Radiocesium Transfer in Forest Insect Communities after the Fukushima Dai-ichi Nuclear Power Plant Accident

Estimation of spatio-temporal distribution of 137Cs concentrations in litter layer of forest ecosystems in Fukushima using FoRothCs model

The nuclear power plant accident in Fukushima had led to pollution of forest ecosystems with ¹³⁷Cs in 2011. In this study, we simulated the spatiotemporal distribution of ¹³⁷Cs concentrations of litter layer in the contaminated forest ecosystems in two decades from 2011, which is one of the key environmental components of ¹³⁷Cs migration in the environment due to the high bioavailability of ¹³⁷Cs in the litter. Our simulations showed that ¹³⁷Cs deposition is the most important factor in the degree of contamination of the litter layer but vegetation type (evergreen coniferous/deciduous broadleaf) and mean annual temperature are also important for changes over time. Deciduous broadleaf trees had higher initial concentrations in the litter layer due to the direct initial deposition on the forest floor. However, the concentrations remained higher than those in evergreen conifers after 10 years due to redistribution of ¹³⁷Cs by vegetation. Moreover, areas with lower average annual temperatures and lower litter decomposition activity retained higher ¹³⁷Cs concentrations in the litter layer. The results of the spatiotemporal distribution estimation of the radioecological model suggest that, in addition to ¹³⁷Cs deposition, elevation and vegetation distribution should also be considered in the long-term management of contaminated watersheds, which can be informative in identifying hotspots of ¹³⁷Cs contamination on a long-term scale.

Imaging Plate Autoradiography for Ingested Anthropogenic Cesium-137 in Butterfly Bodies: Implications for the Biological Impacts of the Fukushima Nuclear Accident

The Fukushima nuclear accident in March 2011 caused biological impacts on the pale grass blue butterfly Zizeeria maha. At least some of the impacts are likely mediated by the host plant, resulting in "field effects". However, to obtain the whole picture of the impacts, direct exposure effects should also be evaluated. Here, we examined the distribution of experimentally ingested anthropogenic cesium-137 (¹³⁷Cs) in adult butterfly bodies using imaging plate autoradiography. We showed that ¹³⁷Cs ingested by larvae was incorporated into adult bodies and was biased to females, although the majority of ingested ¹³⁷Cs was excreted in the pupal cuticle and excretory material during eclosion. ¹³⁷Cs accumulation in adult bodies was the highest in the abdomen, followed by the thorax and other organs. These results suggest that ¹³⁷Cs accumulation in reproductive organs may cause adverse transgenerational or maternal effects mediated by reactive oxygen species (ROS) on germ cells. ¹³⁷Cs accumulation was detected in field individuals collected in September 2011 and September 2016 but not in May 2011, which is consistent with the abnormality dynamics known from previous studies. Taken together, these results contribute to an integrative understanding of the multifaceted biological effects of the Fukushima nuclear accident in the field.

Transfer of elements from soil to earthworms and ground beetles in boreal forest

Data on the transfer of elements (such as heavy metals) and their radionuclides into organisms is needed for assessing environmental risks. The current data on many elements, species and environments is limited, but more information can be obtained both from field studies and experimental laboratory studies. However, it is essential to evaluate whether experimental studies adequately predict transfer in natural conditions. Moreover, because of the sparsity of species-specific empirical data, it is a common practice in current radioecological modelling to use data available for related species under the assumption that transfer into organisms is similar within broader taxonomic groups. Earthworms and ground beetles are examples of important invertebrates living near soil surface in terrestrial ecosystems. In this study, the transfer of 34 elements from soil to these organisms was studied in a field study conducted in boreal forest. The earthworm concentrations were compared to the values obtained in an experimental mesocosm study using soil from the field site and were found to be highly correlated. This indicates that the results of mesocosm studies can be used for predicting the transfer of elements from soil to fauna in natural conditions. Furthermore, concentrations in individual earthworm and beetle species were found to be similar to those observed in broader groups of related species, indicating that the generic approach used in current radioecological models may be useful for predicting uptake of elements into single species.

