Reduction of seaweed beds is a serious problem in coastal areas of Japan and worldwide, and the lack of dissolved iron in seawater may contribute to the destruction of this ecologically important habitat. We have previously developed a method for restoring seaweed beds using a mixture of steelmaking slag and composts containing humic substances. Since October 2004, we have been performing field tests on the Shaguma coast in Mashike-cho, Hokkaido, Japan, to confirm the effectiveness of this method. However, thus far, no studies have been conducted to evaluate the effects of the hydrodynamic conditions by calculating the iron distributions in the area of the field tests. In this study, we evaluated the continuous effects of this iron fertilization method in Mashike by comparing the changes in seaweed bed distributions with the analyzed iron concentrations over a 5-year study period and the simulation results of iron distribution in the study area. Our findings demonstrated that the biomass of seaweeds at the fertilized (experimental) site was larger than that at the reference site. We also found that both the analyzed iron concentration and the calculated iron distribution in the area corresponded to the distribution of seaweed beds.

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