Opening of the closed water area and consequent changes of 228Ra/226Ra activity ratios in coastal lagoon Nakaumi, southwest Japan

•228Ra/226Ra activity ratio in closed lagoon water showed a seasonal change.•228Ra/226Ra activity ratios changed with the opening event of closed area for 28 years.•The opening event made the water circulation more active, but unstable condition in and out the area.

In Lake Nakaumi, the second largest coastal lagoon in Japan, artificially closed (Honjyo) area, which was left untouched for 28 years, was partly opened in May, 2009. 228Ra/226Ra ratio of waters in Honjyo area and Lake Nakaumi showed a well-tuned seasonal variation exhibiting high value in summer. After the opening event, however, the 228Ra/226Ra ratios in the Honjyo water showed an unclear seasonal variation in both surface and deep water. This opening event caused the change of active movement of lake and marine water.