Trophic food chain transfer of radiocaesium from reindeer meat to the blowfly Calliphora vicina and the parasitoid wasp Nasonia vitripennis

The role of insects in the transfer of radionuclides has received little attention, despite their key role in nutrient cycling within ecosystems. In this study, we investigated the trophic food chain transfer of radiocaesium (¹³⁷Cs) from reindeer meat to the blowfly Calliphora vicina (Robineau-Desvoidy) (Diptera: Calliphoridae) and further from blowfly pupae to the parasitoid wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae). Radiocaesium was transferred to blowfly larvae during their feeding stage, with the highest whole-organism to food source concentration ratios (CR_wo-fs) being found in actively feeding third instar blowfly larvae, with CR_wo-fs in the range of 0.68-0.90. CR_wo-fs in blowfly larvae at later developmental stages (i.e., post-feeding stage, prepupal stage, pupal stage, and adult stage) were significantly lower, with CR_wo-fs in adult flies in the range of 0.07-0.10. Modelling of the data indicated that >70 % of the radiocaesium present in third instar feeding stage larvae was removed exponentially with excreta prior to pupariation. Furthermore, on average 49 % of the radiocaesium activity concentration assimilated in the metamorphing blowfly pupa was retained in the pupal case after the adult blowfly had emerged. When blowfly pupae were parasitised by parasitoid wasps, a greater proportion of radiocaesium was retained in the parasitised pupal case, with only 2-3 % of the activity concentration in the meat being found in the wasps, providing further evidence to support that radiocaesium is biodiluted in insect food chains. Our results indicate that carcass decomposition driven by insects directly affects the fate of radiocaesium retained in vertebrate carcasses and we discuss these findings in connection to radiocaesium cycling and dispersion in terrestrial ecosystems.

Factors affecting 137Cs radioactivity and water-to-body concentration ratios of fish in river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant

To elucidate ¹³⁷Cs contamination levels and mechanisms of fish inhabiting river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant, ¹³⁷Cs activity concentrations in fish (15 species, n = 164) and water collected from Maeda River (3.3-8.9 km from the plant) and Shimofukazawa Pond (2.9 km) in 2017 were analyzed. Also, an 8-week rearing experiment using Japanese dace Pseudaspius hakonensis fed on non-contaminated pellets and the pond water (mean ¹³⁷Cs concentration of 2.0 Bq/L) was conducted to evaluate ¹³⁷Cs accumulation from water to fish. The ¹³⁷Cs concentrations in Japanese dace, the only species collected throughout five sampling sites from estuarine to upstream areas in Maeda River, were found to be correlated with ambient air dose rates and fish size, exhibiting large variations (16.5-2.6×10³ Bq/kg-wet). By contrast, dissolved ¹³⁷Cs in river waters increased from the upper to lower course (0.025-0.28 Bq/L), which caused large variations of the water-to-body concentration ratio (CR) in Japanese dace (60.0-35700 L/kg-wet). These CRs (geometric mean of 3670 L/kg-wet) were much higher than the steady-state CR of reared fish (9.7 L/kg-wet), indicating that river fish uptake ¹³⁷Cs mainly from prey items from aquatic and riparian zones, rather than from water. Statistically significant negative correlations between K⁺ concentrations in water and river fish CRs were detected, resulting in the decreasing trend of CRs from upstream to estuarine areas. These results suggest that the large heterogeneity of air dose rates, K⁺ concentration, and estuarine processes in brackish water habitats, in association with the feeding habit and size effect in fish, can engender wide variation of ¹³⁷Cs concentrations and CRs of river fish along a river course. In contrast, ¹³⁷Cs concentrations in pond fish (4.3-14.6 kBq/kg-wet) were higher than in river fish. The CRs of pond fish were constantly high but the range was smaller (1010-3440 L/kg-wet) with larger values in fish of higher trophic levels. These findings suggest that biomagnification within a pond was inferred as the main cause of ¹³⁷Cs contamination of pond fish.

