Jeju Island: a sentinel for tracking ocean warming impacts on high-latitude benthic communities

As climate changes and anthropogenic pressures increase, marine ecosystem assembly and functioning are altered. High-latitude areas may provide opportunities for some tropical and subtropical organisms to survive. Yet, it is difficult to assess ongoing changes as ecological baselines are often missing. Here, we focus on the rapidly warming region of Jeju Island (South Korea), where thermal changes may have already triggered a poleward expansion of tropical and subtropical taxa and changes in community assembly. In 2012–2014, an island-wide quantitative description of its benthic communities (5 m and 15 m depths) was made, whereas patterns in seawater temperatures were examined from 1981 to 2020. Jeju Island algal assemblage was dominated by the regionally endemic kelp, Ecklonia cava. Turf, encrusting coralline algae, alongside tropical, subtropical, and cosmopolitan macro-algae were also major components of the benthic community. Hermatypic coral cover was higher at the 15 m depth than 5 m depth. Increases in seawater temperatures were highly significant, with differences between the average annual SST [1980–2020] and annual SST reaching up to + 1.61 °C in only 40 yr. The thermal rise was more pronounced in winter than summer, and warming was further accompanied by a major decline in the number of days below 14 °C and 18 °C, which are considered critical thermal thresholds for hermatypic coral survival and reef development, respectively. Warming winter seawater temperatures could ultimately lead to a loss of seasonality and profound modifications of benthic assemblages. This study provides the necessary assessment of benthic community conditions at Jeju Island upon which the ongoing changes in the area can be explicitly demonstrated.