Environ. Sci. Technol. 2003, 37, 4597-4602
Global Distribution of Radionuclides (137Cs and 40K) in Marine Mammals R Y U J I Y O S H I T O M E , * ,† T A K A S H I K U N I T O , ‡,§ TOKUTAKA IKEMOTO,‡ SHINSUKE TANABE,‡ HISATAKA ZENKE,† MASANOBU YAMAUCHI,† AND NOBUYUKI MIYAZAKI| Ehime Prefectural Institute of Public Health and Environmental Science, Sanbancho 8-234, Matsuyama, Ehime 790-0003, Japan, Center for Marine Environmental Studies (CMES), Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan, and Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, Akahama, Otsuchi-cho, Iwate 028-1102, Japan
Concentrations of anthropogenic radionuclides were measured in the muscle of marine mammals collected from various locations all over the world, and the global distribution of 137Cs in marine mammals was investigated. 40K was detected in all the specimens of marine mammals with no apparent difference between regions. An anthropogenic radionuclide, 137Cs, was detected in most of the species of marine mammals. With regard to the worldwide distribution of 137Cs, the highest concentration was noticed in the U.K. coast, followed by Lake Baikal, and decreases toward the southern sampling points. A strong positive correlation was observed between 137Cs levels in the muscle of marine mammals and the ambient seawater. Marine mammals feeding on fishes showed a higher concentration factor (CF) for 137Cs than those feeding on cephalopods. To our knowledge, this is the first report on the global distribution of 137Cs and the effect of feeding habits on the CF values of 137Cs in marine mammals.
Introduction Natural and anthropogenic radionuclides occur in the environment. The former, occurring mainly in the earth’s crust, were the only radionuclides on the earth until the first half of 1900s. However, at present, anthropogenic radionuclides have been distributed widely over the world as a result of the fallouts from nuclear tests, accidents at nuclear power plants, etc. Concentrations of anthropogenic radionuclides generally vary between regions, depending on location and magnitude of contamination. The contribution of anthropogenic radioactivity to the surface of the earth is mainly due to global fallout from past nuclear explosions in the * Corresponding author phone: +81-89-931-8757; fax: +81-89934-6466; e-mail:
[email protected]. † Ehime Prefectural Institute of Public Health and Environmental Science. ‡ Ehime University. § Present address: Department of Environmental Sciences, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan. | The University of Tokyo. 10.1021/es030362h CCC: $25.00 Published on Web 09/12/2003
2003 American Chemical Society
atmosphere during 1954-1962. In the Irish and North Seas, concentrations of anthropogenic radionuclides in the marine environment have been largely influenced by discharges from a reprocessing plant in Sellafield, U.K., and the nuclear accident at the Chernobyl nuclear power station has affected the Baltic and Black Seas (1-3). Attention has been focused on their accumulation in ecosystems and transfer through food chain, especially for artificial radionuclide 137Cs because of its long physical half-life and radiotoxicity. Also, 137Cs has been most frequently used in studies of marine processes and radiological assessment on both regional and worldwide scales. 137 Cs concentration in the marine ecosystem is generally lower than that in the terrestrial system (3, 4). Since 137Cs tends to concentrate through the food chain in the marine ecosystem (5-7), marine mammals located at the top of the marine food chain may accumulate large amounts of 137Cs in their tissues. However, studies on top predators such as marine mammals and seabirds are scarce; hence, available information to elucidate the effect of radionuclides on these animals is very little (8, 9). Some reports on local or regional distribution of anthropogenic radionuclides in marine mammals, for example, around U.K. (10) and Greenland (3) are available. However, the worldwide distribution of anthropogenic radionuclides is not well-known. It is reported that marine mammals tend to accumulate 137Cs to very high levels in their muscle rather than in other tissues such as livers and kidneys (10-15). Also, our preliminary study revealed that 137Cs concentration in muscle was 3.7 times higher than in the liver of Caspian seal. Hence, in the present study, we measured 137Cs concentration in the muscle of marine mammals and evaluated its worldwide distribution by referring to other available data. Furthermore, the influence of 137Cs levels in seawater and feeding habits on the 137Cs level in marine mammals was also elucidated.