The second decade of the blue butterfly in Fukushima: Untangling the ecological field effects after the Fukushima nuclear accident

Many field observations of the biological effects of the Fukushima nuclear accident have been reported in the first decade after the accident. A series of observational and experimental studies have demonstrated causal adverse effects on the pale grass blue butterfly even at the low-level radiation exposure in the "field," contrary to the dosimetric view that insects are generally tolerant of radiation exposure. However, it has been demonstrated that the pale grass blue butterfly is tolerant of high oral doses of anthropogenic radioactive cesium (¹³⁷ Cs) under "laboratory" conditions. This field-laboratory paradox can be explained by ecological field effects; for example, radiation stress in the field causes physiological and biochemical changes in the host plant, which then trophically affects butterfly larvae. The second decade of butterfly-based Fukushima research will be devoted to demonstrating how such adverse field effects occur. Changes in the host plant's nutritional contents likely affect butterfly physiology. The host plant may also upregulate secondary metabolites that affect herbivorous insects. The plant may be affected by changes in endophytic soil microbes in radioactively contaminated areas. If demonstrated, these results will reveal that the delicate ecological balances among the butterfly, its host plant, and soil microbes have been affected by radioactive pollution in Fukushima, which has important implications for environmental policies and human health. Integr Environ Assess Manag 2022;18:1539-1550. © SETAC.

Monitoring of radioactive cesium in wild boars captured inside the difficult-to-return zone in Fukushima Prefecture over a 5-year period

Following the Fukushima Daiichi Nuclear Power Plant accident in 2011, tissue samples from wild boar (Sus scrofa) outside the evacuation zone (difficult-to-return zone, DRZ) tended to show high activity concentrations of cesium-137 (¹³⁷Cs). Understanding the ¹³⁷Cs dynamics of wild boar populations inside the DRZ is necessary because they affect ¹³⁷Cs dynamics and wild boar management in areas outside the DRZ. Since few detailed, long-term studies have been conducted inside the DRZ, we measured ¹³⁷Cs activity concentrations in 221 wild boar muscle samples obtained from wild boar caught inside the DRZ and surrounding areas over a 5-year period. Our results showed that the ¹³⁷Cs activity concentration in wild boar from inside the DRZ were higher than those in wild boar outside this zone. No significant difference was observed between muscle and soil ¹³⁷Cs levels, but significant correlations were observed between muscle ¹³⁷Cs activity concentrations and body length and body weight in the low-activity-concentration season, but not between all seasons and the high-activity-concentration seasons. It is considered that the size effects observed during the low-activity-concentration season may be due to factors related to metabolism and changes in food habit. This is the first long-term survey of ¹³⁷Cs in wild boar inside the DRZ.

Developing field-scale, gentle remediation options for nuclear sites contaminated with 137Cs and 90Sr: The role of Nature-Based Solutions

The remediation of contaminated land using plants, bacteria and fungi has been widely examined, especially in laboratory or greenhouse systems where conditions are precisely controlled. However, in real systems at the field scale conditions are much more variable and often produce different outcomes, which must be fully examined if 'gentle remediation options', or GROs, are to be more widely implemented, and their associated benefits (beyond risk-management) realized. These secondary benefits can be significant if GROs are applied correctly, and can include significant biodiversity enhancements. Here, we assess recent developments in the field-scale application of GROs for the remediation of two model contaminants for nuclear site remediation (⁹⁰Sr and ¹³⁷Cs), their risk management efficiency, directions for future application and research, and barriers to their further implementation at scale. We also discuss how wider benefits, such as biodiversity enhancements, water filtration etc. can be maximized at the field-scale by intelligent application of these approaches.

