Seasonal dynamics of epifaunal communities on the Sargassum beds of the coast of Noto Peninsula, Japan

Keito Tsunoda, Yukimasa Higashide, Shouzo Ogiso, Nobuo Suzuki, Kenji Toyota

Epifaunal communities inhabiting seaweeds are key components of temperate rocky reef ecosystems, playing vital roles in nutrient cycling and biodiversity maintenance. However, limited research has addressed their seasonal dynamics in the semi-enclosed Sea of Japan. This study examined the seasonal changes in epifaunal communities associated with two dominant brown algae, Sargassum macrocarpum and S. patens, in Tsukumo Bay, Noto Peninsula. Over the course of one year, we recorded fluctuations in seaweed biomass and associated epifaunal abundance and composition. The wet weight of both host seaweed species peaked in spring and winter and declined during summer and autumn, reflecting their life history traits. Seasonal changes in epifaunal communities were more pronounced than differences between host species. Caprellid and gammarid amphipods exhibited synchronized abundance peaks with seaweed growth in spring and winter, while gastropods remained relatively stable year-round, likely favoring understory habitats. Bivalves increased in summer on S. macrocarpum, potentially due to structural differences facilitating larval settlement. Multivariate analyses (PERMANOVA and nMDS) revealed clear seasonal shifts in community composition, with caprellids, gammarids, and gastropods contributing most to these patterns. Our findings highlight the strong coupling between macroalgal phenology and epifaunal dynamics, suggesting that algal bed seasonality significantly shapes coastal community structure. This study provides a baseline for understanding the ecological functioning of Sargassum beds in the Sea of Japan and informs conservation and management strategies under changing environmental conditions.