[EN] The pulsing of river discharge affects biodiversity and productivity of whole river-floodplain ecosystems, triggering the transport, storage and processing of carbon. In this study we investigate the short-term changes in water chemistry and net pelagic metabolism (NEP) in two floodplain lakes in response to a flood pulse. The two oxbow lakes investigated in the floodplain of the Mediterranean Ebro River (NE Spain) showed a clear temporal shift in their metabolic balance, controlled by the river discharge and associated changes in water physical and chemical characteristics. Water chemistry (turbidity, water organic matter, chlorophyll a and nutrients concentration) returned to pre-flood values after 4 days, highlighting the resilience of the ecosystem to flood pulses. Lake NEP was depressed before and during floods to a minimum of -34 mg O 2 m -3 h -1, and increased after the flood pulse to a maximum of +463 mg O 2 m -3 h -1. The phytoplankton assemblage showed before and after floods a replacement of autotrophic species (e. g. Chlorophyceans) by mixotrophic organisms (e. g. Cryptophyceans, Euglenoids). A linear mixed effects model identified abiotic factors, particularly temperature and river discharge, as significant predictors of the net aquatic metabolism and community respiration during flood conditions. Our results suggest that the role of the Ebro floodplain lakes as sources or sinks of C is complex and relative to the time scale investigated, depending strongly on the river discharge dynamics and the transport of limiting nutrients (phosphorus). © 2012 Springer Science+Business Media B.V.

This research was funded by the Spanish Ministry of Science and Innovation (MICINN CGL2008-05153- C02-01/BOS) and supported by Departments of the Environment and Science, Technology and University—Government of Aragon (Research Group E-61 on Ecological Restoration, and B061-2005 pre-doctoral grant). Thanks are also given to the Nature Reserve’s wardens (ANP-GA), as well as M. L. Dehesa, A. Cabezas, A. de Frutos and specially N. Zapata for their field assistance. I am also grateful to two anonymous reviewers and the handling editor Nick Bond whose comments on a previous version highly improved its quality.

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