Response of community structure to sustained drought in Mediterranean rivers

Mediterranean rivers are characterized by frequent hydrological disturbances, including floods, droughts, and human impacts like water abstraction. During 2008 an exceptionally long drought in NE Spain was irregularly interrupted by rainfall events. We analyzed the relationships between the biological community structure with the ongoing and preceding hydrological patterns during the drought and recovery periods. We aimed: (1) to assess the role of hydrology on different biological assemblages (diatoms, macroinvertebrates and fish), (2) to determine which measures of community structure could be better related to hydrology, and (3) to determine the effects of water abstraction on river communities. Three subsets of variables (hydrological, spatial and environmental) were selected to determine the relative importance of hydrological alterations. Variation partitioning analysis and ordination methods were used to determine which subset of variables had higher effects on 20 parameters and six matrices (taxonomic and functional) of the biological communities. The analyses revealed that both parameters and matrices were significantly related to hydrological effects. Reservoirs intensified the effect of drought on downstream community composition and structure, as a result of the competition between human uses of water and environmental values.