Eleutherococcus sciadophylloides (commonly known as koshiabura) is a deciduous tree whose young buds are a popular wild food in Japan. After the 2011 Fukushima nuclear accident, radiocaesium contamination occurred in edible wild plants in forests, with particularly high levels in E. sciadophylloides buds. Consequently, restrictions on their consumption have impacted local cuisine and cultural traditions. To investigate the mechanisms underlying these high Cs-137 concentrations, we employed a multifaceted approach combining root excavation, environmental DNA (eDNA) analysis, and Sr and Cs isotope analyses to cross-validate root depth and uptake dynamics. Root excavation revealed predominantly shallow root systems. eDNA analysis confirmed that the lateral roots collected from soil cores were most abundant within the 0-5 cm layer of the mineral soil horizon. Sr isotope analysis indicated that small E. sciadophylloides trees primarily absorbed cations from this layer. In contrast, Cs isotope analysis was insufficient to determine the soil depth of Cs-137 uptake. This discrepancy may reflect contributions from both the root uptake of Cs-137 leached from the organic horizon and the supply of Cs-137 to buds through internal translocation within the tree. Our findings indicate that small E. sciadophylloides trees rely primarily on shallow root systems for nutrient uptake, specifically from the 0-5 cm soil layer where Cs-137 has accumulated. This characteristic likely contributes to the high Cs-137 levels observed in E. sciadophylloides buds. Further research is needed to clarify the relative contributions of root uptake and internal translocation to Cs-137 transfer into buds.

Spring-fed tributaries offer stable thermal regimes and high prey density, creating attractive but competitive habitats. This has led to the hypothesis that spring-fed tributary–mainstem linkages complement food supply and seasonal fish habitats. To test this, we monitored the seasonal movements and body condition of juvenile Oncorhynchus masou across six reaches of spring-fed tributary–mainstem links in northern Japan using δ 13 C and δ 15 N signatures. The distinctive isotopic signatures of mainstem periphyton enabled us to estimate their contribution to fish that were consistent among all reaches. This synchrony suggests frequent immigration into tributaries by fish that had previously consumed mainstem-derived prey. Older, larger fish, as superior competitors, were found exclusively in upstream tributary reaches. While fish with lower condition factors appeared in these locations in May and August, fish inhabiting the mainstem and near-confluence tributary reaches exhibited higher condition factors in August. These results suggest that linkages between tributaries and the mainstem complement food supply. Fish with higher condition factors likely began to settle in the tributary in October, whereas the highest mainstem periphyton contribution at all tributary reaches in February suggested the importance of tributaries as wintering habitats. While previous research has focused on spring-fed tributaries as isolated foraging habitats, this study expanded their role to that of interconnected tributary–mainstem networks.

In Japan, mushrooms have long been valued as food resources, but the Fukushima nuclear accident disrupted production and shipment due to radiocesium contamination. Limited knowledge has particularly hindered outdoor fungal bed cultivation. To address this, we conducted cultivation experiments with Lyophyllum decastes and Lepista nuda across 14 broad-leaved deciduous forest sites in Fukushima Prefecture. Air dose rates at 1 m height ranged from 0.04 to 0.89 µSv/h. Radiocesium concentrations (combined 137Cs and 134Cs, expressed as Bq/kg at 90% water content) in fruit bodies were 0.4–12 (mean 2.3) for Ly. decastes and 0.2–43 (mean 8.1) for Le. nuda. Both species remained below Japan’s food safety threshold of 100 Bq/kg, indicating that safe cultivation is feasible across broad areas. Uptake patterns differed: concentrations in Ly. decastes correlated with contamination in litter and multiple soil layers, whereas in Le. nuda it correlated only with litter contamination. These findings suggest that clean soil fills for Ly. decastes and clean litter covers for Le. nuda could serve as mitigation strategies tailored to each species. Aggregated transfer factors (2.76 × 10-5 m2/kg for Ly. decastes and 6.60 × 10-5 m2/kg for Le. nuda) were lower than those reported for wild mushrooms. Overall, this study provides new insights into reducing radiocesium assimilation by cultivated mushrooms and supports the revival of outdoor fungal bed cultivation in contaminated landscapes. ### Competing Interest Statement The authors have declared no competing interest.

