Short- and long-term patterns of ¹³⁷Cs in fish and other aquatic organisms of small forest lakes in southern Finland since the Chernobyl accident

Half-century trends of radioactivity in fish from Danish areas of the North Sea, Kattegat, and Baltic Sea

This work reports comprehensive time-series datasets over the past 50 years for natural (210Po) and anthropogenic (134Cs and 137Cs) radionuclides in three fish species (cod, herring and plaice) from Danish marine areas covering the North Sea, Kattegat, and Baltic Sea. Impact from the global fallout of atmospheric nuclear weapons testing, radioactive discharges from the European nuclear reprocessing plants and release from Chernobyl accident are clearly detected in the fish samples. While 210Po concentrations in each fish species demonstrated comparable levels across the three regions without notable temporal trends, significantly higher median 210Po concentration was observed in the lower trophic level fish, namely herring and plaice, compared to cod. In contrast, 137Cs concentrations in all three species steadily decrease over time after the Chernobyl-attributed peaks in late 1980s in the entire study area, whereas 137Cs always demonstrated higher concentrations in cod than herring and plaice. Our calculated concentration factors (CFs) for 137Cs in this work indicate that the mean CFs for 137Cs over the past 50 years are significantly different across the three species, following the order of cod < herring < plaice. Based on the time-series data, ecological half-lives (Teco) of 137Cs in fish from Danish marine areas were estimated to evaluate the long-term impact of anthropogenic radioactive contamination in different regions. Our results indicate no significant difference in Teco across different fish species, whereas the weighted mean Teco for fish in the Baltic Sea (29.3 ± 3.9 y) is significantly longer than those of the North Sea (9.8 ± 0.9 y) and Kattegat (11.7 ± 1.2 y), reflecting the strong 'memory effect' of the Baltic Sea due to its slow water renewal. However, the dose assessment demonstrates that the contribution of the natural radionuclide 210Po to ingestion dose from fish consumption is 1-2 order of magnitude higher compared to that of 137Cs.

Comparison of 137Cs uptake, depuration and continuous uptake, originating from feed, in five salmonid fish species

The origin of ¹³⁷Cs contamination, contamination levels, and its effective ecological half-life can differ among fish species, but until recently it was unknown whether interspecific differences in radiocaesium metabolism existed. We compare the trophic transfer of ¹³⁷Cs in five salmonid species under controlled conditions. Fish were fed a diet containing 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a fixed period of time. While there were almost no differences in contamination levels among individual fish within a species during fixed periods, the rate of ¹³⁷Cs uptake was statistically different among the species examined, for which the effective half-life of ¹³⁷Cs ranged 49-84 days. No significant difference in rate of decline in the quantity of ¹³⁷Cs was apparent among species in two experiments after correcting for differences in growth. The effective half-life was shortest in Oncorhynchus mykiss and O. masou-two species exhibiting the best growth rate in both experiments-indicating an influence of the dilution effect associated with growth. When continuously fed 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a period of 209-294 days, contamination levels in the five species ranged 216-240 Bq kg^-1 wet wt, and reached a steady state value. Our results indicate metabolic rate, although accompanying a change of body size, did not affect ¹³⁷Cs contamination levels in fish, which implies that contamination levels in the diet strongly determined contamination levels in fish.

Time-dependent trends of artificial radionuclides in biota of the Yenisei River (Siberia, Russia)

We investigated time-dependent trends of artificial radionuclides in aquatic moss, zoobenthos (amphipods and caddisfly larvae), and three abundant wild fish species (Northern pike, Arctic grayling, and Siberian dace) inhabiting the Yenisei River in the vicinity of the radioactive discharge site in 2007-2015, in a period before and after the shutdown of the last nuclear reactor plant at the Mining-and-Chemical Combine (MCC), which occurred in 2010. From our research, we learned that concentrations of short-lived radionuclides, whose discharges to the Yenisei either stopped or declined after the shutdown of the reactor plant at the MCC (²⁴Na, ⁴⁶Sc,⁵¹Cr, ⁵⁴Mn, ⁵⁸Co, ⁵⁹Fe, ⁶⁰Co, ⁶⁵Zn, ¹⁰³Ru, ^141,144Ce, ^152,154Eu, ²³⁹Np), decreased in biota samples as well. The ecological half-life (EHL) of ⁶⁵Zn (0.4-0.7 y) was similar to the physical half-life of this isotope, the EHLs of ⁶⁰Co (1.2-2.1 y) and ¹⁵²Eu (1.8 y) were shorter than the physical half-lives of these isotopes. Concentration of ¹³⁷Cs did not decrease significantly in biota of the Yenisei after the shutdown of the last reactor plant because the discharges of this radionuclide to the Yenisei continued at the same level. On a longer-term scale (since 1973 and since 1991), concentration of ¹³⁷Cs in fish muscle had significantly decreased, following the decrease in annual discharges of this radionuclide to the Yenisei, and the EHL of ¹³⁷Cs was estimated as 6.5-12.8 y. Statistically significant correlation with annual discharges of ¹³⁷Cs was revealed for the concentration of this radionuclide in grayling (whole bodies and muscle); dace (muscle), and amphipods. Despite their ability to accumulate high concentrations of ¹³⁷Cs, aquatic moss and caddisfly larvae (analyzed together with their stony casings) were not sensitive to interannual fluctuations in the releases of this radionuclide to the Yenisei. Among the analyzed fish species of the Yenisei, the highest activity concentration of ¹³⁷Cs was revealed in pike (body and muscle), indicating biomagnification of this radionuclide in the top level of the trophic chain.

