Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident.

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (Cs and Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T) of Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The T in the slow component of the TDMs could converge towards the physical half-life of Cs. Nearly five and a half years after the FDNPP accident, we concluded that Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate Cs contamination level in the river water was high after heavy rains. Overall, Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.