DEEPER REEF BIOEROSION: VARIABILITY AND IMPLICATIONS OF MESOPHOTIC REEF MODIFICATION IN THE US VIRGIN ISLANDS

Like their shallow counterparts, mesophotic coral reefs build intricate geomorphic structures that create vital habitats responsible for many potential ecological and economic benefits. While bioerosion is known to greatly influence the foundational components of shallow-water reefs, little is understood about this critical sedimentary process within mesophotic coral ecosystems. Coral rubble and previously deployed experimental coral substrates of similar composition, exposed for a period of 1 and 2 years, were collected at four MCE habitats and two shallower reefs south of St. Thomas, U.S. Virgin Islands. The collected samples were used to investigate the variability and potential impact of bioerosion on structurally distinct mesophotic habitats.

Following 2 years of exposure, significant variability in experimental substrate bioerosion rates were found between the shallowest (30.7 m) mesophotic reef habitat at -19.6 g/year and the mid-depth mesophotic habitats (39-50 m), averaging 0.534 g/year (SD = 3.01). Experimental substrate weight loss correlated with both decreasing seawater depth and increasing bioeroding parrotfish biomass. Coral rubble macroboring abundance was significantly less in shallow sites than in all coral dominated mesophotic habitats studied, and macroboring diversity was highest at shallow reefs. Study results lead us to conclude that substrate modification is generally dominated by: (1) grazing organisms, along with physical processes, in the shallow-water reefs; (2) grazing organisms (initially) and potential macroboring sponges (long-term) in shallow mesophotic habitats; and (3) macroboring sponges in mid-depth mesophotic habitats. Relatively homogeneous experimental substrate initial bioerosion rates at the deepest study sites suggest that the structural variability of mid-depth mesophotic reefs is mainly dependent on the local quantity, location, and exposure time of available in situ coral framework and coral rubble. These attributes are likely controlled by differences in coral growth rates, partial mortality, and current forces impacting macro-bored reef framework. This study provides one of the first comparisons of substrate modification between multiple mesophotic reef systems.