Aim: We aimed to assess the relative influence of past (Quaternary paleodrainage characteristics) and current factors on the beta diversity of freshwater fishes in coastal lagoons and explore the main processes involved.

Location: Atlantic coast of South America. Taxon: Fishes (173 species)

Methods: We built a dataset of species occurrence in 129 lagoons across eight freshwater ecoregions of the world (FEOWs) located between latitudes 0° and 36° and calculated beta diversity (βjac) and its turnover (βjtu) and nestedness (βjne) components. We used a partial Mantel test and multiple regressions on distance matrices to evaluate the importance of past and current factors, and of geographical distance in determining beta diversity. Past variables were those representing the historical freshwater habitat during the last glacial maximum (LGM), and contemporary variables were those related to current habitat.

Results: We found high values of βjac within the FEOWs, with βjtu prevailing over βjne. Both past (paleodrainage) and current (drainage area, salinity, and lagoon area) factors affected species dissimilarity (βjac = 46%) and its components (βjtu = 44% and βjne =20%), although explanation was, in part, shared with geographical distance. Individually, the influence of past factors was prevalent in beta diversity and its components.

Main Conclusions: The results suggest that major changes in the availability of freshwater habitats and connectivity since the Pleistocene must have affected the colonization, extinction and recolonization processes of fishes along the eastern coast of South America. We suggest that the high beta diversity values may result from limited dispersal after extinctions in the LGM and that the dissimilar freshwater fish communities currently seen were formed mainly by heterogeneous subsets of the regional species pool that persisted in landscape refuges during past sea level increases and then recolonized coastal lagoons.

We compiled fish species composition data for 129 lagoons along the Atlantic coast of South America between latitudes 0° and 36° S, which encompass eight freshwater ecoregions of the world (FEOW). The fish data consisted of a presence/absence matrix of species by lagoon. Most data were compiled by Petry et al. (2016), but we added new data by searching fish collection records in the SpeciesLink database (http://splink.cria.org.br/) and in primary research papers. A taxonomic and nomenclatural verification was performed across the dataset to validate all the species records. We included only lagoons that were subjected to specific community sampling protocols. Exclusively estuarine species or occasional marine species were not included, since we were interested in biogeographical processes related to freshwater species only.