Abstract. The horizontal distribution of radioactive cesium (Cs) derived from the Fukushima Dai-ichi nuclear power plant (FNPP) in the North Pacific is still unclear due to the limitation of direct measurement of the seawater in the open ocean. We present the result of direct observation of radioactive Cs in surface seawater collected from a broad area in the western and central North Pacific in July 2011, October 2011 and July 2012. We also conducted a simple particle tracking experiment to estimate the qualitative spatial distribution of radioactive Cs in the North Pacific. 134Cs was detected at 94 stations out of 123 stations, and 137Cs was detected at all stations. High 134Cs and 137Cs concentrations more than 10 m Bq kg−1 were observed in the area of the northern part of Kuroshio Extension at 144° E and 155° E in July 2011, in the area 147–175° E around 40° N in October 2011, and the northern part of Kuroshio Extension at 155° E and 175°30´ E in July 2012. Combining the result of direct observations and particle tracking experiment, the radioactive Cs derived from the FNPP had been dispersed eastward to the central North Pacific during 2011. It was considered from the horizontal distribution that radioactive Cs was dispersed not only eastward but also north- and southward in the central North Pacific. Pronounced dilution process of radioactive Cs from the FNPP during study period is suggested from temporal change in the activity ratio of 134Cs / 137Cs, which was decay-corrected on 6 April 2011, and relationships between radioactive Cs and temperature.

No abstract

Food sources for the deposit-feeding gastropods Assiminea japonica and Angustassiminea castanea (Gastropoda: Assimineidae) were estimated using stable carbon and nitrogen isotope ratios. We collected animals and potential food materials in reed marshes of the Nanakita River estuary, the eastern part of Miyagi Prefecture, Japan. Feeding experiments were also conducted to confirm whether snails assimilate 4 types of diets. The stable carbon isotope ratios of both assimineid species (mean ± 1 SD: A. japonica, -20.7 ± 0.3 ‰; A. castanea, -19.8 ± 0.5 ‰) were closer to that of deposited organic matter from lagoon water (-20.7 ± 0.3 ‰) than to those of reed litter (-25.4 ± 0.1 ‰) and the surface soil of the reed marsh (-26.3 ± 0.1 ‰). The snails that were fed deposited organic matter showed δ 13 C values similar to the control animals before feeding experiments for both species. The δ 13 C values of the snails fed litter or soil diet, however, revealed that these snails were able to assimilate organic matter from reed detritus under laboratory conditions. These findings suggest that the salt marsh snails utilized mainly deposited organic matter from lagoon water in the field. Microalgae such as phytoplankton and benthic diatoms in deposited matter are considered to be important food sources for 2 species of assimineids inhabiting salt marshes of the Nanakita River estuary.

No abstract

The horizontal distribution of radioactive cesium (Cs) derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP) in the North Pacific is still unclear due to the limitation of direct measurement of the seawater in the open ocean. We present the result of direct observation of radioactive Cs in surface seawater collected from broad area in the western and central North Pacific in July, October 2011 and July 2012. We also conducted a simple particle tracking experiment to estimate the qualitative spatial distribution of radioactive Cs in the North Pacific. ¹³⁴Cs were detected at 94 stations out of 123 stations and ¹³⁷Cs was detected at all stations. The high ¹³⁴Cs and ¹³⁷Cs concentrations more than 10 mBq kg⁻¹ were observed in the area where the northern part of Kuroshio extension at 144° E and 155° E in July 2011, in the area 147° E–175° E around 40° N in October 2011, and the northern part of Kuroshio extension at 155° E and 175° 30´ E in July 2012. Combining the result of direct observations and particle tracking experiment, the radioactive Cs derived from FNPP had been dispersed eastward to the central North Pacific during 2011. It was considered from the horizontal distribution that radioactive Cs was dispersed not only eastward but also north- and southward in the central North Pacific. Pronounced dilution process of radioactive Cs from FNPP during study period is suggested from temporal change in the activity ratio of ¹³⁴Cs/¹³⁷Cs which was decay corrected at 6 April 2011, and relationships between radioactive Cs and temperature