Community reassembly after river regulation: rapid loss of fish diversity and the emergence of a new state

Hydroelectric expansion has affected the functioning of most tropical rivers in the world. However, community reassembly in Neotropical impoundments remains poorly understood, because long-term studies are incipient. In this context, we evaluated temporal changes in fish diversity before and after the construction of Peixe Angical Dam, Upper Tocantins River, Amazon Basin. Specifically, we investigated changes in taxonomic (richness, abundance and species composition) and functional diversity (trait richness and composition) over a period of a decade. Fish assemblages were sampled at five sites along the river, between 2004 and 2014 (before and after river regulation). As predicted, taxonomic and trait richness declined in the impoundment, and the composition shifted over the years, with increasing dissimilarity between sites. We also observed significant changes in species abundance and trait composition. However, changes were abrupt and a novel community state emerged after the fifth year of the impoundment. After this period, richness and abundance tended to stabilize at low levels, with different species/trait composition when compared to the pre-damming period. Our results indicate that major changes in community structure occurred during the first years of the impoundment, with relevant losses in taxonomic and functional diversity, along with a rapid change towards a new state.