Long-term change and disappearance of Lessoniaceae marine forests oS Waka, Ikishima Island, northwestern Kyushu, Japan

Seaweeds belonging to the family Lessoniaceae are one of the major components of marine forests in the warm temperate region of Japan. Over the last few decades, marine kelp forests of Lessoniaceae have been decreasing along the western coast of Kyushu, presumably due to increasing seawater temperatures. We monitored the seawater temperature and distribution of the kelp oS Waka on Ikishima Island, northwestern Kyushu from 2004 to 2018. From 2004 to 2007, the summer seawater temperatures were relatively low, and the distribution and density of the kelp increased. However, in 2010, 2013 and 2016, when the summer seawater temperatures were higher than those measured in the other years, kelp thalli were observed in higher proportions without any blades. The density of adult kelp plants drastically decreased throughout the following year. After the decline in adult density, many recruits grew in 2011 and 2014, and the kelp forests recovered. However, few recruits appeared in 2017, leading to the absence of the kelp at the study site in 2018. The deterioration of the adult community accompanied by higher seawater temperature and subsequent failure of recruitment would be responsible for the disappearance of the kelp forests.