Article pubs.acs.org/est
Ecological Half-Lives of Radiocesium in 16 Species in Marine Biota after the TEPCO’s Fukushima Daiichi Nuclear Power Plant Accident Kayoko Iwata,* Keiko Tagami, and Shigeo Uchida Office of Biospheric Assessment for Waste Disposal, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan S Supporting Information *
ABSTRACT: TEPCO’s Fukushima Daiichi Nuclear Power Plant (FDNPP) accident of March 2011 caused the discharge of a considerable quantity of radionuclides, including 137Cs. Because of its long half-life (30.17 years), the fate of 137Cs in the marine biota is of great interest. This study aims to evaluate, using food monitoring data, ecological half-lives (Teco) of 137Cs in marine biota caught offshore of Fukushima. The data were categorized into two regional groups (north and south) with respect to the FDNPP site, and the regional 137Cs concentration trend and estimated Teco in the marine biota were appraised. Although the 137 Cs concentration in the seawater in the south was higher than that in the north, Teco values remained relatively consistent among common species of both regions. Teco was then compared to biological half-life (Tb) estimated in laboratory settings. The ratios of Teco/Tb were inconsistent among different groups of marine species. The ratios of Teco/Tb for brown seaweed and bivalves were approximately 1, and the ratios of Teco/Tb for demersal fish ranged from 4.4 to 16.1. The reasons for different ratios of Teco and Tb values may be attributed to environmental and ecological factors, such as different trophic levels.
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of 137Cs in marine products are of both ecological and human health concern, but there is little information available about which consumed species are most influenced by the initial contamination. Because marine products play a major role in the traditional Japanese diet, an analysis of reported 137Cs concentrations in marine products as a function of sampling date following the accident was undertaken. To quantify the biological elimination process of 137Cs, the biological half-life (Tb) of 137Cs has been studied in the laboratory settings.12,13 Such investigations have usually been carried out using single species. However, only a limited number of studies have focused on the measurement of effective half-lives of 137Cs in marine biota, using field data. Because laboratory experiments cannot replicate the existing ecological and environmental conditions, analysis of field data is imperative; hence, such studies are now necessary to understand the fate of long-lived 137Cs in the marine ecosystems in the environs of the FDNPP. The purpose of this study is to initiate such analysis using existing data collected in the aftermath of the accident and to bring to light the tendencies for the fate of 137Cs in marine populations and effects of ecological factors.
INTRODUCTION The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident of March 11, 2011 was the result of the Great East Japan Earthquake, a magnitude 9.0 earthquake, and the resultant tsunami, which severely damaged several reactors. Damage was especially severe at reactors 1−4. Hydrogen explosions occurred at three of those reactors. Venting of gases, explosions, and other events occurred during the course of the incident and caused atmospheric discharges of radionuclides.1 One of the major discharged radionuclides was 137Cs (T1/2 = 30.17 years). The total reported amount of 137Cs discharged to the atmosphere was approximately 8.8−15 PBq.2−5 The total amount of 137Cs directly discharged into the sea was estimated to be approximately 3.5 PBq.6 Because of its long half-life, high release amounts, and high solubility in the seawater, the fate in the ocean ecosystems of 137Cs is of great interest. Although there were intermittent winds blowing the atmospheric plume north,2 137Cs deposited from the plume to the surface of the ocean initially spread southward because of the weak current flowing southward along the coastline of Fukushima Prefecture.6,7 Later, 137Cs was spread to the Pacific Ocean by the Kuroshio Current, which flows eastward along the south coast of Japan.5 Consequently, higher 137Cs concentrations in the ocean surface8,9 and sediments10 were detected in the area to the south of the FDNPP site. Furthermore, 137Cs contamination in marine life has been detected after the FDNPP accident.6,9,11 Concentration trends © XXXX American Chemical Society
Received: January 31, 2013 Revised: May 12, 2013 Accepted: June 14, 2013
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dx.doi.org/10.1021/es400491b | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Our study reports on regional 137Cs dispersion trends in marine biota caught offshore of Fukushima Prefecture and ecological half-lives (Teco) in selected species. Furthermore, it aims to show the trends of 137Cs accumulation in the local marine biota and the importance of considering Teco by comparing and contrasting field data (Teco) with laboratory data (Tb).
