Preconditioning effects of intermittent stream flow on leaf litter decomposition
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Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV–VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change.
KeywordsAnoxic pond Drought Mediterranean Organic carbon dynamics Temporary stream UV radiation
We are very grateful for the major support by the staff of all chemical laboratories involved, and we particularly thank Angela Krüger of the Institute for Freshwater Ecology and Inland Fisheries in Berlin for the analysis of ergosterol. We are grateful for the comments of T. Datry and three reviewers that helped to improve the manuscript. This study was funded by the EU-FP 7 project MIRAGE (FP7-ENV-2007-1, http://www.mirage-project.eu). We also acknowledge funding from the Spanish Ministry of Science and Innovation (Warmtemp project, CGL2008-05618-C02-01/BOS) for the experiments at the Fuirosos site. D. von Schiller was supported by a fellowship of the German Academic Exchange Service (DAAD) and the “laCaixa” Foundation.
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