Materials and Methods Samples. Forty-seven muscle samples of 10 species of marine mammals collected from 1981 to 2000 were used in the present study. Northern fur seals (Callorhinus ursinus; n ) 5) were collected from off the coast of Sanriku, Japan, in 1997; Dall’s porpoises (Phocoenoides dalli; n ) 5) were from Otsuchi-cho, Japan, during 1999-2000; Fraser’s dolphins (Lagenodelphis hosei; n ) 5) were from the Philippines in 1996; ringed seals (Pusa hispida) were from the Kara Sea in 1995 (n ) 5) and from Pangnirtung, Canada, in 1999 (n ) 5); spinner dolphins (Stenella longirostris; n ) 5) were from India during 1990-1991; Blainville’s beaked whale (Mesoplodon densirostris; n ) 1) was from Mauritius in 2000; Weddell seal (Leptonychotes weddellii; n ) 1) was from Antarctic in 1981; Baikal seals (Pusa sibirica; n ) 5) were from the Lake Baikal in 1992; harbor porpoises (Phocoena phocoena; n ) 5) were from the Black Sea in 1993; and Caspian seals (Pusa caspica; n ) 5) were from the Caspian Sea in 1998. The samples used in this study were obtained from three sources: (i) strandings along the coasts, (ii) those accidentally killed by fishermen, and (iii) specimens caught for commercial and scientific purposes from large populations (not including endangered species) under appropriate permits. These samples were stored in a deep-freezer at -20 °C until analysis. Analytical Method. Accurately weighed 50-150 g of sample was dried at 105 °C in a porcelain dish and carbonized without raising the temperature excessively, followed by ashing in a muffle furnace at 450 °C for 24 h. The ashed sample was ground and filled into a plastic measurement VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. 137Cs and 40K Concentrations (Bq/kg wet wt) in Marine Mammals of the Present Study and in Seawater (mBq/L) Cited from the Literaturea
species
location
Weddell seal (Leptonychotes weddellii) spinner dolphin (Stenella longirostris) Baikal seal (Pusa sibirica) harbor porpoise (Phocoena phocoena) ringed seal (Pusa hispida) Fraser’s dolphin (Lagenodelphis hosei) northern fur seal (Callorhinus ursinus) Caspian seal (Pusa caspica) ringed seal (Pusa hispida) Dall’s porpoise (Phocoenoides dalli) Blainville’s beaked whale (Mesoplodon densirostris)
n
year
137Cs mean ( SD (range)
Antarctic
1981
1 nd
India
1990-1991 5 nd
5 14 ( 2 (12-17) Black Sea 1993 5 9.0 ( 2.1 (7.3-11) Kara Sea 1995 5 2.0 ( 0.5 (1.6-2.8) Philippines 1996 5 0.30 ( 0.11 (nd-0.41) off Sanriku, Japan 1997 5 0.21 ( 0.09 (nd-0.31) Caspian Sea 1998 5 2.6 ( 0.8 (1.5-2.8) Pangnirtung, Canada 1999 5 0.21 ( 0.11 (nd-0.40) Otsuchi, Japan 1999-2000 5 0.17 ( 0.05 (nd-0.22) Mauritius 2000 1 0.26 Lake Baikal
1992
40K mean ( SD (range)
56.0 91.0 ( 9.6 (77.0-101) 88.6 ( 7.0 (81.0-100) 86.4 ( 6.0 (77.0-91.0) 91.4 ( 6.8 (84.0-99.0) 106 ( 13 (94.0-127) 99.8 ( 2.6 (96.0-102) 88.8 ( 11.7 (75.0-102) 90.0 ( 5.2 (83.0-96.0) 100 ( 12 (92.0-122) 94.0
137Cs level in seawater
place
year
ref
0.4
Antarctic
1986 35
36
Black Sea
1993 19
5
Kara Sea
1996 24
1.8
SW Pacific Ocean
2000 2
2.5
Miyagi, Japan
1997 b
6
Caspian Sea
1995 25
2.2
NW Atlantic Ocean 2000 2
2.2
Miyagi, Japan
2.2
West Indian Ocean 2000 2
1999 b
a For the samples with the value below limit of detection (LD), half of the LD was substituted to calculate mean. In spinner dolphin from India and Weddel seal from Antarctic, the levels were below LD. b The 137Cs level in seawater was estimated by exponential function from data obtained during 1987-1994 (27).
