Time-dependent trends of artificial radionuclides in biota of the Yenisei River (Siberia, Russia).

We investigated time-dependent trends of artificial radionuclides in aquatic moss, zoobenthos (amphipods and caddisfly larvae), and three abundant wild fish species (Northern pike, Arctic grayling, and Siberian dace) inhabiting the Yenisei River in the vicinity of the radioactive discharge site in 2007-2015, in a period before and after the shutdown of the last nuclear reactor plant at the Mining-and-Chemical Combine (MCC), which occurred in 2010. From our research, we learned that concentrations of short-lived radionuclides, whose discharges to the Yenisei either stopped or declined after the shutdown of the reactor plant at the MCC (Na, Sc,Cr, Mn, Co, Fe, Co, Zn, Ru, Ce, Eu, Np), decreased in biota samples as well. The ecological half-life (EHL) of Zn (0.4-0.7 y) was similar to the physical half-life of this isotope, the EHLs of Co (1.2-2.1 y) and Eu (1.8 y) were shorter than the physical half-lives of these isotopes. Concentration of Cs did not decrease significantly in biota of the Yenisei after the shutdown of the last reactor plant because the discharges of this radionuclide to the Yenisei continued at the same level. On a longer-term scale (since 1973 and since 1991), concentration of Cs in fish muscle had significantly decreased, following the decrease in annual discharges of this radionuclide to the Yenisei, and the EHL of Cs was estimated as 6.5-12.8 y. Statistically significant correlation with annual discharges of Cs was revealed for the concentration of this radionuclide in grayling (whole bodies and muscle); dace (muscle), and amphipods. Despite their ability to accumulate high concentrations of Cs, aquatic moss and caddisfly larvae (analyzed together with their stony casings) were not sensitive to interannual fluctuations in the releases of this radionuclide to the Yenisei. Among the analyzed fish species of the Yenisei, the highest activity concentration of Cs was revealed in pike (body and muscle), indicating biomagnification of this radionuclide in the top level of the trophic chain.