Radiocaesium accumulation and fluctuating asymmetry in the Japanese mitten crab, Eriocheir japonica, along a gradient of radionuclide contamination at Fukushima

The 2011 Tohoku earthquake-tsunami and the subsequent nuclear accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) led to large-scale radionuclide contamination of the marine and freshwater environment. Monitoring studies of marine food products in the Fukushima region have generally demonstrated a declining trend in radiocaesium concentrations. However, the accumulation and elimination of radiocaesium and potential biological effects remain poorly understood for freshwater biota inhabiting highly contaminated areas at Fukushima. Consequently, the present study aimed to assess radiocaesium accumulation and developmental effects on the commercially important catadromous Japanese mitten crab, Eriocheir japonica. E. japonica were collected from four sites along a gradient of radionuclide contamination 4-44 km in distance from the FDNPS in 2017. To determine potential developmental effects, fluctuating asymmetry (FA) was used as a measure of developmental stability. Combined ¹³⁴Cs and ¹³⁷Cs values for whole E. japonica from highly contaminated sites 4 and 16 km in distance from the FDNPS were 3040 ± 521 and 2250 ± 908 Bq kg^-1 wet weight respectively, 30 and 22 times greater than the Japanese standard limit of 100 Bq kg^-1. Estimated total dose rates based on radiocaesium concentrations in whole crabs and sediment ranged from 0.016 to 37.7 μGy h^-1. No significant relationship between radiocaesium accumulation and FA was recorded, suggesting that chronic radiation exposure at Fukushima is not inducing developmental effects in E. japonica as measured using fluctuating asymmetry. Furthermore, estimated dose rates were below proposed regulatory limits where significant deleterious effects are expected. The present study will aid in the understanding of the long-term consequences of radiation exposure for non-human biota and the management of radioactively contaminated environments.

Untangling radiocesium dynamics of forest-stream ecosystems: A review of Fukushima studies in the decade after the accident

Forest-stream ecosystems are widespread and biodiverse terrestrial landscapes with physical and social connections to downstream human activities. After radiocesium is introduced into these ecosystems, various material flows cause its accumulation or dispersal. We review studies conducted in the decade after the Fukushima nuclear accident to clarify the mechanisms of radiocesium transfer within ecosystems and to downstream areas through biological, hydrological, and geomorphological processes. After its introduction, radiocesium is heavily deposited in the organic soil layer, leading to persistent circulation due to biological activities in soils. Some radiocesium in soils, litter, and organisms is transported to stream ecosystems, forming contamination spots in depositional habitats. While reservoir dams function as effective traps, radiocesium leaching from sediments is a continual phenomenon causing re-contamination downstream. Integration of data regarding radiocesium dynamics and contamination sites, as proposed here, is essential for contamination management in societies depending on nuclear power to address the climate crisis.

Nutrient Imbalance of the Host Plant for Larvae of the Pale Grass Blue Butterfly May Mediate the Field Effect of Low-Dose Radiation Exposure in Fukushima: Dose-Dependent Changes in the Sodium Content

The pale grass blue butterfly Zizeeria maha is sensitive to low-dose radioactive pollution from the Fukushima nuclear accident in the field but is also highly tolerant to radioactive cesium (¹³⁷Cs) in an artificial diet in laboratory experiments. To resolve this field-laboratory paradox, we hypothesize that the butterfly shows vulnerability in the field through biochemical changes in the larval host plant, the creeping wood sorrel Oxalis corniculata, in response to radiation stress. To test this field-effect hypothesis, we examined nutrient contents in the host plant leaves from Tohoku (mostly polluted areas including Fukushima), Niigata, and Kyushu, Japan. Leaves from Tohoku showed significantly lower sodium and lipid contents than those from Niigata. In the Tohoku samples, the sodium content (but not the lipid content) was significantly negatively correlated with the radioactivity concentration of cesium (¹³⁷Cs) in leaves and with the ground radiation dose. The sodium content was also correlated with other nutrient factors. These results suggest that the sodium imbalance of the plant may be caused by radiation stress and that this nutrient imbalance may be one of the reasons that this monophagous butterfly showed high mortality and morphological abnormalities in the field shortly after the accident in Fukushima.