Article
MATERIALS AND METHODS
Data Sources and Sampling Locations. The data on the 137 Cs concentration in edible marine biota caught offshore of Fukushima Prefecture were taken from open-sourced, governmental food monitoring programs designed to ensure safe food distribution for consumers.11,20 The data were provided on the website of Ministry of Health, Labour and Welfare of Japan (MHLW)20 and Fukushima Prefecture.11 The marine products were caught by the local fishermen and tested for 137Cs concentrations by the technical staff of the prefecture.11 Although 137Cs concentrations only in edible parts are reported, the major portion of Cs taken from the food source was found in the edible parts, mainly muscle, which occupies the large proportion of the whole body.16 In the present study, collected data were categorized into two groups based on the relative sampling areas with respect to the FDNPP site: the northern (Shinchi-cho, Soma-shi, and Minami Soma-shi) and southern (Iwaki-shi) parts. Marine products caught in the Pacific Ocean within an area bounded on the north and south by the prefectural borderlines and extending to the east for approximately 370 km from the coastline of Fukushima were included in the sampling21 (see Figure S1 of the Supporting Information). For marine products caught in this defined coastal area, it was assumed that the samples were collected in the area close to the home city of the fishing fleet based on the sampling maps provided on the website of Fukushima Prefecture.22 Data Selection. By the end of December 2012, 137Cs concentration data on a total of about 170 edible marine species have been reported in the two databases. In this study, the sampling period was 651 days after March 11, 2011 (i.e., an end point of December 21, 2012), and the sampling interval was random and not consistent among different species. Therefore, the species selection to calculate Teco was biased to those species with a relatively large number of samples and fairly continuous sampling data over the sampling period. When there were small portions of data with a large time break from the majority of the data on the plot, those data were omitted because the trend during the break is unclear. For instance, data of 137Cs concentrations in sand crab after the 400th day were excluded because the majority of data were within the first 200 days. In addition, measured values below detection limits were excluded from the data presentation. In the end, 16 species were selected from about 170 species for this study. All 16 selected species were collected in the southern part, as compared to 11 species in the northern part. In both regions, sand crab, ishikawashirauo (Japanese icefish), Japanese sand lance, white rockfish, Japanese jack mackerel, Japanese sea perch, flatfish (bastard halibut), common skate, marbled sole, greenling, and brown hakeling were sampled; in the south region, the following additional species were sampled: arame, abalone, Sakhalin surf clam, northern sea urchin, and whitebait. The sampled species represent several trophic levels. The trophic levels included primary producers and primary and secondary consumers (plankton feeders and benthic invertebrate/fish feeders): primary producer, arame (brown algae); primary consumers, abalone, Sakhalin surf clam, and northern sea urchin; plankton feeders, ishikawashirauo, whitebait, and Japanese sand lance; and benthic invertebrate/fish feeders, sand crab, white rockfish, Japanese jack mackerel, Japanese sea perch,
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DEFINITIONS The time required for the physical decay of the 50% of certain radionuclides (RNs) is called the physical half-life (T1/2). Similarly, the time required for the biological elimination of 50% of RN from a living body, including plants and animals, is called the biological half-life (Tb). When RN is actually taken into a living body, both physical decay and biological elimination occur together; therefore, the term “effective halflife”, which takes into account both T1/2 and Tb, was coined. Furthermore, when the RN contamination occurs in the environment, it is important to investigate how long the RN persists in a population of a certain species in the biota. For such cases, the term “ecological half-life” (Teco) has been used.14 In this study, the term Teco is used to describe the time required for a 50% decline of RN in a population in a natural ecosystem. Our study focuses on the rate of 137Cs loss in each population of multiple species and also the comparisons between Teco and Tb. Teco is estimated by ln 2 λeco−1, where λeco is the ecological loss rate in RN activity in a population and is obtained from the slope of exponential decline in 137Cs in the population over time.14,15 The rate is influenced by ecological factors, including both abiotic and biotic factors. For example, those could be ambient temperature, salinity (if in an aquatic environment), stages of growth, sizes, feed selections, range of contaminated area, and migration range of a population.16 Moreover, the total amount and the duration of RN intake are generally uncertain in a field contamination situation. Consequently, the rates of intake and elimination of RN could fluctuate at the individual level. However, because Teco is estimated on the basis of random sampling of a population, which thereby factors in all of the known and unknown ecological factors, it likely shows the specific trend of the decline in RN in a population of a selected biota. In addition, Teco typically has short- and longterm components.17 The first component likely reflects the biological elimination rate of the peak concentration of RN, while the second component likely reflects the biogeochemical cycles of the RN in the ecosystem; the RN concentration eventually reaches a dynamic equilibrium between organisms and the ecosystem and also decreases as a result of physical decay.14,17 Both short- and long-term components would be affected by the ecological niche of the species.14 On the other hand, the biological elimination rates of RN activities of a species are estimated under the controlled experimental settings. RNs are fed to sample organisms in water and/or a digestible form up to the approximate equilibrium in the tissue of the organisms. Then, organisms are transferred to an uncontaminated environment to monitor the RN loss rate(s) without additional RN feeding.18,19 Tb shows the general ability of each species at a uniform stage of growth to eliminate RN by biological processes under fixed experimental conditions. B
dx.doi.org/10.1021/es400491b | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 1. 137Cs activity (Bq/kg) trend over time in edible part of marine species collected offshore of Fukushima Prefecture after the FDNPP accident/the Great East Japan Earthquake (March 11, 2011).
