public read Rodrigo Federal University of Rio de Janeiro Assitent Professor Fabiano Federal University of Rio de Janeiro Assitent Professor Large rivers createmajor gaps in reef distribution along tropical shelves. The Amazon River represents 20% of the global riverine discharge to the ocean, generating an up to1.3 x 106 km2 plumeand extensive muddy bottoms in the equatorial margin of South America. As a result, a wide area of the tropical North Atlantic is heavily impacted in terms of salinity, pH, light penetration and sedimentation. Such unfavorable conditions were thought to imprint a major gap in Western Atlantic reefs. Here, we present an extensive carbonatesystemoff the Amazon mouth, underneath the river plume. Significant carbonate sedimentation occurred during lowstandsea level, and still occurs in the outer shelf, resulting in complexhard bottom topography. A permanent nearbottomwedge of ocean water, together with the seasonal nature of the plume’s eastward retroflection, conditionsthe existence of this extensive (~9,500 km2) hard bottommosaic. The Amazon reefs transitionfrom accretive to erosional structures, and encompass extensive rhodolith beds. Carbonate structures function as a connectivity corridor for wide depth-ranging reef-asociated species, beingheavily colonized by large sponges and other structure-forming filter feeders that dwell under low light and high levels of particulates.Theoxycline between the plume and sub-plume is associated with chemoautotrophic and anaerobic microbial metabolisms. The system described hereinprovides several insights about the responses of tropical reefs to suboptimal and marginal reef-building conditions,which areaccelerating worldwide due to global changes. All the data from this study can be downloaded in this link: http://marinebiodiversity.lncc.br/files/index.php/s/TaLwiKC6QCiOvnY Amazon River Mouth -53.75 -39.625 7.25 -2.625 1454372991424 1454373027363