Flowing into the unknown: inferred paleodrainages for studying the ichthyofauna of Brazilian coastal rivers

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.

摘要
巴西东部沿海流域为一系列相互隔离的小型水系，均直接注入大西洋。更新世（Pleistocene）冰期引发的海平面退缩使大陆架（continental shelf）裸露，由此形成了规模扩大的古水系（paleodrainage），连通了现今彼此隔离的河流。
本研究借助地理信息系统（Geographic Information System, GIS）推演了巴西东部沿海区域这些古水系的分布与特征。具体而言，研究利用更新世时期最大规模海退事件对应的高程与水深数据，还原了各古水系的分布、面积以及其所连接的现代流域数量。针对本次推演得到的145处古水系，其总流域面积与单古水系覆盖的现代流域面积，以及单古水系涵盖的现代流域数量均呈显著相关关系。沿海区域大陆架暴露程度的差异，影响了现代水系间历史连通性的强弱。
本研究的结果已公开共享，我们在此探讨古水系在生物学研究中的重要应用价值，尤其是在地质数据匮乏的区域。针对巴西沿海丰富多样且特有性极高的鱼类区系（ichthyofauna），我们着重阐明，本研究推演得到的古水系可作为研究框架，用以检验过往河流连通性对生物多样性格局影响的相关假说。