Differential effects of iron enrichment on corals competing with macroalgae and zoantharians

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^-1) for 26 days. We measured photosynthetic efficiencies of all organisms and recorded tissue discoloration and necrosis on corals. The photosynthetic efficiency of all corals reduced along time, regardless of Fe. When contacting macroalgae, Siderastrea sp. was least affected while Mi. alcicornis and Mu. harttii were damaged within the first days and suffered high discoloration. Mu. harttii underwent necrosis and dead areas were significantly larger at the highest Fe concentration. Contacts with the zoantharian caused discoloration in all corals and necrosis in Mu. harttii, without a clear effect of Fe. The concentration of Fe differentially impacted the studied species interactions, but did not cause negative effects alone. Our study shows the vulnerability of SWA corals considering future increases in the abundance of benthic competitors and the consequences for corals following mining disasters, helping to predict the impacts of iron enrichment on reefs.