Long-term distribution and migration of 137Cs in a small lake ecosystem with organic-rich catchment: A case study of Lake Vorsee (Southern Germany)

In the shallow eutrophic Lake Vorsee with an organic-rich catchment the ¹³⁷Cs activity concentration in water samples (n = 250) and different fish species (n = 631) was determined for a period of 30 years starting soon after the Chernobyl fallout. To understand the distribution and migration of ¹³⁷Cs in the ecosystem of Lake Vorsee, also the ¹³⁷Cs inventory in soils of the catchment area, in sediments, in suspended matter, in water plants, as well as its vertical distribution in soils and sediments were studied. The time dependency of the ¹³⁷Cs activity concentration in water and fish was modelled with a compartment model (AQUASCOPE) as well as with the sum of two exponential functions. Results for effective half-lives of ¹³⁷Cs in water are T_eff1  = 1.2 a and T_eff2  = 7.2 a; for small cyprinidae T_eff1  = 0.6 a and T_eff2  = 8.9 a; and for pike T_eff1  = 1.4 a and T_eff2  = 12.1 a. The vertical distribution of ¹³⁷Cs activity concentration in soils was described by a partial differential equation (diffusion constant D = (1.05 ± 1.16) cm²⋅a^-1 and convection speed v = (0.14 ± 0.07) cm⋅a^-1) and its distribution in sediments by a system of two coupled partial differential equations (fixation rate f = 2.0 d^-1; distribution coefficient K_d^ex = (543 ± 140) L⋅kg^-1; sedimentation rate R_S = (0.030 ± 0.010) g⋅(cm²⋅a)^-1; bioturbation constant D_bio, decreasing linearly with depth with D_bioabs = (3.3 ± 1.0) cm²⋅a^-1 at the surface). According to the extended AQUASCOPE model, the following fractions of the total ¹³⁷Cs inventory were found in the different compartments of the lake ecosystem in 2016: 9.0⋅10^-1 in soils of the catchment area, 1.0⋅10^-1 in sediments, 5.5⋅10^-5 in water, 1.5⋅10^-5 in suspended matter, 1.3⋅10^-5 in watermilfoil, and 1.6⋅10^-6 in fish.

Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (¹³⁴Cs and ¹³⁷Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of ¹³⁷Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T_eff) of ¹³⁷Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of ¹³⁷Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of ¹³⁷Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total ¹³⁷Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T_eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The T_eff in the slow component of the TDMs could converge towards the physical half-life of ¹³⁷Cs. Nearly five and a half years after the FDNPP accident, we concluded that ¹³⁷Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of ¹³⁷Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total ¹³⁷Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate ¹³⁷Cs contamination level in the river water was high after heavy rains. Overall, ¹³⁷Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

Fractionation of radiocesium in soil, sediments, and aquatic organisms in Lake Onuma of Mt. Akagi, Gunma Prefecture using sequential extraction

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has resulted in the contamination of the environment in Gunma Prefecture with radioisotope cesium (radio-Cs, ¹³⁴Cs and ¹³⁷Cs). Concentrations of radio-Cs >500Bqkg^-1 were found in wakasagi (Hypomesus nipponensis) in Lake Onuma at the top of Mount (Mt.) Akagi in August 2011. To explain the mechanism of this contamination, monitoring studies have been conducted around Lake Onuma by measuring radio-Cs concentrations in samples of fish, aquatic plants, plankton, lake water, lake sediments, and surrounding soil. The leachability of radio-Cs was evaluated using sequential extraction by Tessier et al. The total concentration of radio-Cs in Lake Onuma ecosystems decreased gradually with time. In the brown forest soil, radio-Cs concentrations of 2000 to 6000Bqkg^-1 were detected. The abundance ratio of the easy-elution form (exchangeable and carbonate forms) in the samples was <10%. The concentrations in phytoplankton samples were 3-6 times higher than those in wakasagi samples. The ratios of easy-elution forms increased by the rank in the food chain; 37% in phytoplankton, 78% in zooplankton, and 97% in wakasagi. It is likely that the lower ratio of the easy-elution form in phytoplankton is related to the adsorption of radio-Cs on suspended substances in the lake, as suggested by the analyses of aluminum and titanium in the phytoplankton, zooplankton, and wakasagi samples. The high concentrations of radio-Cs in wakasagi would be related also to the characteristics of closed mountain lakes.

Radiocesium contamination of greenlings (Hexagrammos otakii) off the coast of Fukushima

We measured the radiocesium (¹³⁴Cs and ¹³⁷Cs) contamination of 236 greenlings (Hexagrammos otakii) off the coast of Fukushima Prefecture in Japan, following the accident at the Fukushima Daiichi Nuclear Power Plant in March 2011. The radiocesium concentrations of greenlings caught approximately 40 km south of the power plant were significantly higher than those of greenlings caught approximately 50 km north of the power plant. The radiocesium concentrations of greenlings caught in southern waters were significantly higher in shallow than in deep waters. Meanwhile, two outlier specimens of greenlings with higher ¹³⁷Cs concentrations, 16,000 Bq/kg-wet on 1 August 2012 and 1,150 Bq/kg-wet on 8 May 2013, were caught approximately 20 km from the power plant. Our calculations suggest that the probability of two such outlier specimens being found off the coast of Fukushima is exceedingly low. By contrast, extremely contaminated greenlings were frequently caught in the power plant port (geometric mean of ¹³⁷Cs = 17,364 Bq/kg-wet). Our results suggest that the two outlier greenlings with higher ¹³⁷Cs concentrations migrated from the power plant port. Continued close monitoring of radiocesium concentrations in the area should be done to ensure the safety of food supplies.