Beta-diversidade taxonômica e funcional como ferramentas para avaliação dos efeitos das pequenas centrais hidrelétricas na ictiofauna de um rio tropical

Abstract

Dams construction blocks lotic systems, creating physical barriers that modify the physical- chemical local conditions and alter the flow regime. The new formed environment prevents the

free migration of the ichthyofauna and other aquatic organisms that inhabits the lotic environment to pass through the filters of the lentic environment. This environment, with lentic characteristics facilitates the entry of non-native species, which may lead to the extinction of native species, especially those that are more specialized and more sensitive to new environmental conditions. Damming of tropical rivers has been one of the main threats to aquatic biodiversity, and it has occurred on an unprecedented scale. One way out for medium and small rivers is the construction of Small Run-of-Run Hydroelectric Power Plants (SPPs), as they present reduced flooding areas and do not form large reservoirs. However, the real influence of SHPs has been questioned as to their effects in ichthyofauna compared to large dams. In this context, the present study aimed to evaluate the influences on the ichthyofauna of two PCHs built in the middle section of the Paraíba do Sul River, a lotic system of great importance for the water supply of the state of Rio de Janeiro. Three periods were selected: 1) a period with a totally lotic scenario, before the construction of the two SPPs (2008 - Pre); 2) a period immediately after the construction of the SPPs (2012 - Post); and 3) a period of a decade after the construction of these dams (2021 - Current). Seasonal comparisons were also made among three hydro-environmental conditions: 1) wet, between January and March; 2) dry, between July and August; and 3) transition, between September and October. To understand these changes in ichthyofauna, taxonomic and functional beta-diversity were used, since they assess changes in ichthyofauna and serve to assess both species/function replacement (turnover) and species/function loss (nestedness) in relation to the pool available regionally. For the determination of 10 functional tracts, 12 quantitative morphometrical measures related to habitat use, feeding and locomotion of the species were taken. The ichthyofauna composition varied between periods, although seasonal variations were less conspicuous. Higher richness was recorded in the Post and lower in the Current period. Abundances decrease considerably in the Current period. The taxonomic and functional beta-diversity increased over the three periods, with the taxonomic having increased significantly in the Post period and remained without significant differences in the Current period. On the other hand, the functional beta- diversity was similar between the pre and post period, with a significant increase in the current

period. The replacement component (turnover) was the one that most influenced the results of taxonomic and functional beta-diversity, whereas no significant temporal differences were observed in the loss of species component (nestedness). There was a process of the ichthyofauna heterogenization over the periods, with an increase in taxonomic and functional B-diversity associated with a decrease in richness and abundance. Our study indicates that the ichthyofauna is undergoing a re-structuration process, due to the appearance of tolerant species and the replacement of native and sensible species, which results in function replacement after one decade of operation of the SPPs. This suggests that small-scale enterprises such as SPPs are harmful to the ichthyofauna, due to the changes in habitat, or the appearance of non-native and generalist species. Little is known about the real effects of SPPs on ichthyofauna and our results suggest that it is important to understand these changes, as they can be harmful to the ichthyofauna. It is suggested to expand these studies in order to provide a basis for biodiversity conservation measures in environments affected by these projects.