Global warming-led spatial competition among kelp forest, coralline algae and hard coral communities in a climatic transition zone (Jeju Island, South Korea)

Jaeseok Chung^1,2, Jaehyeon Kim¹, Sun Kyeong Choi¹, Young Baek Son^1,2, Taihun Kim^1,2

¹Korea Institute of Ocean Science & Technology, Republic of Korea - ²University of Science & Technology, Republic of Korea

Jeju Island, the southernmost island of South Korea, primarily exhibits temperate climate characteristics, and its coastal ecosystem is strongly associated with kelp forest-based habitats. However, the rise in seawater temperature in recent decades has led to the tropicalization of Jeju’s coastal benthic ecosystems. Consequently, kelp forests are declining, coralline algae-driven barren ground is expanding, and tropical stony corals are colonizing the seafloor. In this study, we investigated the effects of increasing water temperature on the survival and growth of Ecklonia cava, a predominant species of local kelp forests, Montipora millepora, a tropical hard coral species that has been rapidly expanding around Jeju Island in recent years, and representatives of Hapalidales, an order of coralline algae causing barren ground. During an incubation experiment, we exposed individuals of these three taxa collected from the wild to a temperature scheme ranging from 7°C to a maximum of 34°C. Survival rate and growth rate were measured, and, in cases of M. millepora and Hapalidales, the level of bleaching was analyzed based on underwater images. M. millepora bleached from <12°C, as well as from >28°C, while Hapalidales exhibited the most pinkish coloration between 10-24°C. E. cava showed notable growth between 7-26°C. Thus, our experiment indicates distinct taxon-specific ranges of optimal water temperature, with global ocean warming likely driving the recent decline of local kelp forest, as well as the expansion of tropical stony corals and coralline algae, allowing them to gain an advantage in spatial competition with kelp. Consequently, the southern coast of Jeju Island appears to be undergoing a rapid stony coral-based habitat shift.