Environmental fate of radiocesium in biota inhabiting a contaminated ecosystem on the U.S. Department of Energy's Savannah River Site

Although biomagnification of radiocesium (¹³⁷Cs) has been reported in food webs, most previous research has been limited to select trophic linkages. Few studies have included a comprehensive survey of fauna associated with aquatic, semi-aquatic, and terrestrial habitats within a single study framework. The objectives of this study were to advance our understanding of the dynamics of ¹³⁷Cs accumulation within food webs by quantifying ¹³⁷Cs activity across a wide range of biota found within a contaminated canal, as well as test the hypothesis that life-stage and body size influence ¹³⁷Cs bioaccumulation in select herpetofauna. With extensive sampling across multiple taxa collected from a contaminated canal system and associated floodplain on the Savannah River Site, we assessed ¹³⁷Cs activity and stable nitrogen isotopes for both aquatic organisms that were restricted to the contaminated effluent canal, and semi-aquatic organisms able to move freely between the contaminated canal and the adjacent uncontaminated terrestrial habitat. We found ¹³⁷Cs activity to be highly variable among species, with evidence for and against biomagnification in semi-aquatic and aquatic organisms, respectively. Furthermore, ¹³⁷Cs activity decreased with life stage and body size in bullfrogs (Lithobates catesbeianus), despite post-metamorphic bullfrogs having a more carnivorous diet compared to tadpoles, while cottonmouths (Agkistrodon piscivorus) retained similar ¹³⁷Cs activity regardless of their age and size. Although evidence of biomagnification has been observed in some contaminated systems, results of our study suggest the extent to which ¹³⁷Cs biomagnifies within food webs is context-dependent and likely influenced by a suite of biotic and abiotic factors. Further, our data indicate sampling of a broad suite of species and environmental attributes are needed to elucidate the fate and dynamics of anthropogenic pollutants within contaminated ecosystems.

Radiocesium concentrations and GPS-coupled dosimetry in Fukushima snakes

One of the largest releases of radioactive contamination in history occurred at Japan's Fukushima Daiichi Nuclear Power Plant (FDNPP). Although the accident happened in 2011, questions still persist regarding its ecological impacts. For example, relatively little is known about radiocesium accumulation in snakes, despite their high trophic status, limited home range sizes, and close association with soil where many radionuclides accumulate. This study presents one of the most comprehensive radioecological studies of snakes published to date using a combination of whole-body radiocesium analyses, GPS transmitters, and optically stimulated luminescence (OSL) dosimeters. The objectives were to: 1) quantify whole-body radiocesium activity concentrations and internal dose rates among several common species of snakes within and around the Fukushima Exclusion Zone (FEZ), 2) determine effects of species, sex, and body size on radiocesium activity concentrations, 3) measure external dose rates using GPS-coupled dosimeters deployed on free-ranging snakes, 4) compare field-derived empirical dose rates to those generated by computer simulation software (i.e., the ERICA tool), and 5) determine if incorporating snake behavior into computer models improve simulated estimates of external dose. Whole-body radiocesium levels for snakes were highly variable among individuals (16 to 25,000 Bq/kg, FW), but were influenced more by levels of local contamination than species, sex, or size. Doses recorded by OSL dosimeters on snakes, as well as modeling in ERICA, suggest that individual movements and behavior have a substantial influence on dose rates to snakes. However, dose estimates produced with ERICA were comparable to dose received by tracked snakes. The average external plus internal dose rate for snakes captured in the FEZ was 3.6-3.9 μGy/h, with external dose contributing 80% to the total. Further research regarding reptile-specific benchmark dose rates would improve risk assessment for reptiles in radiologically contaminated areas.

