Linking terrestrial and aquatic organic matter processing in fluvial ecosystems

Abstract

The decomposition of organic matter is an essential ecosystem process in fluvial ecosystems as it can support heterotrophic food webs and allows the cycling of carbon and nutrients. However, the role of this process in the ecosystem functioning of rivers in arid and semiarid regions is still poorly understood. Arid rivers are highly hydrodynamic systems, characterized by the succession of extreme hydrological fluctuations, ranging from severe drought in summer to floods events during the rainy period. During periods of summer low flow, organic matter remains accumulated on dry riverbeds and floodplain soils, where it is exposed to abiotic and biotic factors which can alter its chemical composition. Then, eventual floods or storm events transport organic matter into the water column, where the aquatic decomposition starts. Therefore, in these fluvial ecosystems, the OM processing is modulated by terrestrial and aquatic interactions lead by flow fluctuations. This dissertation aims to analyse how terrestrial-aquatic interactions control OM processing in fluvial ecosystems, with especial emphasis to arid rivers. The methodological approach used for that included field experiments developed "in situ" in different European floodplains and rivers, experiments developed in outdoor microcosms, or combining laboratory and field experiments. Along this dissertation, we measured organic matter decomposition through mass loss estimations, we analysed the activity of microbial decomposers and detritivores, and also the chemical composition of organic matter. In this regard, it is worth noting the employment of novel techniques for the chemical characterization of dissolved and particulate organic matter such as Fourier-transform ion-cyclotron mass spectrometry (FT-ICR-MS) and Fourier-transform infrared spectrometry (FTIR), respectively. Results of this dissertation highlights that terrestrial phases, or periods of organic matter accumulation on floodplain soils and dry riverbeds, are not static, but active periods of chemical alteration of organic matter. During this period, environmental conditions shape the biodegradability of organic matter by favouring specific abiotic processes, such as photodegradation or rain leaching, or conversely biotic processes like the microbial activity of terrestrial fungi. In the end, changes occurred in OM during the terrestrial phase control its later aquatic decomposition in the river, thus altering energy and nutrient fluxes in the system. Specifically, our results underpin that the exposure of organic matter under hard environmental conditions typical in arid systems, such as intense solar radiation, high temperatures or low soil nutrient content, can exert an overwhelming negative effect on organic matter biodegradability and consequently slows down its processing in the river. Therefore, this dissertation highlights differences in the ecological functioning and organic matter fluxes in arid and temperate rivers. Present findings allow us to advance in the understanding of freshwater ecosystems of arid regions, as well as contributing to the improvement of the environmental management of these systems, giving greater deliberation to the ongoing expansion of arid lands worldwide.