Observations of the 2021/2022 La Niña mass coral bleaching event on the soft coral genus Sarcophyton at Magnetic Island (Australia)

Coral reef ecosystems are declining globally due to climate change. As most ecological studies have focused on stony corals, the distribution of other benthic communities and their susceptibility to bleaching remain largely overlooked. Soft corals can form diverse and ecologically important assemblages; hence it is important to understand their distribution and quantify the impacts of heatwaves on soft coral demographics. We surveyed soft corals on two inshore fringing reefs: Geoffrey Bay (19°09'13.7"S, 146°52'09.5"E) and Alma Bay (19°08'52.9"S, 146°52'13.7"E) at Magnetic Island. We aimed to survey soft corals to genus level. Then, we quantified the size frequency distribution of Sarcophyton colonies as the most abundant genus between substrate types, and examined the susceptibility of Sarcophyton to bleaching during the La Niña mass coral bleaching event in January-March 2022. We found the reefs at Magnetic Island to support diverse soft coral communities. Sarcophyton was one of the dominant soft coral genera on rock, rubble and sand beds, and colonies of all sizes experienced bleaching. Bleaching occurrence was lower in Alma Bay than Geoffrey Bay (26.66% and 52.27% of Sarcophyton colonies bleached, respectively). Bleaching was found to be depth- and size-dependent, as deeper and larger colonies had a greater likelihood to bleach. Since larger Sarcophyton colonies were more susceptible to bleaching, the impacts on reproduction, mortality and ecosystem functioning are likely to be significant. This is the first assessment of the soft corals at Magnetic Island and of the susceptibility of a common soft coral genus on the GBR to bleaching.