Relationship between radiocaesium in muscle and physicochemical fractions of radiocaesium in the stomach of wild boar

After the accident at the TEPCO Fukushima Daiichi Nuclear Power Plant in 2011, it became important to study radiation dynamics, assess internal radiation exposure and specify factors affecting radionuclide variation in wildlife. Therefore, it is necessary to investigate which physicochemical fractions of radiocaesium (¹³⁷Cs) are absorbed from ingested material in species with high activity concentrations of ¹³⁷Cs, such as wild boar. This study analysed the physicochemical fractions of ¹³⁷Cs in the stomach contents of wild boar to evaluate the transfer from ingested food to muscle. The ¹³⁷Cs activity concentration in muscle showed a significantly positive relationship with the ¹³⁷Cs activity concentration in the exchangeable fraction, and the sum of the ¹³⁷Cs activity concentrations in the exchangeable and bound to organic matter fractions. Seasonal variations were also found in the ¹³⁷Cs activity concentration in the exchangeable fraction, and the sum of the ¹³⁷Cs activity concentrations in the exchangeable and bound to organic matter fractions. These findings suggest that the proportions of the physicochemical fractions of ¹³⁷Cs in the exchangeable and bound to organic matter fractions in the stomach contents are important factors affecting the increases and seasonal dynamics of the activity concentrations of ¹³⁷Cs in wild boar muscle.

Overwintering States of the Pale Grass Blue Butterfly Zizeeria maha (Lepidoptera: Lycaenidae) at the Time of the Fukushima Nuclear Accident in March 2011

The Fukushima nuclear accident in March 2011 caused the massive release of anthropogenic radioactive materials from the Fukushima Dai-ichi Nuclear Power Plant to its surrounding environment. Its biological effects have been studied using the pale grass blue butterfly, Zizeeria maha (Lepidoptera: Lycaenidae), but the overwintering states of this butterfly remain elusive. Here, we conducted a series of field surveys in March 2018, March 2019, and April 2019 in Fukushima and its vicinity to clarify the overwintering states of this butterfly at the time of the Fukushima nuclear accident. We discovered overwintering individuals in situ associated with the host plant Oxalis corniculata under natural straw mulch as first-instar to fourth-instar larvae in March 2018 and 2019. No other developmental stages were found. The body length and width were reasonably correlated with the accumulated temperature. On the basis of a linear regression equation between body size and accumulated temperature, together with other data, we deduced that the pale grass blue butterfly occurred as fourth-instar larvae in Fukushima and its vicinity at the time of the accident. This study paves the way for subsequent dosimetric analyses that determine the radiation doses absorbed by the butterfly after the accident.

Web-building spider Nephila clavata (Nephilidae: Arachnida) can represent 137Cs contamination of arthropod communities and bioavailable 137Cs in forest soils at Fukushima, Japan

Large areas of Fukushima's forests were contaminated with radiocesium (¹³⁷Cs) after the Fukushima Dai-ichi Nuclear Power Plant accident. Most of the contaminated forests have not been decontaminated, and bioavailable ¹³⁷Cs is likely to circulate within the forest environment's food web. Nephila clavata (Nephilidae: Arachnida) is a top predator in the forest arthropod community, and this web-building spider potentially consumes many arthropod species presented in the grazing and detrital food chains. We tested whether ¹³⁷Cs in the spider could serve as a proxy for ¹³⁷Cs contamination of these arthropod communities. We also examined whether N. clavata could serve as a proxy for soil bioavailable ¹³⁷Cs. Nephila clavata was similarly or more contaminated with ¹³⁷Cs compared with lower-trophic-level arthropods such as herbivores and other predators at the same trophic level. Thus, the ¹³⁷Cs activity of N. clavata could represent the extent to which the arthropod community was contaminated with ¹³⁷Cs. Data from nine ¹³⁷Cs-contaminated sites in Fukushima showed a significant positive correlation between soil bioavailable ¹³⁷Cs and N. clavata's ¹³⁷Cs activity05 but the coefficient of determination was only moderate (R² = 0.43), suggesting that N. clavata is only a weak proxy of soil bioavailable ¹³⁷Cs. Our results also showed that the bioavailable fraction of ¹³⁷Cs in Fukushima was strongly correlated with the total inventory and that the K and Na contents of the soil determined the soil-to-spider transfer factor for ¹³⁷Cs and the ¹³⁷Cs activity in N. clavata, respectively. These results improve our understanding of ¹³⁷Cs transfer from the soil to arthropod species.