Mushrooms have long been valued as food resources, but past nuclear accidents disrupted production and shipment due to radiocesium contamination. After the Fukushima nuclear accident in 2011, limited knowledge has particularly hindered outdoor fungal bed cultivation. To address this, we conducted cultivation experiments with Lyophyllum decastes and Lepista nuda across 14 broad-leaved deciduous forest sites in Fukushima Prefecture. Air dose rates at 1-m height were 0.04-0.89 µSv/h. 137Cs concentrations (Bq/kg at 90% water content) in fruit bodies were 0.4-12 (mean 2.2) for Ly. decastes and 0.2-42 (mean 8.0) for Le. nuda. Both species remained below Japan's food safety threshold of 100 Bq/kg, indicating that outdoor fungal bed cultivation is feasible for the species across broad areas. Uptake patterns differed: concentrations in Ly. decastes correlated with contamination in litter and multiple soil layers, whereas in Le. nuda, they correlated with upper soil and leaf mold contamination. These findings imply that clean soil fills for Ly. decastes and clean litter covers for Le. nuda may partially serve as mitigation strategies tailored to each species. Aggregated transfer factors (2.72 × 10-5 m2/kg for Ly. decastes and 5.72 × 10-4 m2/kg for Le. nuda) were one order of magnitude lower than those reported for wild mushrooms. Overall, this study provides new insights into reducing radiocesium assimilation by cultivated mushrooms and supports the revival of outdoor fungal bed cultivation in contaminated landscapes.

Sympatric headwater salmonids compete over food items that are the primary 137 Cs sources for fishes. This ecological process driven by competition is expected to be an important determinant of 137 Cs concentrations in headwater salmonids, but little is known regarding the process. Here, we investigated how a stronger competitor salmon ( Oncorhynchus masou ) affects 137 Cs concentrations in them and a sympatric weaker competitor charr ( Salvelinus leucomaenis ) in reaches of a highly contaminated headwater system in Fukushima, Japan. The results indicated that higher proportion of sympatric O. masou individuals within salmon communities lowers 137 Cs concentrations in both species, and the effect is more pronounced for S. leucomaenis . Because stronger salmonid competitors exploit falling and drifting terrestrial prey, which is generally more contaminated than aquatic one, O. masou could monopolize terrestrial prey, thereby be more contaminated than did S. leucomaenis as a consequence of interspecific competition. Although previous studies have reported the effects of abiotic environmental factors such as water quality and 137 Cs deposition on 137 Cs activity concentrations in freshwater salmonids, the present study is the first to discover that competition for food resources is an important determinant of 137 Cs concentrations in salmonids. The findings of this study open a new research area in which the ecological process of interspecific competition can affect radiocesium dynamics in headwater salmonids.

There has been a growing interest in integrating geological and ecological processes for sustainable river management and restoration. Lotic systems are shaped by diverse physical processes, including geology, geomorphology, hydrology, and interactions with terrestrial processes. However, restoration practices often prioritize specific habitats or river forms without fully considering the underlying physical processes that support biological communities and ecosystem functions. In this paper, we synthesize studies that integrally examine geological and ecological processes across different scales and components of riverscapes, including geohydrological processes, which have been less investigated in riverscape studies. We begin by examining processes at broader spatial scales, including river–watershed and river–riparian interactions, and gradually narrow our focus to the dynamics that occur among habitats within river channels, through which we highlight the significance of conceptualizing rivers as dynamic “networks” rather than linear features. Finally, we identify both scientific and practical challenges that can be addressed to bridge the gap between basic-science implications and their implementation in riverscape restorations.

