Facilitation of coral reef biodiversity and health by cave sponge communities

Abstract

Marine caves are understudied ecosystems that are frequently associated with coral reef communities; many are tidally influenced and may host a highly diverse sponge fauna. Although each cave represents a distinct habitat likely structured by site-specific hydrographic processes, a more complete understanding of the ecology of these communities requires com parative studies. Based on a gradient of sponge cover within 5 Bahamian caves, we conducted a natural experiment in sponge-derived nutrient enrichment of nearby patch reefs. We tested the hypothesis that water exiting the cave during low tide provides a nutrient-rich resource that facilitates the diversity and health of nearby reef communities. The percent cover and diversity of corals surrounding the openings of caves were significantly higher than in similar habitats farther removed from these communities. There was a significant correlation between percent sponge cover within the caves and nitrate concentrations in seawater flowing out of the caves, and δ15N stable isotope signatures indicated enrichment of the nearby reefs by sponge-derived nitrate. Zooxanthellae abundance and total protein concentration were higher in corals from reefs near cave entrances, suggesting that those corals benefited more from cave nutrients than did corals farther from cave openings. In addition to corals, percent algal cover increased near cave entrances, but these potential competitors of corals were kept in check by increased levels of herbivory relative to sites removed from cave mouths. As global environmental changes continue to impact coral reef ecosystems, diversity ‘hot spots’, such as these marine caves, could serve as refuges and ‘seedbanks’ for nearby dwindling reef habitats.