container under compression, and the container was sealed and weighed. Measurement of γ-ray spectrum at 50-2000 keV energy range was carried out with a semiconductor (highpurity Ge) detector (ORTEC GEM-40190, etc.). Attenuation was adjusted to the value of a sampling day using the following equation:
A0 ) A/e-λt where A0 is the radioactivity adjusted to the value of a sampling day, A is the radioactivity measured, t is the period from sampling to measurement time, and λ is the decay constant. In this study, limit of detection (LD) was defined as three times the standard deviation of the measurements. For the samples with the value below LD, half of the LD was substituted to the value obtained to calculate the mean. For spinner dolphins from India and Weddell seal from Antarctic, the level was below LD in all the specimens. Quality assurance of the analytical results in the present study is achieved through participation in the intercomparison programs organized by Japan Chemical Analysis Center (Chiba, Japan) using agar MXV-01, soil MXV-02, seawater MXV-03, and marine organism MXV-04, demonstrating that the analytical accuracy is better than (5%.
Results and Discussion 40K
and 137Cs were detected in muscle of marine mammals used in this study. The results of all samples used in this study and also from available literature are shown in Tables 1 and 2, respectively. 40K was detected in all the muscle samples (n ) 47), ranging from 56 to 127 Bq/kg wet wt (Table 1). These levels are comparable to those in other studies in which mean 40K level ranged from 71 to 169 Bq/ kg wet wt (Table 2). Comparison of 40K levels among locations (Tables 1 and 2) revealed no apparent regional difference. In contrast, 137Cs, an anthropogenic radionuclide, was not detected in specimens of some sampling locations (Table 1). Considering that the major source of anthropogenic radionuclides is atmospheric nuclear explosions performed in the past, discharges from the Sellafield nuclear fuel reprocessing plant 4598
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(mainly in mid- to late-1970s) and the Chernobyl accident in 1986, the presence of 137Cs in these samples collected a long period after their releases to the environment might be due to the long physical half-life (30.07 yr) and the accumulative tendency of 137Cs in the muscle tissues of animals. It is well-known that 137Cs is readily taken up from food, and that it preferentially distributes in the muscle at a high concentration because of its chemical similarity to potassium (16). Since the biological half-life of 137Cs in the human body is 2-3 months (16), the 137Cs level in the muscle of marine mammals may reflect the exposure level during the recent past. Figure 1 shows the worldwide distribution of 137Cs in marine mammals including the data of the present study (Table 1) and other studies (Table 2). The 137Cs level was high in Europe and decreased toward the south. The highest 137Cs concentration was observed in the muscle of grey seals from the U.K. coast (mean, 17.6 Bq/kg; 17), followed by Baikal seals from Lake Baikal (mean, 14.0 Bq/kg; Table 1) among the locations shown in Figure 1. The extremely high level of 137Cs in the U.K. coast (Figure 1) may be attributed to the discharges from the nuclear fuel reprocessing plant at Sellafield, U.K. The discharge from Sellafield