Resumen del producto

The arrival of large masses of drifting Sargassum since 2011 has caused changesin the natural dynamics of Caribbean coastal ecosystems. In the summer of 2015,unprecedented and massive mats of S. fluitans and S. natans have been observedthroughout the Mexican Caribbean including exceptional accumulations ashore.This study uses stable isotopes to assess the impact of Sargassum blooms on thetrophic dynamics of the Diadema antillarum sea urchin, a keystone herbivore on manyCaribbean reefs. Bayesian models were used to estimate the variations in the relativeproportions of carbon and nitrogen of assimilated algal resources. At three lagoon reefsites, the niche breadth of D. antillarum was analysed and compared under massiveinflux of drifting Sargassum spp. vs. no influx of Sargassum blooms. The effects of theleachates generated by the decomposition of Sargassum led to hypoxic conditions onthese reefs and reduced the taxonomic diversity of macroalgal food sources available toD. antillarum. Our trophic data support the hypothesis that processes of assimilationof carbon and nitrogen were modified under Sargassum effect. Isotopic signatures ofmacroalgae associated with the reef sites exhibited significantly lower values of 15Naltering the natural herbivory of D. antillarum. The Stable Isotopes Analysis in R(SIAR) indicated that, under the influence of Sargassum blooms, certain algal resources(Dictyota, Halimeda and Udotea) were more assimilated due to a reduction in availablealgal resources. Despite being an abundant available resource, pelagic Sargassum wasa negligible contributor to sea urchin diet. The Stable Isotope Bayesian Ellipses in R(SIBER) analysis displayed differences between sites, and suggests a reduction in trophicniche breadth, particularly in a protected reef lagoon. Our findings reveal that Sargassumblooms caused changes in trophic characteristics of D. antillarum with a negative impactby hypoxic conditions. These dynamics, coupled with the increase in organic matter inan oligotrophic system could lead to reduce coral reef ecosystem function.

Palabras clave: D. antillarum; stable isotopes analysis; Sargassum spp

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