Spatiotemporal patterns in differences between the 137Cs concentrations of forest and stream litters: effect of leaching†

Forest–stream ecotones possess prominent detritus-based food webs, and ¹³⁷Cs-contaminated litter can influence the contamination levels of animals inhabiting such ecosystems. The effects of leaching on contaminated litter induce greater absolute differences between the ¹³⁷Cs concentrations of forest and stream litter in more contaminated sites. Because ¹³⁷Cs concentrations in litter can be attenuated temporally, spatiotemporal patterns in the differences in ¹³⁷Cs concentrations between forest and stream litter may vary depending on both the amount of ¹³⁷Cs deposition and the passage of time. To test this hypothesis, we sampled coniferous needle and broad-leaved deciduous litter in forests and streams at seven forested headwater sites affected by the Fukushima nuclear accident 3.24 and 11.24 years after the accident. We found that ¹³⁷Cs concentrations in the two litter types were one order of magnitude lower 11.24 years after the accident than 3.24 years afterwards. The absolute difference in ¹³⁷Cs activity concentrations of litter between forest and stream ecosystems was higher at more contaminated sites both 3.24 and 11.24 years after the accident. The spatiotemporal changes in litter contamination provide insight into ¹³⁷Cs dynamics and complex transfer in the detritus-based food webs of forest–stream ecotones.