blooms in the Caribbean alter the trophic structure of the sea urchin .

The arrival of large masses of drifting since 2011 has caused changes in the natural dynamics of Caribbean coastal ecosystems. In the summer of 2015, unprecedented and massive mats of and have been observed throughout the Mexican Caribbean including exceptional accumulations ashore. This study uses stable isotopes to assess the impact of blooms on the trophic dynamics of the sea urchin, a keystone herbivore on many Caribbean reefs. Bayesian models were used to estimate the variations in the relative proportions of carbon and nitrogen of assimilated algal resources. At three lagoon reef sites, the niche breadth of was analysed and compared under massive influx of drifting spp. vs. no influx of blooms. The effects of the leachates generated by the decomposition of led to hypoxic conditions on these reefs and reduced the taxonomic diversity of macroalgal food sources available to . Our trophic data support the hypothesis that processes of assimilation of carbon and nitrogen were modified under effect. Isotopic signatures of macroalgae associated with the reef sites exhibited significantly lower values of δN altering the natural herbivory of . . The Stable Isotopes Analysis in R (SIAR) indicated that, under the influence of blooms, certain algal resources (, and ) were more assimilated due to a reduction in available algal resources. Despite being an abundant available resource, pelagic was a negligible contributor to sea urchin diet. The Stable Isotope Bayesian Ellipses in R (SIBER) analysis displayed differences between sites, and suggests a reduction in trophic niche breadth, particularly in a protected reef lagoon. Our findings reveal that blooms caused changes in trophic characteristics of with a negative impact by hypoxic conditions. These dynamics, coupled with the increase in organic matter in an oligotrophic system could lead to reduce coral reef ecosystem function.