Distinct food-web transfers of 137Cs to fish in river and lake ecosystems: A case study focusing on masu salmon in the Fukushima evacuation zone

This study was conducted to elucidate the spatial and size variations, and food-web transfer of ¹³⁷Cs in freshwater fish in the upper reaches of the Ukedo River system, a highly contaminated river system flowing through the Fukushima evacuation zone. Fish collection and environmental surveys were conducted in the summer of 2020 at five forest rivers and at the Ogaki Dam reservoir (an artificial lake) with different air dose rates (mean 0.20-3.32 μSv/h). From the river sites, two salmonid species (masu salmon and white-spotted charr) were sampled, with masu salmon generally exhibiting higher ¹³⁷Cs concentrations, ranging widely (10.6 Bq/kg-wet to 13.0 kBq/kg-wet) depending on the fish size (size effect) and site. The ¹³⁷Cs concentrations in masu salmon were explained by the air dose rates, ¹³⁷Cs concentrations in water, sediments (excluding the lake site), and primary producers, with site-specific variations. In the rivers, masu salmon (fluvial type with parr marks) mainly fed on terrestrial insects with higher ¹³⁷Cs concentrations compared with those of aquatic insects, indicating that ¹³⁷Cs was transferred mainly to fish through the allochthonous forest food-web during summer. In the lake, masu salmon (lake-run type with larger size and silvery body coloration) mainly preyed on smaller fish with lower ¹³⁷Cs concentrations, demonstrating that ¹³⁷Cs is transferred to fish through the autochthonous lake food-web with biomagnification. Differences in ¹³⁷Cs concentrations among masu salmon (mean 441 Bq/kg-wet) and other fish species (mean 74.8 Bq/kg-wet to 2.35 kBq/kg-wet) were also found in the lake. The distinct ¹³⁷Cs transfers to river and lake fish were supported by stable isotope analysis: δ¹⁵N and δ¹³C values enriched stepwisely through the food-webs were, respectively, higher and lower in the lake. Our results obtained using multiple approaches clearly revealed the distinct food-web transfer of ¹³⁷Cs in river and lake ecosystems. These findings can contribute to prediction of radioactive contamination in freshwater fish in the Fukushima evacuation zone.