Temporal variations of 90Sr and 137Cs concentrations and the 137Cs/90Sr activity ratio in marine brown algae, Undaria pinnatifida and Laminaria longissima, collected in coastal areas of Japan

The anthropogenic radionuclides, Sr-90 and Cs-137, were measured in two marine algal species, wakame seaweed (Undaria pinnatifida) and edible kelp (Laminaria longissima), collected in four coastal areas of Japan during 1998-2008. Although Sr-90 and Cs-137 could be detected at all sampling sites, the concentrations of Sr-90 and Cs-137 were at low levels and those in some samples were below the detection limit. These low concentrations and the small variation of both concentrations and the Cs-137/Sr-90 activity ratio indicate that the source of Sr-90 and Cs-137 detected in this study originated from the global fallout deposition following atmospheric nuclear-bomb tests in the past. There were no significant differences in both concentrations of Sr-90 and Cs-137 in wakame seaweed among three sampling sites. Although wakame seaweed is extensively distributed in southern and central Japan, it does not occur in northern areas and so edible kelp was monitored. The concentrations of Sr-90 and Cs-137 in edible kelp were significantly different from those in wakame seaweed in some sampling sites. These differences could be due to the difference in the concentrations of Sr-90 and Cs-137 in the surrounding seawater or the difference in species. The combined data with data from the previous report and the preexisting database showed that wakame seaweed incorporated Cs-137 through a different pathway from that of Sr-90. The combined data also suggested that wakame seaweed responded differently to the source of Cs-137.