Long-term monitoring after the Fukushima Daiichi accident have shown that 137Cs concentrations in freshwater fish do not always decrease exponentially; they level off at relatively high values more than a decade after the fallout. In forested headwater streams, insectivorous salmonids are closely associated with forest-derived prey species. Because 137Cs is redistributed among forest compartments and declines at different rates over time, the compartments and pathways that control long-term contamination in fish remain unclear. We quantified temporal changes in 137Cs across forest-stream compartments, including masu salmon (Oncorhynchus masou) and white-spotted charr (Salvelinus leucomaenis), in two contaminated reaches of the Ota River, Fukushima, from 2018 to 2025 (7-14 years post-accident). Leaf, litter, periphyton, and dissolved 137Cs generally declined, with effective half-lives of 2-7 years. In contrast, masu salmon showed no clear decline, whereas white-spotted charr decreased slowly, with a half-life of 7-13 years. Terrestrial insects showed no consistent decline, and exhibited highly variable 137Cs concentrations among the feeding groups, reflecting uptake from heterogeneously contaminated forest compartments. Correlation network analysis indicated that masu salmon was more strongly coupled to terrestrial insects than to white-spotted charr, suggesting that terrestrial insects are a key pathway for continued exposure to forest-derived 137Cs. These results indicate that 137Cs stored in near-surface forest soils remains a persistent source for terrestrial fish prey. Our findings highlight the striking persistence of 137Cs in masu salmon, raise concerns for fishery recovery, and suggest pathways sustaining this contamination. Clarification of these pathways will provide a basis for more accurate long-term predictions.

In this study, the effect of temperature, water quality, and the impact of an intense typhoon event on change in 137Cs concentration in the water of agricultural and forested rivers near the Fukushima Daiichi nuclear power plant (Japan) was evaluated using monthly stationary observations obtained under baseflow conditions 2.8-10.6 years after the nuclear accident in 2011. The dissolved 137Cs concentration fluctuated seasonally with water temperature in all rivers, and the increase in dissolved 137Cs concentration for unit increase in temperature was higher in forested rivers than in agricultural rivers. The relationship between water temperature and the apparent distribution coefficient of 137Cs well followed the van 't Hoff equation in the two agricultural rivers, where the enthalpy of reaction was estimated as-15.6 and-19.6 kJ mol-1. The van 't Hoff equation was not well followed for a forested river, where the suspended solids mainly comprised organic matter, suggesting that the dominant process determining dissolved 137Cs concentrations in forested rivers is not only water temperature effect on ion exchange, but rather the input of 137Cs and K+ (competing with 137Cs for exchange sites on mineral particles) into the water phase via litter leaching. Suspended solids concentrations in agricultural rivers correlated negatively with 137Cs concentrations in suspended solids, suggesting an increased proportion of coarse particles or the input of soils with low 137Cs concentration from decontaminated agricultural land. At some sites, 137Cs concentrations in dissolved form and in suspended solids were reduced sharply in association with the passage of Typhoon Hagibis in October 2019, suggesting that Typhoon Hagibis caused large-scale surface erosion that removed the source of 137Cs.

In Japan, small traditionally managed rice paddies in small valleys (yatsuda) are rarely subject to land consolidation and have the potential to retain well-connected wetland habitats. In this study, we investigated frog assemblages in various lentic habitats including cultivated and fallow rice paddies, irrigation ponds, and biotopes in a yatsuda area in Saitama, Japan. Monitoring was conducted weekly for 44 weeks, and then, multivariate analysis was applied to assess frog assemblages. The structure and phenology of frog assemblages were similar among habitat types, indicating the importance of permanently retained lentic habitat in maintaining frog assemblages regardless of habitat type or management history. The forest green tree frog (Zhangixalus arboreus) characterized frog assemblages in forested irrigation ponds, suggesting that a combination of open spaces, i.e., rice paddies, and shady spaces i.e., irrigation ponds, enhances the species diversity of frogs in yatsuda areas. This study highlights the importance of permanent lentic environments in small traditional rice paddies and connected surrounding area for conserving amphibians in an era of depopulation and rice paddy abandonment in Japan.