started in 1951, reached the maximum in the midto late-1970s, and then decreased rapidly (3). Indeed, Watson et al. (10) reported that 137Cs concentration was highest near Sellafield and decreased with the distance from Sellafield in the muscle of harbor seals, grey seals, and harbor porpoises stranded at the U.K. coast during 1988-1995. The 137Cs concentration in the muscle of harbor porpoises in the Black Sea (mean, 9.0 Bq/kg wet wt; Table 1) was relatively high and was similar to those of harbor porpoises around the U.K. (6.9 Bq/kg wet wt; 10). The high level in harbor porpoises of the Black Sea despite the fact that these samples were collected in 1993 (7 yr after the Chernobyl accident) suggests that the Black Sea is still contaminated with 137Cs. According to Egorov et al. (18) and Kanivets et al. (19), the water of the Black Sea had been contaminated by anthropogenic radionuclides before the Chernobyl accident, and the 137Cs level of the Black Sea further increased mainly
TABLE 2. 137Cs and 40K Concentrations (Bq kg-1 wet wt) in Muscle of Marine Mammals and in Seawater (mBq L-1) Cited from the Literature
species
location
fin whale (Balaenoptera physalus) harp seal (Pagophilus groenlandicus) common dolphin (Delphinus delphis) spinner dolphin (Stenella longirostris) pantropical spotted dolphin (Stenella attenuata) seal northern fur seal (Callorhinus ursinus) northern fur seal (Callorhinus ursinus) Dall’s porpoise (Phocoenoides dalli) grey seal (Halichoerus grypus) grey seal (Halichoerus grypus) harbor porpoise (Phocoena phocoena) harbor seal (Phoca vitulina) harbor porpoise (Phocoena phocoena) whale marine mammals ringed seal (Pusa hispida) harbor seal (Phoca vitulina) white whale (Delphinapterus leucas) Bowhead whale (Balaena mysticetus) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) Minke whale (Balaenoptera acutorostrata) harp seal (Pagophilus groenlandicus) ringed seal (Pusa hispida) bearded seal (Erignathus barbatus) harbor porpoise (Phocoena phocoena)
Blandford, Canada
harbor porpoise (Phocoena phocoena) harbor porpoise (Phocoena phocoena)
year 1968
n
137Cs mean (range)
40K mean (range)
1.8 (0.3-2.8) Gulf of St. Lawrence 1968 11 1.1 (0.3-1.6) Eastern Tropical 1977-1983 8 0.39 Pacific Ocean Eastern Tropical 1977-1983 7 0.44 Pacific Ocean Eastern Tropical 1977-1983 37 0.52 Pacific Ocean Barents Sea 1980 1 0.33 off Sanriku, Japan 1984 1 0.37 1984
2
Hokkaido, Japan
1984-1997 2
U.K. coast
1987
U.K. coast
1988-1995 19
U.K. coast
1988-1995 30
U.K. coast
1988-1995 19
British and Irish coastal waters Greenland Northern Canada North Alaska and Resolute, Canada North Alaska
1989-1993 25
132
11b
6.5
ref
118
11b
6.5
130
11b
6.5
31b 12
Eastern Tropical Pacific Ocean Eastern Tropical Pacific Ocean Eastern Tropical Pacific Ocean
1977-1983 c
4.4
Miyagi, Japan
1984
27
23
3.0
Hokkaido, Japan
1996
23
17 10
25 50 8.4
Firth of Forth 1987 5 West Coast of Scotland 1987 5 U.K. coast 1988-1995 d
10
21.6
U.K. coast
1988-1995 d
10
11.3
U.K. coast
1988-1995 d
5
50.5
Irish Sea
1993
3 3 14
2
Bering, Chukchi and Beaufort Seas Bering, Chukchi and Beaufort Seas Bering, Chukchi and Beaufort Seas Bering, Chukchi and Beaufort Seas
1994-1995 e
1977-1983 c 1977-1983 c
12
1990-1994 1990-1994 1995-1997 11 1996
0.34
14
2
1
year
13a
0.31 (0.25-0.37) 0.19 (0.15-0.23) 17.6 (11.08-27.5) 2.7 (nd-57.2) 6.9 (nd-66.6) 3.6 (nd-13.3) 7.0 (
