Significant changes in seaweed community structure revealed by a nationwide long‐term monitoring survey in Japan ‘ Monitoring‐Site 1000 ’ over the past 15 years (2008–2022)

SUMMARY Significant changes in seaweed communities were identified at six sites in Japan through the long‐term monitoring program ‘ Monitoring Sites 1000 .’ This study summarizes the results of 15 years of surveys conducted from 2008 to 2022. Annual nondestructive permanent quadrat observations and line‐transect surveys, carried out perpendicular to the shoreline, were used to assess species composition, coverage, and vertical distribution. Canopy‐forming seaweeds were present at all sites from 2008 to 2015; however, Ecklonia radicosa at the Satsuma‐Nagashima site on Kyushu, facing the open ocean, suddenly disappeared in 2016 and has not recovered since. As this species currently persists only in a nearby semi‐enclosed area, monitoring continues at both the disappeared and remaining locations. Similarly, Ecklonia cava and Eisenia bicyclis at the Shimoda site in central Honshu, facing the Pacific Ocean, gradually declined after 2018 and had completely disappeared by 2022. At the Shizugawa site on the Pacific coast of northern Honshu, the upward shift of Eisenia bicyclis communities to shallower depths—caused by seafloor subsidence following the 2011 earthquake—has persisted, with no recovery at the species' former deepest distribution. Saccharina japonica at the Muroran site in Hokkaido was abundant nearshore but showed considerable interannual variation offshore. At the remaining two sites, Takeno and Awaji‐Yura, located on the coasts of the Sea of Japan and the Pacific Ocean in central Honshu, respectively, Ecklonia and Sargassum communities appeared relatively stable. Nonetheless, shifts in canopy‐forming species were frequently observed, suggesting ongoing gap dynamics driven by community succession and episodic disturbances such as typhoons and heavy rainfall. Over the 15‐year period, canopy‐forming species disappeared from two of the six sites, apparently due to direct or indirect impacts of environmental change. Even at the other four sites, persistent communities were influenced by abiotic and biotic factors, underscoring the need for continued long‐term monitoring.