Barren grounds are increasing along the coast of Japan and all over the world. Kelp and other useful large seaweeds are depleted in barren ground as rocky beds become covered with small algae called crus-tose coralline algae whose main constituent is calcium carbonate. There are a number of different factors responsible for barren ground, including changes in the environment such as elevation of water tempera-tures and grazing by sea urchins. However, the lack of iron (ferrous ions) is thought to be another factor of barren ground, particularly in the coast of Japan Sea in Hokkaido. The effect of iron for recovering from barren ground was evaluated in this study. We utilized slugs and composts including humic substances (HSs) as iron supplier in seawater, and complex formation of iron with humic substances was examined. At first, the amount of irons emitted from slugs was investigated and it was confirmed that iron-humate could be produced by mixing humic substances and ferrous ions. Next, mixtures of slugs and composts including HS were supplied to a barren ground in shallow water in Hokkaido, in order to investigate the effect of irons for recovering from barren ground. It was indicated that supplying irons resulted in establishing kelp bed significantly.
