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Understanding the factors influencing 137Cs concentrations in freshwater salmonids is crucial for reviving inland fisheries in polluted regions. We studied seasonal variations of 137Cs concentration in charr (Salvelinus leucomaenis) through uptake and metabolism in forested headwaters at Fukushima and Gunma sites. Charr consumed both terrestrial and aquatic animals, and terrestrial prey was predated more in summer at both sites. The 137Cs concentrations in litter, which is a dominant basal food resource of both forest and stream ecosystems, differed between forest and stream due to 137Cs leaching effect on litter submerged in streams. The concentration difference in both litter and prey was greater at Fukushima site than at Gunma site. The estimated prey 137Cs concentration at Fukushima site peaked in summer when terrestrial preys are most available, whereas it remained relatively constant at Gunma site because of the small difference of 137Cs concentrations in between terrestrial and aquatic preys. The specific metabolic rate of charr was commonly changed with stream water temperature, greatest in summer, and lowest in winter at both sites. Because both prey 137Cs concentrations and specific metabolic rates peaked in summer, the combination of uptake and metabolism at Fukushima site largely negated seasonal 137Cs fluctuations in charr, whereas specific metabolic rate fluctuations could be the major determinant of charr 137Cs concentrations at Gunma site. Our results suggested that 137Cs concentrations in prey items, whose seasonality are varied due to initial 137Cs fallout volume, were expected to be an important determinant for 137Cs concentrations in charr. [ABSTRACT FROM AUTHOR]

This study examined 137Cs detachment from needle litter of Japanese cedar (Cryptomeria japonica) through leaching and decomposition in a headwater stream located 45 km from Fukushima Daiichi Nuclear Power Plant. We placed litter bags in pools and riffles and removed them after 4, 15, 75, 150, and 240 days. The 137Cs activity concentration in litter collected after 4 and 15 days was 48–72 % of the initial 137Cs activity concentration in litter. The 137Cs activity concentration in litter after 240 days in the pools decreased by 26 %, while that in litter of riffles decreased by 9 % compared with the initial 137Cs activity concentration. The dry weight of litter became 71 and 86 % of initial weight after 75 days, and from 32 to 51 % after 150 days. The decay coefficient of litter (k = 0.0060) in the riffles was 1.6-fold higher than that in the pools (k = 0.0037). The greater decrease in litter weight with longer submergence time was possibly associated with a combination of leaching and decomposition, in particular in litter that was submerged for 100 days in the water column. The difference in the rate of decomposition and 137Cs detachment from litter between the riffles and pools was due to the substrate and flow conditions at the litter bag sites. Examining the different patterns of 137Cs detachment from litter among channel morphologies is necessary for comprehending the mechanisms of 137Cs accumulation in aquatic biota. The findings of this study suggest that elucidating leaching and decomposition processes in litter of stream channels is important for understanding 137Cs dynamics in forested headwater ecosystems. [ABSTRACT FROM AUTHOR]

It is important to understand the changes in the 137 Cs concentration in litter through leaching when considering that 137 Cs is transferred from basal food resources to animals in forested streams. We found that the difference of 137 Cs activity concentration in litter between forest and stream was associated with both litter type and 137 Cs fallout volume around Fukushima, Japan. The 137 Cs activity concentrations in the litter of evergreen conifers tended to be greater than those in the litter of broad-leaved deciduous trees because of the absence of deciduous leaves during the fallout period in March 2011. Moreover, 137 Cs activity concentrations in forest litter were greater with respect to the 137 Cs fallout volume. The 137 Cs activity concentrations in stream litter were much lower than those in forest litter when those in forest litter were higher. The 137 Cs leaching patterns indicated that the differences in 137 Cs activity concentration between forest and stream litter could change with changes in both fallout volume and litter type. Because litter is an important basal food resource in the food webs of both forests and streams, the 137 Cs concentration gradient reflects to possible 137 Cs transfer from lower to higher trophic animals. Our findings will improve our understanding of the spatial heterogeneity and variability of 137 Cs concentrations in animals resident to the contaminated landscape. [ABSTRACT FROM AUTHOR]

