Contamination of sea urchin Mesocentrotus nudus by radiocesium released during the Fukushima Daiichi Nuclear Power Plant accident.

Countless marine organisms were polluted with radioactive materials that were dispersed when the Fukushima Daiichi Nuclear Power Plant (FDNPP) was damaged in 2011 by the Great East Japan Earthquake. The aim of this study was to determine the degree to which marine herbivorous sea urchins, Mesocentrotus nudus, were contaminated with radiocesium because of the accident. We collected samples of sea urchins from four locations in Fukushima prefecture (at the coast and offshore from the Yotsukura and Ena stations) and investigated how the 137Cs activity concentrations changed. The biological half-life (Tbio) of 137Cs in the individual sea urchins was between 121 and 157 days. The ecological half-life (Teco) of 137Cs was 181-423 days and was high in places close to the FDNPP. The Teco values in the sea urchins were longer than previously reported. The results infer that the food sources of the sea urchins around the Fukushima coast strongly influenced their uptake of 137Cs.

Temporal Variation of Radionuclide Contamination of Marine Plants on the Fukushima Coast after the East Japan Nuclear Disaster.

As a result of the Great East Japan Earthquake and associated tsunami in March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) released a large amount of radioactive material into the environment, resulting in contamination of many marine organisms. In this study, 15 marine algal species and a seagrass species were collected from the sublittoral zone of the Iwaki Coast of Fukushima Prefecture from May 2012 to June 2015 and analyzed for variations in 110mAg, 134Cs, and 137Cs over time. The results indicated that (1) 110mAg, 134Cs, and 137Cs were present in all marine plants collected in May 2012, (2) the concentration of 110mAg in the seagrass Phyllospadix iwatensis decreased significantly over time while the ecological half-life of 110mAg in P. iwatensis was longer at locations closer to the FDNPP, and (3) the 110mAg/137Cs radioactivity ratio of P. iwatensis was remarkably high until 2015, indicating that detectable 110mAg was present in the coastal environment 4 years after the accident. The concentration of 110mAg in P. iwatensis was higher than those in other marine algae, demonstrating a species-specific mechanism of accumulation.

The effects of sediment transport on temporal variation in radiocesium concentrations in very shallow water off the southern coast of Fukushima, Japan.

We studied the very shallow coastal water off Iwaki City, Fukushima Prefecture, and investigated: (1) temporal variation in 137Cs concentrations; (2) particle-size distribution of sediments; and (3) the effect on variation by waves, component-fractionated concentration of radiocesium and mineral composition at three sampling stations (Yotsukura, Ena rocky reef and Ena sandy station). There was a decline in 137Cs concentrations in sediment samples at all sampling stations between 425 and 1173 days after the accident. All stations had fluctuations in 137Cs concentrations between 425 and 800 days. At Ena sandy station and Ena rocky reef stations the declines in 137Cs concentrations slowed from about 800 days after the accident. Fluctuations in particle median diameters were seen, as well as in 137Cs concentrations. At Yotsukura, where the fluctuation in median diameter was small, a constant decrease in 137Cs concentrations was observed. We considered that bioturbation may contribute the constant decrease. At Ena sandy station, where the fluctuation of the median diameter was large, the fluctuation in 137Cs concentrations was also large. The movement of sediments was evaluated by the Shields parameter, and results indicated that at any station where the sediment was moved more frequently, the fluctuation in 137Cs concentrations was also large. The highly contaminated small particles moved from our stations due to wave action between 425 and 800 days after the accident. The remaining relatively large particles might contribute to the slowing down in reduction of 137Cs concentrations from 800 days after the accident. However, the 137Cs concentrations in sediments in very shallow water off the southern coast of Fukushima may continue to decline over time.

Biological half-life of radioactive cesium in Japanese rockfish Sebastes cheni contaminated by the Fukushima Daiichi nuclear power plant accident.

Since the Fukushima accident in March 2011 the concentration of radioactive cesium in Japanese rockfish (Sebastes cheni) has been decreasing slower than other fish species. The aim of this study was therefore to investigate the possibility of slow elimination rate (i.e., relatively longer Tb) as one of the reasons for the slow decrease in (137)Cs concentrations in Japanese rockfish (S. cheni). To do this, we reared twenty-three individuals of this species for a period of about 1 year, during which time we measured the (137)Cs concentrations and γ-ray spectra 14 times by using a high-efficiency NaI(Tl) scintillator. We then examined the relationship between the (137)Cs concentrations and the total length of each individual. We estimated the biological half-life (Tb, day) for each individual using the total number of (137)Cs counts in the energy region, and examined the effects of total length and (137)Cs concentration on Tb by generalized linear model (GLM). We also examined the effect of sex, total length, seawater temperature, and the (137)Cs concentration of seawater on temporal changes in the (137)Cs count reduction rate by GLM. There was no clear relationship between the corrected whole-body (137)Cs concentrations and the total length in females, however there was a significant positive correlation between these two variables in males. The difference between males and females may be attributable to variation in the degree of dilution because of variable growth of individuals, and suggests that the (137)Cs concentrations of small individuals may be greatly diluted because of faster growth. However, there was no significant difference in Tb between sexes. The mean Tb (±SD) in all individuals was 269 (±39) days; this Tb value is 2.7-5.4 times longer than past Tb values (marine fish: 50-100 days), and is thought to be one of the reasons for the slower decrease in (137)Cs concentrations in this species than other fish species on the coast of Fukushima. The GLM showed significant effects of both total length and (137)Cs concentration on Tb, which may reflect a reduction in the metabolic rate with increased body size (i.e., aging) and gradient of concentration against seawater. The GLM also showed a significant positive effect of seawater temperature on the reduction rate of the (137)Cs counts (D, day(-1)). Therefore, D was clearly related to seasonal variations in the temperature of seawater, and this relationship may be attributable to changes in the metabolic rate that are controlled by variations in the seawater temperature. From these measurements, we examined the processes that control reductions in (137)Cs radioactivity.