Heatwave hit phase shifted coral reefs: Zoantharian mass mortality record.

Phase shift is characterized by an abrupt change in the structure of a community in response to a disturbance that can break its resistance, displacing it from its natural variation. This phenomenon has been recognized in several ecosystems and often points to human activities as the main cause. However, reactions of shifted communities to anthropogenic impacts have been less studied. In recent decades, heatwaves resulting from climate change have strongly affected coral reefs. Mass coral bleaching events are recognized as the main cause of coral reef phase shifts on a global scale. In 2019, an unprecedented heatwave hit the southwest Atlantic Ocean causing mass coral bleaching in non-degraded and phase-shifted reefs of Todos os Santos Bay, at an intensity never recorded in a 34-year historical series. We analyzed the effects of this event on the resistance of phase-shifted reefs, dominated by the zoantharian Palythoa cf. variabilis. Using benthic coverage data from 2003, 2007, 2011, 2017, and 2019, we analyzed three non-degraded reefs and three phase-shifted reefs. We estimated the coverage and bleaching of corals and P. cf. variabilis on each reef. There was a reduction in coral coverage in non-degraded reefs before the 2019 mass bleaching event (i.e., heatwave). However, there was no significant coral coverage variation after the event and the structure of non-degraded reef communities did not change. In phase-shifted reefs the coverage of zoantharians did not change significantly before the 2019 event, however, after the mass bleaching, there was a significant reduction in the coverage of these organisms. Here we revealed that the resistance of the shifted community was broken, and its structure was altered, indicating that reefs in this condition were more susceptible to bleaching disturbance than non-degraded reefs.

The role of competition in the phase shift to dominance of the zoanthid Palythoa cf. variabilis on coral reefs.

Phase shift phenomena are becoming increasingly common. However, they are also opportunities to better understand how communities are structured. In Southwest Atlantic coral reefs, a shift to the zoanthid Palythoa cf. variabilis dominance has been described. To test if competition drove this process, we carried out a manipulative experiment with three coral species. To estimate the natural frequency of encounters we assess the relationship between the proportion of encounters and this zoanthids coverage. The contact causes necrosis in 78% of coral colonies (6.47 ± SD 7.92 cm(2)) in 118 days. We found a logarithmic relationship between the proportion of these encounters and the cover of P. cf. variabilis, where 5.5% coverage of this zoanthid is enough to put 50% of coral colonies in contact, increasing their partial mortality. We demonstrate that zoanthid coverage increase followed by coral mortality increase will reduce coral cover and that competition drives the phase shift process.