Disentangling dam impacts in river networks.

Damming is one of the most widespread impairments of river systems around the world. The formula-tion of scientifically justified guidelines for restoration and remediation of impairments requires betterunderstanding of the relationships between alteration and stream condition. The present study examinesrelationships between the presence and density of dams and biological metrics of river health in the con-text of a variety of environmental drivers over the Loire river basin. We hypothesized that dam densitymeasured at supra-reach level would show more significant influence on river health than the local leveldensity, and further that the impact of dams is best estimated with measures for the functional traits ofbiotic assemblages. An extensive dataset of fish (169 sites) and invertebrate (211 sites) communities inthe Loire river basin, described with metrics of density of ecological guilds, taxonomic richness and lifehistory traits, and coupled with reach hydromorphology and catchment characteristics was constructed.Generalized linear modeling was performed in order to quantify dam impact and investigate the impor-tance of regional- and local-scale measures of dam density to the structure of biotic communities. Theanalysis showed that community structure at the basin scale responded significant to dam presence andconfirmed that the strongest relationships were observed for specific functional trait-based metrics. Forthe macroinvertebrates the observed impact counts up to 25% of the variance in the trait-based qual-ity indices, whereas for fish communities the dam density only explains up to 12%. Macroinvertebrateresponses were stronger at higher scale level, and especially the upstream context explained on its own70% of the observed impairment. For fish communities, the local context prevails and explained up to70% of the dam impact. These results can be explained by the biotic processes ruling community assem-bly in the specific groups, passive dispersal for the invertebrates and migrations between habitats forfish. The geographic context furthermore explains the differentiation in these responses, reflecting themetacommunity structure of invertebrate assembly over the river basin. We conclude that for upstreamparts of the river basin, locally based management actions can be successful in restoring biotic integrity,whereas more downstream, dam removal actions require more integrated measures at regional ratherthan local scale.