Time-series of radiocesium (Cs-137) in forest–stream ecosystem compartments of the Ota River catchment (Fukushima, Japan), 2018–2025

This dataset provides multi-compartment time-series measurements of radiocesium (Cs-137) in a headwater forest–stream system in Fukushima, Japan (Ota River catchment). The data were collected to support understanding of long-term contamination dynamics, including the slowing of declines (“leveling off”) reported for some freshwater biota. Samples were collected concurrently at two upstream sites (ST1 and ST4) from 2018 to 2025. The dataset covers major forest-derived materials and in-stream compartments: periphyton, riparian leaves, terrestrial litter, aquatic litter, terrestrial insects, aquatic insects, crustaceans, stream water (dissolved Cs-137), and streambed sediment. Cs-137 activity concentrations were quantified by gamma-ray spectrometry using an HPGe detector. Over the monitoring period, Cs-137 concentrations in many environmental compartments (e.g., periphyton, riparian leaves, and litter) generally declined, whereas those in stream salmonid fish tended to decline more slowly and showed signs of leveling off. The Excel file contains four data worksheets (Periphyton_Leaves_Litter, Insects_Crustaceans, Water, Sediment) and a “Data Description” sheet summarizing variables, units, and notes.