Strong contrast of cesium radioactivity between marine and freshwater fish in Fukushima

A proper understanding of radioactive contamination levels of food resources near the Fukushima Dai-ichi Nuclear Power Plant is necessary to estimate the potential effects of radionuclide contamination on human health. This study was conducted to present a direct comparison of radiocesium (¹³⁴Cs and ¹³⁷Cs) concentrations in marine and freshwater fish inhabiting different water bodies in Fukushima Prefecture (coastal waters, 6.3-54.5 km from the plant; forest rivers and irrigation ponds, 1.4-71.6 km), and to reveal plausible contamination mechanisms for each habitat. In contrast to marine demersal fish (7 species, n = 50), which showed lower and less variable radiocesium concentrations (0.234-3.41 Bq kg^-1-wet), freshwater fish (6 species, n = 463) showed higher and more site-specific variations for each species and habitat (4.09 Bq kg^-1-wet - 25.6 kBq kg^-1-wet) in 2015-2016. The apparent concentration ratio (aCR, L/kg) of ¹³⁷Cs in fish to water is higher for fish of freshwater habitats (mean 1240-12900 for each site) than in those of coastal waters (mean 200). Radiocesium contamination is more severe and persistent in freshwater fish, especially those distributed within the designated evacuation zone (salmon in rivers and bass in ponds). Continuous radiocesium uptake through the food web in relation to fish feeding habits and size (size effect), and biotic/abiotic characteristics in water and surrounding environments are main factors affecting site/habitat-specific bioaccumulation of radiocesium in freshwater fish. By contrast, uniformly lower radiocesium concentrations in marine demersal fish are mainly attributable to decreased radiocesium transfer intensity from the benthic food web because of lowered radiocesium contamination in sediments, and low physiological ability to retain radiocesium. Our results revealed a strong contrast of radiocesium contamination levels and mechanisms between marine and freshwater fish in natural habitats. Particularly, a close relation between ¹³⁷Cs accumulation in river salmon and contamination of prey items in forest ecosystems (mainly terrestrial and aquatic insects) is peculiar to the upstream areas affected by the Fukushima accident.

Bioaccumulation of 137Cs in anuran larvae utilizing a contaminated effluent canal on the U.S. Department of Energy's Savannah River Site

As a result of activities such as nuclear weapons testing, nuclear power generation and waste disposal, and nuclear accidents, radiocesium (¹³⁷Cs) is a widely distributed radio-contaminant of concern that readily accumulates in exposed wildlife. Although bioaccumulation of ¹³⁷Cs is an important factor for understanding its fate within the environment, there are currently limited data available on bioaccumulation patterns of ¹³⁷Cs in amphibians, despite their widespread distribution and potential to transport contaminants between aquatic and terrestrial ecosystems. Therefore, the objective of this study was to determine the amount of time necessary for anuran larvae experimentally placed in a contaminated system to reach a steady-state whole-body ¹³⁷Cs concentration, and to determine the threshold at which that steady-state ¹³⁷Cs concentration occurred for tadpoles within our study system. By restricting uncontaminated bullfrog (Lithobates catesbeianus) larvae to three experimental enclosures located along a¹³⁷Cs contaminated effluent canal on the U.S. Department of Energy's Savannah River Site, we modeled ¹³⁷Cs uptake through time using the von Bertalanffy modification of the Richards Model. The results of our modified Richards Model indicate that bullfrog tadpoles achieved steady-state ¹³⁷Cs concentrations of 3.68-4.34 Bq/g¹³⁷Cs dry whole-body weight after 11.63-15.50 days of exposure among sampling sites, with an average of 3.94 Bq/g after 14.07 days exposure. Radiocesium accumulation in bullfrog tadpoles was more rapid than that reported for other biota studied from other contaminated systems, likely due to incidental ingestion of sediments and a diet consisting of periphyton and other items that accumulate high levels of ¹³⁷Cs. Given their rapid accumulation of ¹³⁷Cs and inability to leave aquatic environments prior to metamorphosis, our data suggest amphibian larvae may be useful indicators for monitoring ¹³⁷Cs distributions and bioavailability within aquatic systems.

