Long-term changes in seaweed assemblages in Nagashima Island, Kagoshima Prefecture: Disappearance of the canopy-forming kelp and fucoid algae from coastal areas facing the East China Sea

Abstract

Ecklonia radicosa (Laminariales), a canopy-forming brown alga, was reported to have disappeared since 2016 at a site surveyed under a nationwide long-term monitoring survey for the natural ecosystem (Monitoring Site 1000, Japanese Ministry of Environment) in Nagashima Island, Kagoshima Prefecture, Japan. The distribution of seaweed assemblages along the coast of the island was surveyed in 2019 to compare with that in 2005. In 2005, E. radicosa and the temperate species of Sargassum (Fucales) were widely distributed, and comprised the large seaweed assemblages along the coast facing both the East China Sea and Yatsushiro Bay. However, in 2019, these species, including an edible alga, Sargassum fusiforme, were not found oŠ the coast of the former, suggesting the degradation of kelp and fucoid assemblages. These algae were still found at the sites in Yatsushiro Bay as large seaweed assemblages; however, some tropical species of Sargassum that were uncommon in 2005 were also observed in 2019. The reason for the disappearance of these canopy-forming algae has not been elucidated yet; however, the herbivory impacts of fish on the algae may have caused the elimination of these algae from the coast facing the East China Sea in relation to climate change.