Significant changes in the southernmost temperate canopy-forming macroalgal communities in Japan: A comparison of their status in 2024 with that a decade ago

To reveal the significant decline and species shift in macroalgal communities over the past decade in Kagoshima Prefecture, Kyushu, Japan, we investigated the spatial distribution of kelp and fucoid communities at nine sites in early summer 2024, where data from previous surveys (conducted in the late 2000s and early 2010s) were available, and compared the current status with that of a decade ago. The line-transect method was applied from the shore up to 150 m offshore at the same sites as in the previous surveys, and the species composition and coverage of the canopy-forming species were recorded. The results showed that the decline or shift of macroalgal communities toward subtropical species was not uniform but varied across different biological, geographic, and coastal environments. A kelp forest of Ecklonia radicosa, which represents the southern distributional limit for this alga and the lowest latitude of any Japanese Ecklonia species, had completely disappeared at two sites facing the East China Sea. In fucoid communities, a characteristic shift from temperate Sargassum species (S. fusiforme and S. hemiphyllum) to subtropical species (S. ilicifolium and S. crispifolium) was observed, particularly at the mouth of Kagoshima Bay, where the influence of oceanic water from the warm current is evident. In contrast, temperate species remained stable in more enclosed areas—particularly on islands sheltered within the Yatsushiro Sea and in the inner part of Kagoshima Bay—suggesting that these areas experience less impact on their macroalgal communities. These differences may reflect the varied and complex influences on macroalgal communities, such as rising seawater temperatures and herbivory by fish.