Comparison of Sargassum community succession between artificial and natural reefs in Wakasa Bay, Japan

The threat of declining seaweed beds has been a concern around the world. Seagrass and seaweed (brown algae) beds are essential habitats supporting fisheries. However, approximately 22% of these habitats have been lost in Japan due to increased coastal landfill sites and ports. This study aims to rehabilitate the depletion of these habitats by constructing an artificial reef in Wakasa Bay, Japan, and monitoring brown algae ( Sargassum sp.) succession in 1 year and 3 years after the reef construction. In this study, we set up six sites on the artificial reef as a treatment area and three sites on the natural reef as a control area and then identified the seaweed species composition of the Sargassum community and their coverage on each reef using underwater visual observation by scuba divers. Significant differences between the artificial and natural reefs were identified. The seaweed species composition and coverage of the Sargassum community on the artificial reef differed from those on the natural reef in 3 years after the reef construction. On the artificial reef, the presence of S. horneri decreased and that of S. hemiphyllum / S. patens increased, whereas on the natural reef, S. patens dominated and S. macrocarpum increased. The seaweed species composition and coverage on the artificial reef had not caught up with those on the natural reef. The seaweed on the artificial reef is thus considered to be in the vegetation succession process, which has not yet reached the climax condition, while the natural reef has almost reached the climax phase. The Sargassum community monitoring on the artificial reef appears necessary to be continued in the next few years.