Abstract. Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived ¹²⁹I, the ¹²⁹I concentrations in seawater before and after the accident were compared.

Before the accident (2008–2009), the ¹²⁹I concentrations in the western margin of the North Pacific between 32° N and 44° N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average ¹²⁹I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of ¹²⁹I in surface seawater, the accident-derived ¹²⁹I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively.

As of October 2011, depth profiles of ¹²⁹I revealed that ¹²⁹I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived ¹²⁹I were estimated to be (1.6–9.6) × 10¹² atoms m⁻² in this study area.

On the basis of the ¹²⁹I data in the seawater near Fukushima, the effective dose of ¹²⁹I from seafood ingestion was much smaller than the annual dose limit.