Comparison of seaweed species composition and coverage of Sargassum and Myagropsis communities between artificial and natural reefs in Wakasa Bay, Japan

Abstract

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 and Myagropsis) succession in 2 years and 4 years after the construction was completed. In this study, we set up five sites on the artificial reef as a treatment area and one site on the natural reef as a control area and then identified the seaweed species composition of the Sargassum and Myagropsis communities and their coverage on each reef using underwater visual observation by scuba divers. The seaweed coverage on the artificial reef was already close to that on the natural reef in 2 years after construction. However, the dominant species on the artificial reef was not conformable to that on the natural reef in 2 years after construction. The dominant species on the artificial reef changed to S. horneri/S. confusum in 2 years after construction and M. myagroides in 4 years after construction. On the other hand, the dominant species on the natural reef changed to S. patens in 2 years after construction and M. myagroides in 4 years after construction. That is, the species composition on the artificial reef was close to that on the natural reef in 4 years after construction. Thus, the recovery of species composition takes longer than that of seaweed coverage on the artificial reef.