Horizontal distribution of Fukushima-derived radiocesium in zooplankton in the northwestern Pacific Ocean

Abstract

The magnitude of the 9.0 Tohoku earthquake and the ensuing tsunami on 11 March 2011, inflicted heavy damage on the Fukushima Dai-ichi nuclear power plant (FNPP1). Fission products were emitted, falling over a broad range in the northern hemisphere, and water contaminated with radionuclides leaked into the ocean. In this study, we described the horizontal distribution of the Fukushima-derived radiocesium in zooplankton and in seawater in the western North Pacific Ocean (500–2100 km from the FNPP1) 10 months after the accident. ¹³⁴Cs and ¹³⁷Cs were detected in zooplankton and seawater from all the stations. Because of its short half-lives, ¹³⁴Cs detected in our samples could only be derived from the FNPP1 accident. The highest ¹³⁷Cs activity in zooplankton was same order of magnitude as that one month after the accident, and average activity was one or two orders of magnitude higher than ¹³⁷Cs activities observed before the accident around Japan. Horizontally, the radiocesium activity concentrations in zooplankton were high at around 25° N while those in surface seawater were high at around the transition area between the Kuroshio and the Oyashio Currents (36–40° N). We observed subsurface radiocesium maxima in density range of the North Pacific Subtropical Mode Water and occurrence of many diel vertical migratory zooplanktons. These suggested that the high activity concentrations in the subtropical zooplankton at around 25° N were connected to the subsurface radiocesium and active vertical migration of zooplankton. However, the high activity concentrations of radiocesium in subsurface seawater did not necessarily follow the higher radiocesium activity in zooplankton. Biological characteristics of zooplankton community possibly influenced how large was contamination of radiocesium in the community but it is still unknown what kind of biological factors were important

Intrusion of Fukushima-derived radiocaesium into subsurface water due to formation of mode waters in the North Pacific

“…North of 38 °N along the 155 °E transect, the FNPP-derived radiocaesium core was within CMW. These findings are consistent with those of previous studies 25 26 27 , which have reported the detection of 134 Cs in STMW around 149 °E in the winter of 2012, in STMW at 137 °E during April of 2012 and March of 2013 24 , and in CMW at 165 °E in June of 2012 30 . Concentrations of 137 Cs in the subsurface peaks in this and previous studies have also been roughly comparable, ranging between 5 and 15 Bq m −3 ( Fig.…”

“…Concentrations of 137 Cs in the subsurface peaks in this and previous studies have also been roughly comparable, ranging between 5 and 15 Bq m −3 ( Fig. 3 ) 24 25 26 27 28 30 .…”

“…Our observations showed that a large amount of FNPP-derived radiocaesium existed in the subtropical region south of the KE 19 months after the FNPP accident. Subsurface peaks of FNPP-derived radiocaesium associated with STMW have also been reported during December 2011 and February 2012 on a transect along 149 °E 27 , in December 2011 at 20–30 °N on a transect along 135 °E 26 , between August 2011 and March 2013 far south of the Japanese islands around 138 °E 24 , and in June 2012 off the coast of Taiwan 29 . FNPP-derived radiocaesium has also been reported in CMW in the transitional region in June 2012 30 .…”

“…The newly formed STMW can reach southern part of 137 °E section half a year after the formation 33 . The subsurface peaks of FNPP-derived radiocaesium were also located in the STMW and CMW 24 25 26 27 28 29 30 . Preliminary estimates of the total amount of radiocaesium trapped in the STMW are 6 PBq during March and April 2011 based on observations from December 2011 to February 2012 27 , and an additional 5 PBq entered prior to April 2012 during the winter of 2011–2012 based on observations during September 2012 24 .…”

“…However, there was atmospheric deposition of radiocaesium onto the area south of the KE 17 22 23 . Some papers have reported dispersion of FNPP-derived radiocaesium in subsurface waters south of the KE, but the data are limited spatially and temporally 24 25 26 27 28 29 30 . Our previous study documented southward transport of FNPP-derived radiocaesium as far as 18 °N, 135 °E in subsurface waters by September 2012, 18 months after the FNPP accident 24 .…”