This study aims to elucidate factors affecting variations in 137Cs concentrations of masu salmon Oncorhynchus masou inhabiting a forest river and its connected dam reservoir (an artificial lake) in the upper reaches of the Ota River system during summer–autumn (August and October 2017). A clear size effect was observed in river-resident individuals, with significantly higher 137Cs concentrations in autumn, particularly in 1-year-old (age-1) prespawning adults. River residents (mean 1990 Bq/kg) showed significantly higher 137Cs concentrations than the larger lake migrants (mean 703 Bq/kg). The main prey items of river residents were terrestrial and aquatic insects, whereas lake migrants predominantly consumed small fish (pond smelt), as indicated by distinct δ13C and δ15N values. Larger age-1 river residents consumed a greater proportion of terrestrial prey than smaller age-0 individuals. A significant linear relationship between 137Cs concentrations in diets and those in age-1 river residents was observed in autumn, when terrestrial insects had higher 137Cs concentrations than aquatic insects. These results suggest that larger river residents efficiently fed on terrestrial insects with elevated 137Cs concentrations, resulting in rapid 137Cs accumulation during summer–autumn. By contrast, the lower variation in lake migrants likely reflects their consistent feeding on small fish with lower 137Cs concentrations. Potential factors contributing to interspecific differences in 137Cs concentrations and their variability among masu salmon and other river and lake fish are also discussed.

This study examined the influence of dam lakes on 137Cs dynamics in downstream river systems based on monthly observations of inflow/discharge water over 10 years at the Matsugabo Dam and Yokokawa Dam, which were affected by the Fukushima Daiichi nuclear power plant accident in 2011. Dissolved 137Cs concentrations in the discharge water of both dams showed seasonal fluctuation, but the phase of the fluctuation lagged that of water temperature, and the relationship between the distribution coefficient of 137Cs and temperature in the dam discharge water was poorly reproduced by the van't Hoff equation. These discrepancies indicate that dissolved 137Cs concentrations in the dam discharge are not determined predominantly by in situ ion exchange between solid and aqueous phases. In autumn 2019, dissolved 137Cs concentrations in the discharge of both dams decreased markedly, and the role of the Yokokawa Dam lake shifted from that of a source to a sink of dissolved 137Cs for the following three years. This occurred because Typhoon Hagibis brought intensive rainfall and caused watershed erosion, which resulted in accumulation of large amounts of sediment and changed the 137Cs balance between the water and sediment in the dam lake. When dam water storage decreased, notable increase in dissolved 137Cs concentration was observed in the discharge water. Multiple partial regression indicated that water level decline is more influential relative to water temperature rise regarding the increase in dissolved 137Cs concentrations in lake water, suggesting that water level management could mitigate the elevation of 137Cs concentration in discharge water.

国内における遊水地の生物多様性に関する文献のシステマティックレビューを実施し,治水と生物多様性を両立させる遊水地の創出に関する知見を収集した.今回,様々な分類群の生物に対する遊水地の効果を検討するため,鳥類,両生類,魚類,昆虫類,貝類,植物の 6 分類群を対象とした.9,183 件の文献を精査した結果,本システマティックレビューには 242 件の文献を選抜した.対象の遊水地は 33 か所に限定され,これは国内にある遊水地全体の 22%に過ぎなかった.最も文献数が多か ったのは鳥類の 94 件で,最少は両生類の 6 件だった.どの分類群についても,希少種や新種の分布を記載する内容が多かった.また,外来種の定着を記載する文献も鳥類を除く分類群で確認された.全体のうち BACI デザインに基づく研究事例はわずか 15 件に留まり,遊水地の生物多様性評価に関する科学的知見の不足が示唆された.そのため,遊水地の創出が生物多様性に及ぼす正負の効果について十分な検証を行うことができなかった.ただし,いくつかの研究は,湿地創出や植生管理,土壌掘削が複数の在来種の種数増加や個体数増加,希少種の分布拡大などの正の効果をもたらすことを示唆していた.

Collection and cooking of wild vegetables have provided seasonal enjoyments for Japanese local people as provisioning and cultural ecosystem services. However, the Fukushima Daiichi Nuclear Power Plant accident in March 2011 caused extensive radiocesium contamination of wild vegetables. Restrictions on commercial shipments of wild vegetables have been in place for the last 10 years. Some species, including buds of Aralia elata, are currently showing radiocesium concentrations both above and below the Japanese reference level for food (100 Bq/kg), implying that there are factors decreasing and increasing the 137Cs concentration. Here, we evaluated easy-to-measure environmental variables (dose rate at the soil surface, organic soil layer thickness, slope steepness, and presence/absence of decontamination practices) and the 137Cs concentrations of 40 A. elata buds at 38 locations in Fukushima Prefecture to provide helpful information on avoiding collecting highly contaminated buds. The 137Cs concentrations in A. elata buds ranged from 1 to 6,280 Bq/kg fresh weight and increased significantly with increases in the dose rate at the soil surface (0.10-6.50 μSv/h). Meanwhile, the 137Cs concentration in A. elata buds were not reduced by decontamination practices. These findings suggest that measuring the latest dose rate at the soil surface at the base of A. elata plants is a helpful way to avoid collecting buds with higher 137Cs concentrations and aid in the management of species in polluted regions.

