TRANSFER OF RADIOCAESIUM FROM CONTAMINATED BOTTOM SEDIMENTS TO MARINE ORGANISMS THROUGH BENTHIC FOOD CHAIN IN POST-FUKUSHIMA AND POST-CHERNOBYL PERIODS
Authors: Bezhenar Roman , Jung Kyung Tae , Maderich Vladimir , Willemsen Stefan , de With Govert , Qiao Fangli 
Published: January 2016

Abstract. After the earthquake and tsunami on 11 March, 2011 damaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), an accidental release of a large amount of radioactive isotopes into both the air and the ocean occurred. Measurements provided by the Japanese agencies over the past four years show that elevated concentrations of 137Cs still remain in sediments, benthic organisms and demersal ﬁshes in the coastal zone around the FDNPP. These observations indicate that there are 137Cs transfer pathways from bottom sediments to the marine organisms. To describe the transfer quantitatively, the dynamic food chain model BURN has been extended to include benthic marine organisms. The extended model takes into account both pelagic and benthic marine organisms grouped into several classes based on their trophic level and type of species: phytoplankton, zooplankton, and ﬁshes (two types: piscivorous and non-piscivorous) for the pelagic food chain; deposit feeding invertebrates, demersal ﬁshes feeding by benthic invertebrates and bottom omnivorous predators for the benthic food chain; crustaceans, molluscs and coastal predators feeding on both pelagic and benthic organisms. Bottom invertebrates ingest organic parts of bottom sediments with adsorbed radionuclides which then migrate up through the food chain. All organisms take radionuclides directly from water as well as food. The model was implemented into the compartment model POSEIDON-R and applied to the Northwestern Paciﬁc for the period of 1945–2010 and then for the period of 2011–2020 to assess the radiological consequences of releases of 137Cs due to FDNPP accident. The model simulations for activity concentrations of 137Cs in both pelagic and benthic organisms in the coastal area around the FDNPP agree well with measurements for the period of 2011–2015. The decrease constant in the ﬁtted exponential function of simulated concentration for the deposit ingesting invertebrates (0.45 y–1) is close to the decrease constant for the sediment observations (0.44 y–1), indicating that the gradual decrease of activity in the demersal ﬁsh (decrease constant is 0.46 y–1) was caused by the transfer of activity from organic matter deposited in bottom sediment through the deposit feeding invertebrates. The estimated from model transfer coefﬁcient from bulk sediment to demersal ﬁsh in the model for 2012–2020 (0.13) is larger than that to the deposit feeding invertebrates (0.07) due to the biomagniﬁcation effect. In addition, the transfer of 137Cs through food webs for the period of 1945–2020 has been modelled for the Baltic Sea that was essentially contaminated due to global fallout and the Chernobyl accident. The model simulation results obtained with generic parameters are also in good agreement with available measurements in the Baltic Sea. Due to weak water exchange with the North Sea of the semi-enclosed Baltic Sea the chain of water-sediments- biota slowly evolves into a quasi-equilibrium state unlike the processes off the open Paciﬁc Ocean coast where the FDNPP is located. Obtained results demonstrate the importance of the benthic food chain in the long-term transfer of 137Cs from contaminated bottom sediments to marine organisms and the potential of a generic model for the use in different regions of the World Ocean.