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In the Mexican Caribbean, the decomposition of unusual amounts of Sargassum (Sargassum fluitans and S. natans) has caused changes in the benthic community. The aim of this study was to evaluate the impact of massive pelagic Sargassum arrivals on the trophic structure of sea urchins Echinometra lucunter and E. viridis, using stable isotopes of carbon (δ13C) and nitrogen (δ15N) to identify variations in relative proportions of assimilated primary producers in two reef lagoons (Xahuayxol and Xcalak) in the southern Mexican Caribbean. The survey considered two scenarios: (1) with massive Sargassum arrivals (WIMA) in 2018 and (2) without massive Sargassum arrivals (WOMA) in 2017. Results revealed a decreased in δ15N and δ13C values during WIMA in primary producers and sea urchins. Furthermore, the isotopic mixing models (MixSIAR) indicated that both E. lucunter and E. viridis preferably consumed Halimeda spp. and Thalassia testudinum. The isotopic niche width obtained from Stable Isotope Bayesian Ellipses in R (SIBER) analysis showed differences between species and localities. In Xahuayxol during WIMA the corrected standard ellipse area (SEAc) of E. lucunter increased while that of E. viridis decreased. However, the latter species had a significant increase in trophic level. In Xcalak during WIMA, for both sea urchins, the SEAc increased and overlapped while the trophic levels increased significantly. Results suggest that E. viridis in Xahuayxol was better adapted to changes in resource availability, while the niche expansion of E. lucunter in Xahuayxol and E. lucunter and E. viridis in Xcalak suggest that sea urchins exhibited a starvation stage due to a decrease in food resources. Inglés