Contaminated Marine Sediments As a Source of Cesium Radioisotopes for Benthic Fauna near Fukushima

Marine animals, seawater, and sediment near Fukushima, Japan have become contaminated with Cs-134 and Cs-137 released in March 2011 from the damaged Fukushima Dai-ichi nuclear power plant. Radiocesium concentrations in some benthic fauna declined more slowly than in pelagic fish in the same region. We tested the hypothesis that benthic fish remained more contaminated due to the bioavailability of radiocesium in sediments. Laboratory experiments demonstrated that the assimilation efficiency of Cs-137 was 16% in polychaetes ingesting Fukushima sediment, up to 55% in crabs ingesting polychaetes, and about 80% in fish ingesting worms. In addition, all animals acquired Cs directly from the aqueous phase, but this accounted for only 1.2-2.5% of their total body burden. Thus, diet accounted for nearly all of the total body burden of Cs in these animals. Rate constants of Cs loss from animal tissues were 20% d(-1) for polychaetes, 10% d(-1) for crabs, and 6% d(-1) for fish after acquisition of Cs from water; rate following dietary uptake were 45% d(-1), 14% d(-1) and 5% d(-1) for polychaetes, crabs, and fish, respectively. A bioaccumulation model indicated that the transfer factors of Cs from sediments and the trophic transfer factors from worms to predators were about 1. Overall, sediment-bound Cs is sufficiently bioavailable to deposit-feeding polychaetes, and macrofauna assimilate Cs from these polychaetes to account for >90% of their body burden. This helps to explain the longer retention of Cs in bottom dwelling fish near Fukushima.