Bleaching Dynamics of Crustose Coralline Algae in Jeju Island, Korea

Crustose coralline algae (CCA) are key reef-building organisms, yet their bleaching dynamics and associated environmental factors in natural ecosystems remain poorly understood. This is particularly relevant to Jeju Island, Korea, where declines in canopy-forming macroalgae and the expansion of CCA-dominated barren grounds are reshaping benthic communities, highlighting the need to monitor how CCA respond to environmental stress. In this study, bleaching dynamics were monitored monthly using Autonomous Reef Monitoring Structures (ARMS) deployed at Bomok, southern Jeju Island, Korea, during a 13-month period (August 2022–August 2023). Total CCA cover, bleached CCA cover, and bleached CCA ratio were quantified and compared with seawater temperature and daytime light intensity. Despite persistently high total CCA cover, the bleached CCA ratio varied markedly over time, with multiple peaks and subsequent declines, reaching a maximum of 65.8%. The bleached CCA ratio was positively correlated with daytime light intensity (ρ = 0.709, p = 0.015), whereas no significant association was detected with seawater temperature. Nine bleached specimens were assigned to the genus Porolithon based on psbA phylogenetic analysis, representing the first molecular identification of bleached CCA from Korea. These findings provide a temporal baseline for understanding CCA bleaching within the rapidly changing benthic communities of Jeju Island and indicate that light intensity may be an important environmental correlate of bleaching.