Abstract
ABSTRACT The eastern coastal basins of Brazil are a series of small and isolated rivers that drain directly into the Atlantic Ocean. During the Pleistocene, sea-level retreat caused by glaciations exposed the continental shelf, resulting in enlarged paleodrainages that connected rivers that are isolated today. Using Geographic Information System (GIS), we infer the distribution of these paleodrainages, and their properties for the east Brazilian coast. Specifically, using elevation/bathymetric data for the largest sea-level retreats during the Pleistocene, the paleodrainages, their area and the number of contemporary basins connected by each palaeodrainage, was inferred. For the 145 inferred paleodrainages, total paleodrainage area is strongly correlated with the contemporary area encompassed by each paleodrainage, as well as with the number of contemporary basins encompassed by a paleodrainage. Differences in the continental shelf exposure along the coast affected the degree of past connectivity among contemporary rivers. With our results freely available, we discuss how paleodrainages have tremendous utility in biological studies, especially in regions with limited geologic data. With respect to the diverse ichthyofauna of the Brazilian coast, and its high endemism, we highlight how the inferred paleodrainages provide a backdrop to test hypotheses about the effect of past riverine connectivity on diversity patterns.
Highlights
With the steep slope of mountain chains isolating the east coast from the inland (e.g., Serra do Mar and Chapada Diamantina; Fig. 1), the eastern Brazilian coastal basins are a series of small and isolated rivers that drain directly into the Atlantic Ocean
We inferred 145 paleodrainages along the Brazilian coast (Fig. 3), many of which may have provided temporary past connections among 1,034 contemporary river basins that are currently isolated from each other, as well as small basins formed by a single stream
The inferred paleodrainages represent an application of GIS techniques with tremendous utility in biological study, with the potential to inform conservation strategies. These inferences are invaluable to regions where there is a lot of uncertainty about possible past connections among contemporary river basins e180019[11]
Summary
With the steep slope of mountain chains isolating the east coast from the inland (e.g., Serra do Mar and Chapada Diamantina; Fig. 1), the eastern Brazilian coastal basins are a series of small and isolated rivers that drain directly into the Atlantic Ocean. Freshwater fishes in these coastal basins have evolved in isolation with sporadic dispersal with inland basins of Brazil (Weitzman et al, 1988; Ribeiro, 2006). This biogeographic history is hypothesized to underlie the extraordinary diversity of the region, with current estimates of more than 500 valid species of freshwater fishes Comm.; Fricke et al, 2017; Fig. 2), many of which are endemics (95% according to Bizerril, 1994) These estimates span the basins between Laguna dos Patos and Rio São Francisco (i.e., no inland basins), which is currently recognized by six Freshwater Ecoregions: Tramandai-Mampituba (335), Southeastern Mata Atlantica (331), Ribeira de Iguape (330), Fluminense (352), Paraiba do Sul (329) and Northeastern Mata Atlantica The formation of paleodrainages during pronounced sea level changes associated with glacial periods would have provided connections on the exposed continental shelf during sea level retreat (Fleming et al, 1998; Miller et al, 2011)
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