Patterns of benthic cover with depth on Caribbean mesophotic reefs

Abstract

Tropical mesophotic reefs, which span the transition between shallow reefs and the bottom of the photic zone (30–150 m), are an understudied ecosystem. It has been proposed that sponge abundance increases with depth through 150 m on reefs throughout the Caribbean and perhaps globally (the “sponge increase hypothesis”). A recent review concluded that there was not sufficient evidence in the literature to support this hypothesis for the Caribbean and that further data quantifying changes in sponge abundance with depth on mesophotic reefs are needed. In this study, percentage cover of sponges, macroalgae, and hard corals was estimated using 1123 images taken by remotely operated vehicles on 58 dive tracks on mesophotic reefs off the coasts of Puerto Rico (to 180 m) and St. Thomas (US Virgin Islands; to 100 m), and on the Flower Garden Banks (Gulf of Mexico; to 100 m). Off Puerto Rico, sponge cover decreased with increasing depth below ~ 100 m, driven primarily by a decline in the cover of emergent (non-encrusting) sponge species. Sponge cover did not change with depth off St. Thomas or on the Flower Garden Banks and was an order of magnitude lower on the Flower Garden Banks. The data from these three locations did not support the sponge increase hypothesis. Mean cover of hard coral off Puerto Rico and St. Thomas was less than 8% between 20 and 80 m and negligible below these depths, but much higher ( ~ 15–30%) between 30 and 40 m on the Flower Garden Banks and negligible below these depths. Off Puerto Rico, cover of macroalgae decreased with depth in a manner similar to sponge cover, which may reflect a positive relationship between sponges and macroalgae in the lower mesophotic zone ( > 100 m). Off St. Thomas and on the Flower Garden Banks, which were both shallower sites with more abundant macroalgae, sponge abundance was likely limited due to competition for space. Generalizations about patterns of sponge abundance across mesophotic depths await further studies, but our data suggest a depth-dependent relationship with macroalgal abundance.