Monitoring 137Cs concentrations in bird species occupying different ecological niches; game birds and raptors in Fukushima Prefecture

This study was conducted to assess radiocesium accumulation in birds after the accident at Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station in 2011, with a particular focus on ¹³⁷Cs, which has a long physical half-life. Results of ¹³⁷Cs monitoring in four game bird species including two pheasant species and two duck species (copper pheasant, green pheasant, spot-billed duck and mallard) were assessed in Fukushima Prefecture. We also obtained samples from rescued raptors that died during treatment or rehabilitation at the Wildlife Symbiosis Centre in Fukushima Prefecture because of severe injury. We measured the muscle concentrations of ¹³⁷Cs in four of these raptor species (black kite, northern goshawk, peregrine falcon and ural owl). Comparison of the two pheasant species showed that the copper pheasants inhabiting forested areas had higher ¹³⁷Cs concentrations in muscle (Bq/kg, fresh mass) than the green pheasants inhabiting mountainous areas near human habitation (i.e., Satoyama). No clear tendencies were observed in ¹³⁷Cs concentration in muscle of copper pheasants over time, but a tendency to decrease was observed in green pheasants over time. The difference in tendencies between species may be attributable to differences in their food habits and its ¹³⁷Cs concentration, and also differences in the situation of ¹³⁷Cs accumulation in their habitat. No significant differences were observed in ¹³⁷Cs concentration in muscle between the resident spot-billed duck and migratory mallard because of the comparatively short biological of effective half-life of radiocesium. Analysis of ¹³⁷Cs concentration in muscle of raptor revealed that the concentration was similar to, or lower than, those of pheasants and ducks.

Concentrations and biological half-life of radioactive cesium in epigeic earthworms after the Fukushima Dai-ichi Nuclear Power Plant accident

To understand the long-term behavior of radiocesium in the biological processes of a forest ecosystem, its concentration in Japanese epigeic earthworms (Megascolecidae), litter, and soil, and the ambient dose equivalent rates, were investigated after the Fukushima Dai-ichi Nuclear Power Plant accident. The metabolism of radiocesium in the earthworms was also investigated in the laboratory, and its biological half-life (T_b) was estimated. The concentration of ¹³⁷Cs in the habitat soil and litter changed from 2014 to 2016, with levels in the litter going from 44.9 Bq/g dw (in 2014) to 45.3 Bq/g dw (2015) and 10.7 Bq/g dw (2016); in soil, these values were 9.79 Bq/g dw, 7.14 Bq/g dw and 18.0 Bq/g dw, respectively. By contrast, no significant changes were observed in the concentrations in the earthworms, which were 4.87 Bq/g fw, 5.30 Bq/g fw and 4.67 Bq/g fw in 2014, 2015, and 2016, respectively. The ambient dose equivalent rates at the sampling site declined significantly over these three years, going from 2.15 μSv/h to 1.68 μSv/h and 1.35 μSv/h, mostly corresponding to physical decay of radiocesium. The majority (95%) of the ¹³⁷Cs in the earthworms, observed via autoradiography, was concentrated primarily in the intestine. The clearance of ¹³⁷Cs from the earthworms was described by dual exponential functions: the half-life in the rapid loss due to gut clearance was 0.10 days and a second slower loss due to physiological clearance was 27.4 days.