Ten years of monitoring the seaweed community in Nomo, Nagasaki Prefecture, southwestern Japan

Abstract

　Over the last few decades, the extent of marine kelp forests has decreased along the west coast of Kyushu in Japan. We monitored the distribution of seaweeds and seawater temperatures off Nomo, Nagasaki, western Kyushu from 2010 to 2019, to examine the relationship between seaweed distribution and abundance, and water temperature, with particular attention to Ecklonia radicosa. In 2010, Ecklonia cava ssp. kurome, E. radicosa and Sargassum macrocarpum dominated along the transect. However, E. cava ssp. kurome and S. macrocarpum disappeared by December 2013. Moreover, E. radicosa drastically decreased in 2016 and has not been observed since 2017. In contrast, small macroalgae such as Zonaria diesingiana increased in abundance after the larger macroalgal species disappeared. Water temperatures in the spring of 2016 were the highest recorded in the decade from 2010 to 2019 (18.2–19.7℃) and blades of E. radicosa were frequently lost in 2016, probably because of browsing by herbivorous fish. Previous studies showed that feeding rates of herbivorous fish increase, while relative growth rates of E. radicosa are stable, within the recorded temperature range. Changes in the balance between fish herbivory and seaweed productivity due to increasing water temperatures could be responsible for the decline of E. radicosa.