Spatial and seasonal variation of benthic fish assemblages in whitewater rivers of Central Amazon

Abstract

Despite the high number of fish species described for the Amazon region, the ichthyofauna that inhabits the depths of the main channels of large tropical rivers is one of the least known. In order to know the diversity patterns of these fish in whitewater rivers of the Central Amazon, we used data from the main channel benthic fish assemblage of the Japurá, Purus and Madeira rivers and tested the hypothesis that there are marked spatial and seasonal differences in the composition of the fish community among them. For this, we used a multivariate dispersion test, total β diversity and its decomposition into local (LCBD) and species contribution to β diversity (SCBD). Additionally, we tested for relationships between LCBD values and richness, total abundance, and environmental variables. We categorized species with higher SCBD values into resident or migratory to investigate the potential importance of floodplains to benthic fish assemblage of the main channel of whitewater rivers. Our results corroborate the proposed hypothesis, showing that there are seasonal and inter-river differences in benthic ichthyofauna, being more evident for the Purus River. LCBD showed strong negative relationships with species richness and total abundance, particularly in the Japurá and Madeira rivers in rising season, indicating that rivers and season with high uniqueness in their composition also had low richness and abundance. LCBD was negatively correlated with conductivity and pH, which increased with declining these environmental variables, as observed mainly in Japurá River in both seasons. Approximately one third of the species had higher than average SCBD values and were considered major contributors to β diversity, as well as classified as migratory. This demonstrates the importance of conducting studies that use spatial and seasonal variables, in addition to including the background fish fauna in conservation studies, expanding the protected area and taking into account the different patterns of diversity between rivers. Furthermore, these differences in assemblage composition might be explained by the asymmetrical spatial use of habitats during different seasons, strongly suggesting the importance of the flood-pulse cycle for maintaining diversity in this environment.

Effects of anomalous climatic events on the structure of fish larvae assemblages in the eastern Amazon

“…For example, the associations of slow benthic and fast benthic fish larvae, as well as sedentary ones with the high fluxes in El Niño corroborate previous studies that demonstrate that species of these functional groups require minimum hydrological levels to trigger their reproduction (for example, greater than 5 m) (Cañas and Waylen, 2012;Hermann et al, 2021). This allows the larvae to enter the floodplain areas and explore the spatial patterns and nature of resources essential for successful recruitment, as reported by Duarte et al (2020Duarte et al ( , 2022 and Zacardi and Ponte (2021). Thus, the period and extent of the flooding over the river plain significantly contribute to the generation and maintenance of the biodiversity of the Amazon ichthyofauna (Röpke et al, 2017;Frederico et al, 2021).…”

“…We expect that the response of species to climate change will be influenced by their functional traits. For example, pelagic fish larvae must be associated with different periods than benthic species, because the former may be more sensitive to changes in water masses than the latter, which have more geographically fixed, nearbottom habitat requirements (Duarte et al, 2020(Duarte et al, , 2022Silva et al, 2020). In addition, periodic species, as well as medium-distance migrators, should be more abundant during La Niña, due to favorable conditions for the reproduction of these fish, such as greater amounts of rainfall and river flow (Zacardi et al, 2017b;Humphries et al, 2020).…”