Radiocesium-bearing microparticles cause a large variation in (137)Cs activity concentration in the aquatic insect Stenopsyche marmorata (Tricoptera: Stenopsychidae) in the Ota River, Fukushima, Japan

After the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Plant accident in Japan, freshwater ecosystems near the site remained contaminated by radiocesium (RCs). Clarifying RCs concentrations in aquatic insects is crucial because fishes consume these insects that transfer RCs into freshwater ecosystems. As aquatic insects are usually measured for radioactivity in bulk samples of several tens of insects, variation in RCs concentration among individuals is not captured. In this study, we investigated the variability in (137)Cs activity concentration in individual aquatic insects in detritivorous caddisfly (Stenopsyche marmorata) and carnivorous dobsonfly (Protohermes grandis) larvae from the Ota River, Fukushima. Caddisfly larvae showed sporadically higher radioactivity in 4 of the 46 caddisfly larvae, whereas no such outliers were observed in 45 dobsonfly larvae. Autoradiography and scanning electron microscopy analyses confirmed that these caddisfly larvae samples contained radiocesium-bearing microparticles (CsMPs), which are insoluble Cs-bearing silicate glass particles. CsMPs were also found in potential food sources of caddisfly larvae, such as periphyton and drifting particulate organic matter, indicating that larvae may ingest CsMPs along with food particles of similar size. Although CsMP distribution and uptake by organisms in freshwater ecosystems is relatively unknown, our study demonstrates that CsMPs can be taken up by aquatic insects.