Trapa spp. dominate many shallow eutrophic lakes in Japan, which must affect the nutrient dynamics in lakes. Trapa spp. are utilized by several animals, in particular the leaf beetle, Galerucella. To quantify the trophic links and material transfers between Trapa spp. and G. nipponensis, their seasonal dynamics, the amount of leaves being consumed, and carbon and nitrogen stable isotope ratios (δ13C and δ15N) in Lake Inba were tracked. Trapa spp. began to increase in May, reached their highest biomass in September, and disappeared in October. G. nipponensis exhibited continuous reproduction from May until October. Grazing by G. nipponensis did not seem to affect the abundance of Trapa, as the average percentage of Trapa leaf consumed was 3%. Rather, G. nipponensis was influenced by Trapa, as δ13C and δ15N of Trapa spp. and G. nipponensis showed very similar seasonal fluctuations. The difference in δ15N was 3–4‰, which indicates that Trapa leaves were a main food source for G. nipponensis and a trophic linkage exists between them. The estimated nitrogen and phosphorus amounts in Trapa corresponded to 1% of nitrogen and 18% of phosphorus in the entire lake. If G. nipponensis migrate and die in a terrestrial environment, G. nipponensis can transport certain nutrients from lake to terrestrial environments. The nutrients from Trapa spp. are mostly regenerated, except for that biomass transported to terrestrial ecosystems by humans.