Invasive species and trophic pathways: long-term shifts in food chain dynamics of a fragmented river's floodplain

Impoundments within the Paraná River basin have significantly impacted the Upper Paraná River Floodplain (UPRF). The Itaipu Hydropower Plant, installed in 1982, removed a natural barrier, enabling fish species from the Lower Paraná River to colonize the Upper Paraná River. The completion of the Porto Primavera Dam in 1998 upstream of the UPRF facilitated the invasion of a submerged aquatic macrophyte ( Hydrilla verticillata (L.f) Royle) in 2005. We aimed to assess the long-term impact of this invasive macrophyte on food chain dynamics in the UPRF, with a focus on effects to an invasive herbivore and native detritivore fish species ( Schizodon borellii Boulenger, 1900 and Prochilodus lineatus Valenciennes, 1836; respectively). Impacts were assessed across three periods spanning 20 years (early 2000s, 2010s, and 2020s), capturing periods before, shortly after, and long after the macrophyte invasion. We hypothesized that the invasive macrophyte's contribution to both the green trophic pathway (energy flow from living primary producers through herbivores to predators) and the brown trophic pathway (energy flow from decomposed organic matter through detritivores to predators) would increase over time, eventually becoming the primary energy source. Using stable isotope analysis, we examined food chain energy sources, trophic niches, and trophic positions of both consumers associated with the UPRF. Our findings indicate that the herbivorous invasive fish incorporates carbon from the invasive macrophyte into the green food chain, while the native detritivore fish likely relocate from the main channel to floodplain lakes, utilizing detritus from native emerged macrophytes located only in these environments. Additionally, both species experienced an increased trophic niche shortly after invasion, possibly relating to behavioral and foraging adaptations following disturbances to the floodplain. Our study underscores how invasive species can replace native food sources, reshape local food webs, and exacerbate the ecological impacts of impoundments. • Trophic dynamics of a floodplain downstream of a dam change after macrophyte invasion. • Invasive fish incorporate carbon from an invasive plant into the green food chain. • The trophic niche of native and invasive fish expands shortly after a macrophyte invasion. • Invasive plants replace native food sources and reshape local food webs. • Invasive species exacerbate the ecological impacts of impoundments.