Flow regulation by dams affects ecosystem metabolism in Mediterranean rivers

Summary Large dams regulate river hydrology and influence water chemistry, sediment dynamics, channel form and biotic communities. These effects may translate into important changes in river ecosystem processes, especially in rivers naturally subject to strong seasonality, such as those under Mediterranean climate. The effects of flow regulation on ecosystem metabolism [i.e. gross primary production (GPP) and ecosystem respiration (ER)] were analysed by means of open‐stream measurements. Organic matter accrual and metabolism were measured in reaches upstream and downstream from large reservoirs in three tributaries of the Ebro River (NE Iberian Peninsula) during three sampling campaigns. Dams reduced downstream hydrological variability, dampened floods and increased the duration of interflood periods. Benthic organic matter increased twofold and chlorophyll‐a eightfold. GPP increased by 59% on average, whereas ER increased by 75%. In general, flow regulation intensified the capacity of downstream river reaches to store and process materials and energy, therefore increasing the amount of organic carbon processed and altering the whole flux of materials and energy along the river continuum.