Invasive bivalves alter energy pathways in freshwaters, but their trophic dynamics in floodplain systems under varying levels of human impacts remain unclear. We investigated how trophic status, river regulation and flood pulse shape the diet and isotopic niche of the invasive bivalve Limnoperna fortunei (Dunker 1857) in two large rivers of a Neotropical floodplain: an oligotrophic, dam-regulated, highly impacted river (HIR) and a meso-eutrophic, free-flowing, moderately impacted river (MIR), across dry, rising and flood phases. We sampled L. fortunei and food resources (phytoplankton, zooplankton, periphyton, macrophytes, riparian vegetation, sediment, particulate organic carbon and methanotrophic bacteria) and used stable isotopes of carbon (δ13C) and nitrogen (δ15N), Bayesian mixing models and isotopic niche metrics (standard ellipse areas) to quantify dietary contributions and trophic niche breadth. Autochthonous resources dominated the diet in both rivers, but their identity and relative importance varied: in the HIR, L. fortunei shifted from a phytoplankton-dominated diet (dry) to increasing reliance on macrophytes and zooplankton (flood); whereas in the MIR, macrophytes remained the main dietary source year-round, with the consumption of riparian vegetation increasing during floods and methane-derived carbon contributing modestly to the diet. Isotopic niche area was larger in the HIR and expanded from dry to flood phases in both systems, doing so abruptly in the HIR and gradually in the MIR. Overall, L. fortunei tracks spatiotemporal variation in resource availability and maintains a broad trophic niche, particularly in degraded, regulated environments. This trophic plasticity enhances invasion success and highlights how hydrological alteration and land use change can favour invasive consumers in floodplain rivers worldwide.
