The total radioactive Cs concentration in fnal-instar Stenopsyche marmorata larvae from the Mano River, Fukushima Prefecture, in 2013 was found to be >1000 Bq kg−1 in dry weight. The concentration of radioactive Cs in the river water itself was on the order of 0.01 Bq kg−1, while its concentration in drifting fne particulate organic matter (FPOM) was on the order of 10,000 Bq kg−1 in dry weight. When excretion experiments were performed in the summer and winter, signifcant decreases in the Cs concentrations in the whole body of S. marmorata were observed in both seasons. The biological half-life of Cs-137 was 5.3 and 8.8 h for the nonwintering generation and the wintering generation, respectively. Another elimination test using a stable Cs solution was performed, and the Cs concentration was observed to decrease rapidly to 7.5% of its original level in the digestive tract of S. marmorata after 72 h, but stable Cs remained in muscle and extra alimentary tissue of S. marmorata. As the total radioactive Cs in the water was quite low, its concentration in S. marmorata from the Mano River refects the fact that it feeds on FPOM. The radioactive Cs concentration in the FPOM signifcantly declined in 2015, but a corresponding decline was not observed for S. marmorata. We therefore infer that ultrafne particulate matter such as suspended vermiculite clay is strongly contaminated with radioactive Cs, and that this radioactive Cs passes continuously through the digestive tract of S. marmorata.

EB00000003652K

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