Algae. 1993; 8(2): 191-198.

Succession of Seaweed Communities on Artificial Reefs in Ashizuri, Tosa Bay, Japan

Masao Ohno

Usa Marine Biological Institute, Kochi University

The survey site had a substratum encrusted only with coralline algae in waters which are affected by the Kuroshio Current. A total of 209 artificial reef units (3 tons each of concrete trapezoid blocks0 which had been developed for fishery resources were set up on the sandy, boulder and rocky bottom substrata at 3-10 m depth in Ashizuri, Tosa Bay, Japan. Plastic mats offered favourable substrata for seaweeds and attaching organisms. Mature thalli of Sargassum, Gelidium and other seaweeds were transported by mesh bags from other coastal areas. Slow-eluting fertilizers were placed among the blocks to support growth of seaweeds. The qualitative survey of seaweeds was carried out on the artificial reefs in different bottom types and natural rocks adjacent to reefs during mature seasons of seaweeds. The total number of species at any site ranged from 22 to 35, two years after placements. Fertilizers were effective in increasing growth of seaweeds. Sargassum became the dominant population at different bottoms after 6 years. The biomass of seaweeds on the artificial reef in sandy area was 9,998g wet wt m super(-2) and that on the reefs in rocky area was 229 g wet m super(-2) after one year. After 6 years a biomass of 5,137 g wet wt m super(-2) in the sandy area was recorded. In two other areas, i.e. boulder and rocky, a biomass of 2,666 and 2,848 g wet wt m super(-2), respectively, were recorded. The Sargassum communities expanded from the reefs to adjacent rocky area year after year. At our study site, especially the rocky area, sea urchins were the most active grazer of seaweeds. The first step in creating artificial seaweed beds may be to look for a good site in sandy areas where sea urchins are scarce.

Keywords : succession, seaweed beds, artificial reefs