A freshwater wall in the Atlantic

The central role of the Amazon River in the evolution of Western Atlantic reef fishes.

Above: Amazon River mouth, where the plume of freshwater and sediment reaches the Atlantic Ocean. Photo by Coordenação-Geral de Observação da Terra/INPE.

The magnitude of the Amazon River, by far the largest river of the world, can be illustrated by its mean annual water discharge – 2.5 times greater than the combined discharges of the world’s second and third largest rivers (Congo and Orinoco rivers, respectively). Actually, the 300 million liters released by the Amazon River every second in the northern portion of South America represent about 20% of the freshwater input of all rivers of the world into the ocean. The amount of freshwater and sediments is so impressive that the physicochemical properties of a huge area of the Atlantic are modified, forming the most important biogeographic filter responsible for the faunal discontinuity between the Caribbean and Brazilian Provinces – the Amazon-Orinoco plume, which is an extensive layer of fresh muddy water about 30 meters deep that extends for about 200 km into the sea.

Editors’ choice article: (Free to read online for two years.)
Araujo, G. S., Rocha, L. A., Lastrucci, N. S., Luiz, O. J., Di Dario, F., & Floeter, S. R. (2022). The Amazon-Orinoco Barrier as a driver of reef-fish speciation in the Western Atlantic through time. Journal of Biogeography, 49, 1407-1419. https://doi.org/10.1111/jbi.14398 

The effectiveness of the Amazon-Orinoco plume as a biogeographic barrier varied over time according to sea-level height: during high sea-level stands, like in modern times, the less dense freshwater plume floats high above salt water on the continental shelf, allowing marine organisms to use the relatively extensive “normal” marine layer below the plume as a corridor between the Caribbean and Brazilian provinces. However, during low sea-level periods in geological time, the continental shelf is shallower, and as a consequence this corridor is practically closed by the huge freshwater plume. It has been proposed that adult body size of fishes is important for the ability to cross the Amazon-Orinoco barrier today. However, the Amazon River has a very long and complex history, and just about 9.4 million years ago the amount of freshwater and sediment discharge in the Atlantic was much lower (about 24 times) than what we see today.

With that in mind, the question we addressed in this paper is: considering the evolution of the Amazon River itself and the dramatic increase in freshwater and sediment discharge throughout its history, what is the influence of the Amazon-Orinoco Barrier on the diversification of the Western Atlantic reef fish fauna over time?

Our results revealed a complex and dynamic barrier with temporally variable permeability. Almost all the cladogenetic events addressed in our meta-analysis that resulted in sister species or populations of reef fishes in the Caribbean and Brazilian Provinces can be associated with the emergence and development of the Amazon-Orinoco barrier. This panorama suggests that the Amazon River was likely the main promoter of diversification between the reef fish fauna of the Caribbean and Brazil. In other words, the main possible driver of speciation of marine species in the Western Atlantic was a river.

The results indicate that the maximum total length of ancestral populations of reef fishes was also a decisive feature in the ability to overcome the Amazon-Orinoco Barrier in the past, as it is nowadays. We observed a strong correlation between body size and dates of cladogenesis, with smaller body sizes primarily associated with older cladogenetic events. Older speciation events appear to be probably more associated with the onset and early development of the plume itself rather than with fluctuations in sea level, since only milder variations of sea level occurred between the Middle Miocene and early Pleistocene.

Examples of sister fish species found in Caribbean reefs (left bar) and Brazilian (right bar): first line: saddled blenny Malacoctenus triangulatus and rusty scaly blenny Malacoctenus zaluari, second: rainbow parrotfish Scarus guacamaia and greenbeak parrotfish Scarus trispinosus, third: redfin parrotfish Sparisoma rubripinne and gray parrotfish Sparisoma axillare. Photos by F. Krasovec, J.P. Krajewski, J. Lyle, C. Sampaio, and S.R. Floeter.

In the last 2.4 million years, the plume has become a more formidable barrier. More than half of the cladogenetic events detected in our study occurred only in this time-period, which is characterized by higher sedimentation rates and greater sea-level fluctuations. Of the 31 cladogenetic events recorded in the last 2.4 million years, 13 involved species larger than 50 cm, in contrast to only two events with species of similar sizes in the previous 7 million years. It seems therefore that these two factors together, higher sedimentation rates and greater sea-level fluctuations, fostered the isolation capacity of the Amazon-Orinoco plume, to the point that now if affects populations of larger fishes by restricting their dispersal ability, and therefore contributing to the differentiation of populations on both sides of the barrier.

Our data also revealed a substantial number of possible cryptic species still nor formally recognized as such. Surprisingly, 20% of the species addressed in the study that are reported to occur on both sides of the Amazon-Orinoco barrier have populations that are genetically distinct at the species level. We can therefore conjecture that there is still a surprisingly large hidden diversity of reef fishes awaiting formal recognition in the Caribbean and Brazilian provinces, despite recent advances in the taxonomy of western Atlantic fishes in general.

The importance of the Amazon-Orinoco plume as a biogeographic barrier began to be noticed in the 1970’s. Nearly half a century of research has shed light on the significant influence of the Amazon River on the marine environment. Despite recent advances, there is still a considerable lack of data on the distribution, taxonomy, and other basic aspects of the biology of Western Atlantic reef fish communities, especially in the Brazilian Province. The uniqueness of the Amazon plume region goes far beyond its role as a biogeographic barrier and main driver of diversity in the Western Atlantic over the last 9.4 million years. This region has a magnificent mesophotic reef system adapted to life under the influence of the plume, the Great Amazon Reef System. Today, we know only about 5% of this outstanding ecosystem, which is seriously threatened by the prospection and eventual exploitation of oil. Therefore, we draw attention to the urgent need for broad baseline studies to better know and preserve this fascinating and unique region of the world’s ocean.

Written by:
Gabriel Soares Araujo (1) & Fabio Di Dario (2)
1. PhD candidate, Programa de Pós-Graduação em Ciências Ambientais e Conservação, Universidade Federal do Rio de Janeiro, Instituto de Biodiversidade e Sustentabilidade – NUPEM/UFRJ, Macaé, Rio de Janeiro, Brazil.
2. Associate Professor, Universidade Federal do Rio de Janeiro, Instituto de Biodiversidade e Sustentabilidade – NUPEM/UFRJ, Macaé, Rio de Janeiro, Brazil

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Published by jbiogeography

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