Habitat characteristics and environmental factors related to boring sponge assemblages on coral reefs near populated coastal areas on the Mexican Eastern Pacific coast

Abstract

Coral reefs are undergoing global decline due to the environmental degradation of coastal areas through increasing human impacts from urban coastal development. Increases in the abundance of boring organisms are suggested to be a direct consequence of coral mortality and the exposure of calcareous substrates associated with deteriorated reef environments. The Mexican Pacific coast harbors coral reefs undergoing rapid degradation. In this study, we analyzed species richness and abundance of boring sponges associated with living corals, dead coral framework, and coral rubble from five coral reefs near the tourist destination of Zihuatanejo Bay, Guerrero. Environmental and habitat parameters indicative of human impacts on coral reefs were also recorded and analyzed. All coral reefs exhibited similar species richness (10–12 species), but sponge abundance (25–60 %) varied among the study sites. While dead coral framework and coral rubble were the preferred substrate types for boring sponges, the sites with the highest coverage of dead corals did not show the highest abundance of boring sponges. In fact, the reefs closest to the tourist site, which were weakly or moderately conserved, exhibited the highest levels of sponge abundance. These reefs also had the highest levels of δ15N (up to 9.5 ‰) and δ13C (−13.3 ‰) and high concentrations of chlorophyll a (1.8 mg m−3), which are indicative of wastewater impacts. In contrast, sites more distant from the tourist area, which were moderately or highly conserved, had less well-developed sponge assemblages. The analysis of environmental indicators showed comparatively high environmental quality for these sites, with water clarity values up to 7.5 m and chlorophyll a concentrations as low as 0.27 mg m−3. Differences in species composition were also evident among coral reef sites, with species such as Pione carpenteri, Thoosa calpulli, Siphonodictyon crypticum, Cliona mucronata and C. tropicalis dominating the more highly impacted sites. These results indicate that anthropogenic sources of nutrients may have a greater positive influence on the abundance of boring sponges on coral reefs near human populated sites.