo tanto, indicamos que la relevancia de las entradas de hojarasca como fuente de materia orgánica para las comunidades del arroyo puede estar sujeta a las características hidro-morfológicas de dicho ecosistema, ya que determinan la heterogeneidad espacial de la velocidad del agua dentro del mismo. Además, sugerimos que la calidad de la hojarasca influye en las propiedades químicas y estructurales de los lixiviados de hojarasca y también cómo estos lixiviados son utilizados por los microorganismos del arroyo. La calidad de la hojarasca también puede dictar las demandas microbianas de C y N disuelto de la columna de agua. Finalmente, sugerimos que la influencia de las concentraciones de nutrientes del arroyo en la descomposición de la hojarasca puede estar sujeta a la calidad de la hojarasca. Teniendo en

cuenta todos nuestros resultados, sugerimos que la composición del bosque de ribera, que determina la calidad de las entradas de hojarasca, puede controlar el vínculo entre el procesamiento de la hojarasca y la dinámica de solutos C y N del arroyo. Sin embargo, la heterogeneidad de la velocidad del agua puede ser el factor más relevante para determinar la importancia de la hojarasca en las corrientes, ya que puede controlar la probabilidad de que las hojas queden retenidas en el arroyo o se exporten aguas abajo, perdiéndose como fuente de materia y energía.