Understanding the seasonality of 137Cs concentrations in aquatic animals is crucial for reviving local inland fisheries. The seasonality of 137Cs concentrations in animals is expected to vary, even if focal species consume similarly contaminated foods because the 137Cs excretion rate is species-specific, and 137Cs uptake by foraging autochthonous food resources also vary among seasons. Here, we conducted a seasonal monitoring survey of dissolved 137Cs concentrations as an indicator of the contamination level of food resources and measured 137Cs concentrations in two carnivorous aquatic animals (Palaemon paucidens and Rhinogobius sp.) that share a trophic niche in a stream connected to a dam reservoir. The dissolved 137Cs concentration had clear seasonality—high in summer and low in winter. The 137Cs concentrations in the animals revealed a different seasonal pattern—it peaked in October in P. paucidens and peaked in February in Rhinogobius. Overall, the 137Cs concentration was relatively higher in P. paucidens than in Rhinogobius , suggesting that P. paucidens has a lower excretion rate than Rhinogobius. Consequently, the seasonality of the 137Cs concentration in P. paucidens showed temporal changes similar to those of the dissolved 137Cs concentration, which were likely affected by 137Cs uptake through foraging, whereas that in Rhinogobius was controlled by 137Cs excretion. This study shows that the seasonality of 137Cs concentration can differ between sympatric animals that share a trophic niche. Accumulating knowledge and comparing the seasonality of 137Cs concentrations in fisheries species based on the balance between uptake and excretion will be valuable to determine the appropriate seasons to obtain less-contaminated products. [Display omitted] • Two sympatric species that belong to different phyla was compared. • Stable carbon and nitrogen ratios showed that the two species share a trophic niche. • Dissolved 137Cs concentration peaked in summer and dropped in winter. • The two species showed different seasonality of 137Cs concentrations. • 137Cs seasonality varies with interspecific variations of excretion rates in animals. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms of 137 Cs movement across different ecosystems is crucial for projecting the environmental impact and management of nuclear contamination events. Here, we report differential movement of 137 Cs in adjacent forest and stream ecosystems. The food webs of the forest and stream ecosystems in our study were similar, in that they were both dominated by detrital-based food webs and the basal energy source was terrestrial litter. However, the concentration of 137 Cs in stream litter was significantly lower than in forest litter, the result of 137 Cs leaching from litter in stream water. The difference in 137 Cs concentrations between the two types of litter was reflected in the 137 Cs concentrations in the animal community. While the importance of 137 Cs fallout and the associated transfer to food webs has been well studied, research has been primarily limited to cases in a single ecosystem. Our results indicate that there are differences in the flow of 137 Cs through terrestrial and aquatic ecosystems, and that 137 Cs concentrations are reduced in both basal food resources and higher trophic animals in aquatic systems, where primary production is subsidized by a neighboring terrestrial ecosystem. [ABSTRACT FROM AUTHOR]

We developed a food web-based transfer factor ( TF web ) to study contaminant movements from multiple prey items to a predator based on the dietary contributions of prey items with their respective contamination levels. TF web was used to evaluate the transfer of 137 Cs into whitespotted char ( Salvelinus leucomaenis ) from the trophic structure of a stream–riparian ecosystem in headwater streams draining a Japanese cedar forest. We also examined the applicability of this method by comparing sites with different contamination levels but similar surrounding environments in Fukushima and Gunma. All samples were collected from August 2012 to May 2013. The dietary contributions from both aquatic and terrestrial prey items to whitespotted char were analyzed using stable carbon and nitrogen isotope ratios. 137 Cs activity concentrations in char ranged from 704 to 6082 Bq kg −1 -dry in Fukushima and from 193 to 618 Bq kg −1 -dry in Gunma. Dominant prey taxa such as mayflies ( Ephemera japonica ), spider crickets (Rhaphidosphoridae gen. spp.), and freshwater crabs ( Geothelphusa dehaani ), each of them accounted for 3–12% of the fish diet, based on lower and upper estimates, respectively. TF web ranged from 1.12 to 3.79 in Fukushima and from 1.30 to 4.30 in Gunma, which suggested bioaccumulation from prey items to predator. Widely used ecological parameters TF s by media–char and TTF by single prey–char showed high variability with both dilution and accumulation. TF web is applicable for 137 Cs transfer in predator-prey systems with complex food web structures of stream-riparian ecosystems. [ABSTRACT FROM AUTHOR]