Abstract
Freshwater streams are fascinating habitats that not only host a striking diversity of animals, plants and microbes, but also fulfill a series of complex functions that are ultimately necessary for the sustenance of life as we know it. Streams are tightly interwoven with the surrounding terrestrial matrix, as flows of organisms, material and energy repeatedly cross the boundaries between these two markedly distinct ecosystems. A key resource connecting the terrestrial realm to the aquatic one is leaf litter. Abscised yearly in temperate climates, leaf litter is the food source of a complex community of stream detritivores, which, in turn, decompose this plant detritus ensuring the cycling of the nutrients and carbon therein. In this thesis, I explored the linkages between stream and land by investigating key flows within and across ecosystems that are associated with the cycling of leaf litter. Using a combination of field and laboratory methods, extensive timeframes and spatial replication, I identified distinct temporal signatures in the magnitude and quality of leaf litter flows and in the assemblage of dominant detritivores, freshwater amphipods. Specifically, I first demonstrate the influence of land use in shaping the temporal dynamics of amphipod assemblages. Thanks to multiple field campaigns spanning 8 years and covering 12 small freshwater streams, I found that agricultural landscapes are associated with more pronounced temporal changes in assemblages, which suggests a destabilization effect of anthropogenic activities and highlights the susceptibility of stream communities to terrestrial land use. Second, I used a mesocosm laboratory experiment to illustrate nutrient stoichiometry changes in leaf litter throughout a year of aquatic microbial decomposition, and show how the duration of this decomposition affects the subsequent shredding activity of amphipods. I provide evidence that, in comparison to labile leaf litter, recalcitrant litter can not only store higher absolute amounts of nutrients in the system over longer time periods, but also peak in palatability at a later time. Third, I investigated the seasonal patterns of leaf litter decomposition by conducting 10 consecutive 6-weeks aquatic and 10 consecutive 12-weeks terrestrial decomposition assays at two small freshwater stream catchments. The results validate the marked temperature control on in-stream decomposition and the influence of precipitations on terrestrial decomposition, but they also suggest underlying effects of the temporal changes in the viability of in-situ litter. This draws attention to the need to account for the whole ecosystem phenology and not only abiotic variables in order to understand the cycling of leaf litter within and across ecosystems. Lastly, I provide a temporally resolved and empirically derived overview of key flows that control the intra-annual cycling of leaf litter within and across the stream-forest meta-ecosystem. I show the clear seasonal patterns of plant litter inputs to terrestrial and aquatic systems, the hydrological dynamics linked to precipitation events, and the general stability of the macroinvertebrate community structure, emphasizing the urgency of achieving a more holistic understanding of the temporal interdependency of these processes. My thesis offers an in-depth examination of leaf litter flows and detritivore dynamics across time and uses the knowledge acquired through the individual investigations—i.e. chapters—to build a more comprehensive understanding of the multifaceted stream-terrestrial linkages. In particular, this work sheds light on often-overlooked temporal and phenological trajectories that drive the magnitude and quality of plant litter flows, therefore advocating for a better integration of the temporal dimension in future research. In a time of increasing anthropogenic stressors, land use conversions and climate change, the flows connecting aquatic and terrestrial realms and their temporal synchrony are bound to be transformed, with critical and cascading consequences for the overall functioning of both ecosystems. Therefore, we urgently need a more resolved understanding of the drivers, mechanisms and interactions underlying these flows to recognize and hinder their degradation or at least mitigate the consequences of it.
Cereghetti, Eva