Seasonal dynamics of gammarid assemblages associated with <i>Sargassum macrocarpum</i> and <i>S</i>. <i>patens</i> beds in Tsukumo Bay, Noto Peninsula, Sea of Japan

Sargassum beds provide important habitats for diverse epifaunal organisms including gammarid amphipods that are closely associated with macroalgae. This study aimed to describe the seasonal dynamics of gammarid assemblages associated with two predominant temperate seaweeds, Sargassum macrocarpum and S. patens in Tsukumo Bay, Noto Peninsula, central Sea of Japan. Because tidal amplitude in the Sea of Japan is minimal, this region offers an ideal setting for evaluating seasonal biological patterns with limited tidal influence. Seaweed samples were collected seasonally from winter to autumn in 2023 and associated gammarids were identified to the family level to assess abundance, richness, and Shannon diversity. Statistical analyses revealed significant seasonal variation in gammarid assemblage composition, whereas no significant differences were detected between host seaweed species. Gammarid richness, diversity, and abundance were highest in winter and spring, while lowest in summer and autumn. Ampithoidae consistently dominated the assemblages across seasons. These seasonal shifts closely corresponded with the phenological changes of the host seaweeds, suggesting that seasonal variation in habitat availability and food resources is primary driver of gammarid assemblage structure. This study provides baseline information on gammarid phenology in temperate Sargassum beds of the Sea of Japan.