Shifts in fish community composition and structure linked to seasonality in a tropical river

Abstract

Abstract The seasonality of tropical rivers, induced mainly by water level changes, shapes many interrelated aspects of ecological communities and the populations they contain, including animal movement, feeding, growth, and reproductive activity. However, the role played by seasonality in structuring the diversity of tropical assemblages is not yet fully understood. We examined the effects of seasonality on community structure and composition of benthic fish assemblages comparing two consecutive receding and rising water seasons in a major tributary of the Amazon basin. We quantified seasonal shifts in species abundance distributions and in composition using 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. Many benthic fish species were rare in terms of numerical abundance. Rarity was most pronounced in the rising seasons, which had a higher proportion of singletons. A logseries was the best‐fit model for both the receding and one of the rising season's species abundance distributions, while a lognormal was selected for the second rising season. We detected variation in species composition between seasons—the rising seasons were distinct from one another in terms of species composition, as well as differing from the receding seasons. LCBD showed strong negative relationships with species richness and total abundance, particularly in the rising seasons, indicating that seasons with high uniqueness in their composition also had low richness and abundance. LCBD was negatively correlated with temperature, while depth presented a positive relationship, as observed mainly in rising seasons with colder temperatures and greater water depth. Approximately one third of the species had higher than average SCBD values and were considered major contributors to β diversity. These significant seasonal differences in both species relative abundances and 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. Studies that seek to identify the species inhabiting a particular environment (e.g. bottom rivers habitats) and the factors that affect the dynamics, structure and composition of these communities are fundamental for future management and conservation. This is particularly urgent for understudied tropical freshwater ecosystems, as human pressures including anthropogenic climate change are expected to become increasingly severe. Rising temperatures and changing precipitation patterns modify water temperature and flow regimes, thus affecting the hydrologic regime that determines the structure and dynamics of the ecological communities, with potential consequences for their integrity.