Radiocesium contamination from the Fukushima Daiichi Nuclear Power Plant has affected freshwater ecosystems for years. By studying two rivers and two lakes in Fukushima, researchers found that radiocesium levels decrease from plants to fish in rivers, but increase with higher trophic levels in lakes. Fish in rivers get more radiocesium from periphyton, while those in lakes get it from zooplankton. Lakes also receive radiocesium from the pelagic food web. This study shows how stable isotope analysis can help understand radiocesium dynamics in freshwater food webs and identify important sources of contamination.