Sargassum blooms in the Caribbean alter the trophic structure of the sea urchin Diadema antillarum

Nancy Cabanillas-Terán; Héctor A Hernández-Arana; Miguel-Ángel Ruiz-Zárate; Alejandro Vega-Zepeda; Alberto Sanchez-Gonzalez

doi:10.7717/peerj.7589

Sargassum blooms in the Caribbean alter the trophic structure of the sea urchin Diadema antillarum

PeerJ. 2019 Aug 30:7:e7589. doi: 10.7717/peerj.7589. eCollection 2019.

Authors

Nancy Cabanillas-Terán¹, Héctor A Hernández-Arana², Miguel-Ángel Ruiz-Zárate², Alejandro Vega-Zepeda², Alberto Sanchez-Gonzalez³

Affiliations

¹ Consejo Nacional de Ciencia y Tecnología México- El Colegio de la Frontera Sur, Chetumal, Quintana Roo, México.
² Departamento de Sistemática y Ecología Acuática, El Colegio de la Frontera Sur, Chetumal, Quintana Roo, México.
³ Centro Interdisciplinario de Ciencias Marinas del Instituto Politécnico Nacional, La Paz, Baja California, México.

Abstract

The arrival of large masses of drifting Sargassum since 2011 has caused changes in the natural dynamics of Caribbean coastal ecosystems. In the summer of 2015, unprecedented and massive mats of S. fluitans and S. natans have been observed throughout the Mexican Caribbean including exceptional accumulations ashore. This study uses stable isotopes to assess the impact of Sargassum blooms on the trophic dynamics of the Diadema antillarum 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 D. antillarum was analysed and compared under massive influx of drifting Sargassum spp. vs. no influx of Sargassum blooms. The effects of the leachates generated by the decomposition of Sargassum led to hypoxic conditions on these reefs and reduced the taxonomic diversity of macroalgal food sources available to D. antillarum. Our trophic data support the hypothesis that processes of assimilation of carbon and nitrogen were modified under Sargassum effect. Isotopic signatures of macroalgae associated with the reef sites exhibited significantly lower values of δ¹⁵N altering 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 available algal resources. Despite being an abundant available resource, pelagic Sargassum 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 Sargassum blooms caused changes in trophic characteristics of D. antillarum 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.

Keywords: Coral reefs; Echinoids; Mexican caribbean; Niche breadth; Pelagic macroalgae; Stable isotopes; Trophic ecology.

Grants and funding

This work was funded by Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT) through the project: OGRMIS- DAC-UCR #001/2015 ECOSUR/SEMARNAT. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.