Variation in basal sources contribution to the diet of a predator fish in an altered flood pulse area in the Amazon

This study examined the diet and assimilation of basal resources by the redeye piranha, Serrasalmus rhombeus, in a low-flow section of the Xingu River, Brazil. Samples of both individuals and basal resources were collected monthly from December 2020 to November 2021, encompassing a complete hydrological cycle. The stomach contents of the individuals and tissue samples of the species were submitted to stable isotope analysis for δ¹³C and δ¹⁵N. The species exhibited a predominantly piscivorous diet, with no observed variation in diet composition or feeding intensity across different hydrological periods. However, δ¹⁵N levels in the species varied between periods, indicating potential shifts in the availability and assimilation of basal resources during controlled flood pulses. The trophic position of the species was higher during flood periods and lower during low-water periods. Mixing models revealed the assimilation of different basal sources in each period, with the significance of terrestrial producers evident in almost all periods except during ascending. These findings are crucial for understanding the trophic dynamics of predators in flooded forests with reduced flow, highlighting the reliance of S. rhombeus on piscivory while being supported by terrestrial basal sources in its trophic chain.