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

Abstract

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 (¹³⁴Cs and ¹³⁷Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of ¹³⁷Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T_eff) of ¹³⁷Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of ¹³⁷Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of ¹³⁷Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total ¹³⁷Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T_eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The T_eff in the slow component of the TDMs could converge towards the physical half-life of ¹³⁷Cs. Nearly five and a half years after the FDNPP accident, we concluded that ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate ¹³⁷Cs contamination level in the river water was high after heavy rains. Overall, ¹³⁷Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

Keywords: Decay function model; Effective ecological half-life; FDNPP accident; Freshwater fishes; Lake Onuma; Radiocesium ((137)Cs).