The Nagoya Daruma pond frog Pelophylax porosus brevipodus (formerly Rana porosa brevipoda ) requires a wet environment year-round, but such habitats have generally been lost due to improved rice paddy drainage such that the frog populations have been decreasing. There have been attempts to create permanent pools in rice paddy areas to help the populations recover, but the basic life history patterns and population dynamics in both environments have not been well studied. We captured frogs in rice paddies and adjacent biotopes. Using capture–mark–recapture data with 816 marked individuals, we compared frog demographics and population structure using a Jolly–Seber POPAN model. Constructed biotopes had conditions favoring long-term persistence. For example, biotopes had larger frogs of both sexes than rice paddies. The ratio of juveniles to adults was lower in biotopes than rice paddies. By contrast, rice paddies were an important habitat for breeding and producing new frogs. The two habitats complemented each other to support the local frog population. Because P . p . brevipodus is now exclusively distributed in rice paddy areas, the creation of permanent pools is a feasible way to improve habitat quality, especially in modernized rice paddy areas with few permanent lentic habitats.

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.

Groundwater discharge along channels can affect stream discharge, chemistry, and ecological communities. Although the spatial distribution of groundwater springs along wide rivers can be investigated by areal thermal infrared (TIR) remote sensing, this technique is difficult to apply to mapping at a high spatial resolution and under riparian tree canopies. We present a real‐time monitoring and postprocessing method of ground‐based TIR video for determining groundwater discharge sampling points and mapping the surface water temperature. We applied this method to mapping two headwater streams in Hokkaido, Japan, in the summer. The first site was a 1.3‐km‐long reach underlain by Pleistocene andesite lava. Almost all of the springs were colder and had a different chemistry compared to that of the stream water, which supports the usefulness of TIR monitoring for determining groundwater discharge zones. Video postprocessing showed that cold groundwater springs were spaced every ∼100 m, and their distribution did not follow the topography. At the second site, cold and warm springs were underlain by Holocene volcanic ash. The cold springs mainly seeped from hyporheic and riparian zones downstream, while warm springs were at the footslope. Some cold springs had much higher solute concentrations than the stream and warm springs, which suggests that the water temperature is useful for inferring sources of groundwater discharge. At this site, the video postprocessing could map not only the locations of the cold springs but also the spatial heterogeneity of the stream temperature associated with groundwater inputs.

AbstractCollection and cooking of wild vegetables have provided seasonal enjoyments for Japanese local people as provisioning and cultural ecosystem services. However, the Fukushima Daiichi Nuclear Power Plant accident in March 2011 caused extensive radiocesium contamination of wild vegetables. Restrictions on commercial shipments of wild vegetables have been in place for the last 10 years. Some species, including buds ofAralia elata, are currently showing radiocesium concentrations both above and below the Japanese reference level for food (100 Bq/kg), suggesting that there are factors decreasing and increasing the137Cs concentration. Here, we evaluated easy-to-measure environmental variables (dose rate at the soil surface, organic soil layer thickness, slope steepness, and presence/absence of decontamination practices) and the137Cs concentrations of 40A. elatabuds at 38 locations in Fukushima Prefecture to provide helpful information on avoiding collecting highly contaminated buds. The137Cs concentrations inA. elatabuds increased significantly with increases in the dose rate at the soil surface. Meanwhile, the137Cs concentration inA. elatabuds were not reduced by decontamination practices. These findings suggest that measuring the latest dose rate at the soil surface at the base ofA. elataplants is a helpful way to avoid collecting buds with higher137Cs concentrations and aid in the management of species in polluted regions.