flatfish, common skate, marbled sole, greenling, and brown hakeling. Teco Calculation. On the basis of previous studies, there is likely a two-component exponential decline in the 137Cs concentration in biota after contamination of its environment occurred. The first component shows a relatively fast decline over a short period of time, and the second component shows a relatively slow and long-term decline of 137Cs in an aquatic population of each species in the biota. Previous studies following the Chernobyl accident showed that the first component lasted approximately for 3−5 years for 137Cs in lake fish.14,17 This time frame may differ to some extent in marine systems; however, it is probable that two-component exponential decline also occurs in marine systems. A large portion of Cs is removed from surface water by physical processes, such as hydraulic dilution and sedimentation, in lake systems in the beginning.14 Similarly, the initial concentration is significantly decreased by advection in marine systems. As a consequence, the Cs concentration eventually reaches a dynamic equilibrium between organisms and the ecosystem in marine systems as well.23 Becausee our studied monitoring period is less than 2 years and a dynamic equilibrium is unlikely to be completely attained because of the rapid decline in 137Cs in both organisms and the environment, a single-component decay function was applied.14 Using the concentration data, a best fit exponential trend line for each species in a sampling area was computed as y = Ao exp(−λx), using KaleidaGraph software, where Ao was the original activity, λ was the loss-rate constant for 137Cs of a particular species, and x was the sampling day after March 11, 2011.15 The correlation between the 137Cs activity (after converted to log scale) and time was tested, and the probability of the correlation (p values) was obtained, using KaleidaGraph software. The ecological half-life of 137Cs in a population of a species, Teco, was estimated for each species as Teco = ln 2 λeco−1.
Regional Comparisons. Regional comparisons of 137Cs concentrations in marine organisms were carried out to assess the regional differences in the 137Cs concentration in each population. A total of 11 species were included in the comparison. Using the trend line calculated above, the 137Cs concentrations in each population on the 100th day, an arbitrarily selected point of comparison, were determined. Then, a comparison of Teco for each common species of two regions was performed to show whether Teco varies regionally within the studied biota. Furthermore, Teco and Tb from refs 16, 24, and 25 were compared among organisms of similar groups, categorized mainly as seaweeds, crustacean, and demersal (bottom-feeding) fish. Teco of arame (a type of kelp) and Tb of Fucus (brown algae) for brown seaweed and Teco of Sakhalin surf clam and Tb of scallop for crustacean (bivalves) were compared. For demersal fish, Teco of marbled sole and Tb of plaice and Teco and Tb of flatfish were compared separately. A comparison between Teco and Tb may help to show the trends of 137Cs accumulation and loss in the local marine biota caught offshore of Fukushima Prefecture. For example, a Teco/Tb ratio larger than 1.0 would show that ecological factors are extending apparent residence time of 137Cs in the species by either increasing intake and/or uptake rate, restricting the metabolic elimination efficiency, or possibly both.
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RESULTS AND DISCUSSION The collected data for each species of each region were plotted on a logarithmic scale, and some examples of 137Cs activity (Bq kg−1 of fresh mass) trends over time in marine species are shown in Figure 1. These selected species showed a decreasing trend in 137Cs concentrations with time. The best fit exponential trend line for each species of each region was also plotted, and the correlation coefficient (r) and the corresponding probability (p) of the computed exponential lines are shown in Table 1. Most of the selected populations C
dx.doi.org/10.1021/es400491b | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
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Table 1. Sample Details and Ecological Half-Lives (Teco) of 137Cs for Each Species Caught in the Offshore Waters of Fukushima species
scientific name
arame
Eisenia bicyclis
abalone
Haliotis sorenseni
Sakhalin surf clam
Pseudocardium sachalinesnse
northern sea urchin
Strngylocentrotus nudus
sand crab
Ovalipes punctatus
ishikawashirauo (Japanese icefish)
Salangichthys ishikawae
whitebait (Japanese anchovy)
Engraulis japonicus
Japanese sand lance
Ammodytes personatus
white rockfish
Sebastes cheni
Japanese jack mackerel
Trachurus japonicus
Japanese seaperch
Lateolabrax japonicus
flatfish/bastard halibut
Paralichthys olivaceus
Japanese common skate
Okamejei kenojei
marbled sole
Pseudopleuronectes yokohamae
greenling
Hexagrammos otakii
brown hakeling
Physiculus maximowiczi
areaa
number of samples
correlation coefficient
probability
Teco (days)b
south north south north south north south north south north south north south north south north south north south north south north south north south north south north south north south north
19 NDc 25 ND 46 ND 60 ND 4 4 23 10 39 ND 7 17 60 35 24 23 49 62 223 222 120 100 113 139 141 157 86 54
−0.854
