Effectiveness of iron supply in the coastal areas of Tsushima for restoring seaweed beds

Abstract

The formation of barren grounds due to the degradation of seaweed beds is a serious problem along the coast of Japan and other parts of the world. A new method has been developed for the restoration of seaweed beds that utilizes a mixture of steelmaking slag and humus materials (Fe supply unit). Although the effectiveness of this method was confirmed in a field test within the Japan Sea in Hokkaido, further studies are needed to put this method to practical use. We must confirm a possibility of unforeseen negative effect and should understand the dependence of seaweed beds on seawater quality, especially the role of iron. The possibility of negative impact on coastal environment by utilizing Fe supply unit was investigated at the site of the field test in Tsushima. The possibility of heavy metal elution in seawater from Fe supply unit was evaluated. We analyzed six kinds of heavy metals (Cd, Pb, Cr, Cu, Zn and Mn) in seawater at 10 m and 30 m offshore from shorelines at Fe supply unit installation and control sites after 3.5 years from the beginning of the experiment. It was confirmed that concentrations of all the heavy metals were under the determination limit and the environmental standards in Japan. Secondly, the dependence of seaweed beds on water quality in Tsushima was investigated. We chose six sites within the coastal sea area in Tsushima for monitoring water quality. Nine water quality variables, such as pH, DO, T-N, T-P and D-Fe, were analyzed. We found that T-N and D-Fe concentration values showed a distinctive characteristic behavior. T-N concentrations within all sites of the coastal sea areas were not at a low level (> 0.3 mg/l) throughout the year. Meanwhile, measured D-Fe concentrations at the six sites were at a relatively low level throughout the year (< 4 μg/l) except one site. There is a high possibility that the Fe fertilization method would be an effective method for the restoration of seaweed beds in Tsushima.