Unprecedented erosion of Mussismilia harttii, a major reef-building species in the Southwestern Atlantic, after the 2019 bleaching event

Reefs are diverse environments because of the structural complexity provided by the tridimensional coral-built framework. However, they are sensitive environments that face multiple stressors including global warming, which triggers bleaching and mortality episodes. After death, coral skeletons are overgrown by a microbial film, which degrades and erodes the reef framework. Although erosive processes have been investigated in the Caribbean and Indo-Pacific, they remain poorly addressed for the unique Southwestern Atlantic reefs. Therefore, we investigated through field surveys of three Brazilian reefs if colonies of the endemic and regionally dominant coral Mussismilia harttii underwent erosion after the 2019 bleaching episode. We also collected corallite fragments from healthy, mildly bleached, severely bleached and dead M. harttii colonies for microcomputed tomography and densitometry analyses to assess whether microporosity and skeletal mineral density are reduced following bleaching. Our findings show that > 90% of the colonies underwent bleaching and loss of live cover was higher than 60% for all three reefs. All reefs also underwent severe erosion, with an area loss of intact colonies ranging from 33.8 to 85.2%. Furthermore, we detected higher total microporosity for dead skeletons and, together with severely bleached colonies, lower skeletal mineral density. Our results also suggest that bleaching, mortality and erosion processes are connected. These findings show that Southwestern Atlantic reefs are facing unprecedented degradation, although they are often considered climate refugia. In addition, because M. harttii is among the most important reef-builders in the region, carbonate budgets and structural complexity may face declines in the Southwestern Atlantic.