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Dams modify river physical and chemical attributes. This disruption hampers fish migration, leading to the introduction of non‐native species and losses of specialised native species. Small hydropower plants (SHPs) have been suggested for replacing large dams to minimise reservoir size and preserve natural flow regimes. We evaluated the influences of two SHPs on the ichthyofauna in a tropical river, comparing fish taxonomic and functional beta‐diversity in three periods. Period‐1 preceded SHP construction (Pre‐2008), when the river was free‐flowing. Period‐2 was 1 year after completion of SHP construction (Post‐2012). Period‐3 was after 10 years of SHP operation (Current‐2021). We calculated 10 functional ichthyofaunal attributes based on 12 quantitative morphometric measurements related to habitat use, feeding, and locomotion. The ichthyofaunal composition varied among the periods (p =.001) and seasons (p =.009), with the highest species richness in the Post period compared to the other two periods (p =.002). Abundance significantly decreased over time (p =.004). Taxonomic beta‐diversity increased in the Post period remaining stable in the Current period. Functional beta‐diversity did not change between the Pre and Post periods but significantly increased in the Current period. The turnover component had the greater influence on both taxonomic and functional beta‐diversity, with no temporal changes observed in nestedness. The ichthyofauna appears to have undergone heterogenisation and restructuring. Changes include the emergence of more tolerant species (e.g., Hoplosternum littorale and Pimelodus maculatus), displacement of native and sensitive species (e.g., Pogonopoma parahybae and Steindachneridion parahybae), and colonisation by a non‐native predator (Plagioscion squamosissimus). This indicates that small‐scale projects such as SHPs harm fish populations by altering habitats and restructuring the ichthyofauna. [ABSTRACT FROM AUTHOR]

Functional characteristics of species are of great importance for understanding their roles in ecosystems and can be used to detect long‐term chances in the environment. We evaluated temporal changes (1983–1985 and 2017–2019) in taxonomic and functional indices of the fish fauna in shallow areas of a tropical bay heavily impacted by anthropogenic activities in recent decades. The hypothesis that functional indices change over time as a result of environmental degradation was tested. Our results showed a significant decrease in species richness and abundance over time, and in functional richness, while others functional diversity indices (divergency, evenness, and originality) remained stable. Thirteen functional groups were detected, some of which contained only one species, raising concerns about the loss of ecosystem functions due to ongoing changes. We also observed an increase in beta diversity over time, which may be the result of a decrease in local richness without leading to regional extinctions. Turnover was the most important process in structuring the fish fauna at the evaluated time scale. The relative stability of the functional structure and the higher levels of turnover seem to be related to the dominance of functional groups, within which species replace each other according to their responses to environmental filters that select for specific functional traits. Incorporating functional diversity indices and beta diversity variations in the fish community helped to enhance the existing information about this coastal system by offering improved estimates of biological diversity through diverse approaches. The predominance of turnover identified in the preset study suggests a dynamic and fluctuating species composition within the habitat. In this sense, habitat preservation should prioritize the protection of diverse habitats to accommodate a broad spectrum of species. [ABSTRACT FROM AUTHOR]

Damming of large rivers has promoted well‐documented ecosystem changes that constitute critical threats to freshwater fish biodiversity. However, it is unclear how trait–environment relationships drive community assembly in older (>50 years) tropical river‐reservoir systems. We assessed physical features filtering functional traits (RLQ and fourth‐corner approaches) in seven tropical reservoirs along a major river, in southeastern Brazil, and correlated taxonomic and functional counterparts of local and beta diversity to assess niche‐based assembly processes under prevailing environmental conditions (null model approach). Lower littoral habitat availability (smaller area, volume, and riverine influence) favored species inhabiting open areas that feed on the bottom (mostly migratory species). Downstream position (lower elevation and inflow of water from the mainstem) allowed migrators to enter the reservoirs and promoted environmental heterogeneity, favoring detritivores, invertivores, and carnivorous generalists. Total blocking of the main channel excluded most migratory species from reservoirs at upstream reaches, whereas higher riverine influence and hydrological stability (larger area, volume, and water residence time) favored small‐sized sedentary species (omnivores and piscivores with parental care, mostly non‐native species) in the littoral zone. Correlations differing from null expectations for alpha diversity and nestedness revealed that niche‐based processes support species richness and loss under higher riverine influence and hydrological stability. Our study supports the importance of physical features to set functional spaces available for fish species, but random processes that boost taxonomic differences between reservoirs, largely related to non‐native species introductions, are the primary drivers of fish assemblage structure in tropical reservoirs long‐term disconnected from riverine dynamics. [ABSTRACT FROM AUTHOR]

Resource partitioning is essential for coexistence of phylogenetically related species in altered environments, where changes in biotopes can limit resources and change ecological strategies. We investigated resource partitioning between two congeneric catfishes (Hypostomus affinis and H. luetkeni) in three niche dimensions (trophic, spatial, and temporal) in a Neotropical reservoir. We tested the hypothesis that the use of different mesohabitats (spatial dimension) is more relevant for the coexistence of species, and that morphological differences between the species confer different advantages in the mesohabitats occupation. We evaluated species morphology, diet composition and abundance along a spatial-temporal gradient. A high dietary overlap and a low temporal variability in species abundance were found, indicating that these two dimensions play a minor role in the species distribution. However, a strong spatial segregation modulated apparently by morphological differences was observed. Hypostomus luetkeni has a larger oral disk area, and a flatter body that confer advantages in the occupation of lotics habitats, whereas H. affinis has relatively larger fins that provide more efficient displacements in lentic habitats. Spatial partitioning was the main driver modulating mechanism for the coexistence of these species and differences in the occupation of the mesohabitats probably have been amplified in this altered environment. [ABSTRACT FROM AUTHOR]

The dynamics of fish assemblages in reservoirs depend on factors such as the morphometry of the catchment, habitat structure, and the uses of the reservoir. In the Paraíba do Sul (PSR) basin in southeastern Brazil, there are several types of reservoirs, some on the main channel, some are cascade reservoirs originated from a diversion of the waters of the PSR, and others are isolated reservoirs from tributaries. This study aimed to evaluate the influences of these three types of reservoirs on the taxonomic and functional distinctness of the fish assemblages. It was hypothesized that reservoirs on the main channel (more inflowing waters and habitat availability) have higher taxonomic and functional distinctness compared to cascade reservoirs, which in turn, have greater distinctness than isolated reservoirs. In addition, we expect assemblage structure to differ among these types of reservoirs because of the close relationship with local environmental conditions and habitat structure. Thirty measurements (26 quantitative and 4 categorical) from 34 fish species representing functional traits associated with locomotion, feeding, life strategy, and habitat use were taken. Fish assemblages differed among the three types of reservoirs, which was probably associated with different environmental and local habitat conditions. Higher taxonomic and functional distinctness were found for the isolated reservoirs, and lower for reservoirs on the main channel. This suggests that the fish fauna in this latter type of reservoir were probably composed of phylogenetically close and morphologically similar species. It is likely that limitation of the available resources induces fish to partition the available niches to coexist, favoring assemblages with species adapted to different functions. Our results demonstrate that assessing fish functional and taxonomic distinctness can be used to advance understanding of fish communities from reservoirs in Neotropical regions. [ABSTRACT FROM AUTHOR]