Abstract
The Amazon River Basin, one of the world’s most threatened ecosystems, has an enormous diversity of fish species, a result of temporally and spatially complex habitat containing biogeochemically different river systems. The annual hydrologic cycle results in floodplain lakes during low water and inundates forests during high water, exposing fish to different resources and environmental conditions. The two principal river systems in the central Brazilian Amazon are blackwater, with nutrient-poor acidic water, and nutrient-rich whitewater. Although species-rich, the Amazon Basin is data-poor in terms of comparative studies on a regional scale. We analyzed data sets from independent sampling studies of pelagic fish in 16 floodplain lakes, nine whitewater (Rio Solimões) and seven blackwater (Rio Negro), in the central Amazon Basin of Brazil. Our findings suggest striking similarities in pelagic fish diversity patterns. Species richness was virtually equal (165 in whitewater and 168 in blackwater). Both species richness, and number of migratory species, per lake increased toward the confluence of the rivers in both systems in our study. The proportion of unique species was also similar in whitewater lakes and blackwater (41 and 43%, respectively), boosting total regional richness to 237 species. However, species composition in whitewater lakes was more homogenous (lower β diversity), and species composition was associated with conductivity and pH in whitewater, but with dissolved oxygen and transparency in blackwater. Therefore, regional fish diversity cannot be represented by sampling one lake or even one drainage system, but must include multiple lakes from both systems. These two systems may differ in sensitivity to anthropogenic stressors such as damming and deforestation.
Highlights
The Amazon River Basin is the world’s largest freshwater ecosystem, with an aquatic surface area of 93,000 km2 (Allen and Pavelsky, 2018)
Rarefaction curves were asymptotic, showing that the sampling effort was adequate to represent the bulk of pelagic fish species richness (Appendix 3)
Fish community composition (Appendix 4) revealed that pelagic species richness was nearly identical between blackwater lakes and whitewater lakes (S = 161) there were substantial differences in species composition between the systems
Summary
The Amazon River Basin is the world’s largest freshwater ecosystem, with an aquatic surface area of 93,000 km (Allen and Pavelsky, 2018). At least 2,716 fish species, including 1,696 of which are endemic to the basin, have been identified so far (Reis et al, 2016; Dagosta and Pinna, 2019) It is one of the earth’s most endangered ecosystems, largely from ongoing anthropogenic stressors to its hydrology and biodiversity such as dam building that reduces connectivity among habitats (Kahn et al, 2014; Hurd et al, 2016; Santos et al, 2018), oil and gas development (Finer et al, 2008), overfishing of both food species and ornamental fish (Campos et al, 2015; Costa-Pereira and Galetti, 2017; Castanho et al, 2019), invasive alien species (Latini and Petrere, 2004), deforestation (Lobón-Cerviá et al, 2015), urbanization (Tregidgo et al, 2017), and droughts caused by deforestation and global warming (Castello et al, 2013; Freitas et al, 2013; Nazareno and Laurance, 2015). Our aim is to discern how pelagic fish species diversity, which includes many species harvested for human consumption, is distributed between physically different drainage systems that flow through the central portion of the Amazon River Basin in Brazil
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