Marine anthropogenic eutrophication from nutrient and metal inputs has been linked to shifts in reef communities, benefitting fast-growing organisms that can outcompete corals. In 2015, the collapse of a mining dam in Brazil containing iron (Fe) waste reached the Southwestern Atlantic (SWA) Ocean and adjacent environments, but its effects on competitive interactions of corals are still unknown. We assessed the impacts of seawater enrichment with dissolved Fe on benthic competition with three reef-building corals (Siderastrea sp., Millepora alcicornis, Mussismilia harttii) against a common macroalga (Lobophora variegata) and a zoantharian (Palythoa caribaeorum) in a mesocosm experiment. Organisms physically interacted while submitted to four Fe concentrations (0, 100, 300, and 900 μg L