Bioerosion of Mesophotic Reefs in the U.S. Virgin Islands

Weinstein, David K.; Klaus, James S.; Smith, Tyler B.; Reid, R. P.; Kiene, William E.

Bioerosion is a fundamental sedimentary process known to impact the development and persistence of shallow reef structures and ultimately influence facies properties of reef systems. The deterioration of shallow coral reef health and the potential connectivity of deeper reefs to shallow reefs have renewed interest in mesophotic coral ecosystems (MCEs). However, little is known regarding the impact and variability of mesophotic bioerosion, or how it might affect MCE sedimentology and geomorphology. To investigate the potential role of bioerosion in the structural development of mesophotic reefs, experimental coral substrates were used to calculate bioerosion rates at four distinct MCE habitats as well as two shallow water reefs. Additionally, coral rubble collected at each site was used to evaluate the long-term effect of macroboring.

Results suggest significant differences in both the initial bioerosion and long-term coral rubble macroboring of different mesophotic reef habitats, and also with nearby shallow-water reefs. Substrate weight change is dominated by bioerosional grazing in the shallow water reefs and the shallowest mesophotic reef (30.7 m) while the primary long-term substrate alteration is dominated by sponges in mid-depth mesophotic reefs (39-50 m). Substrate weight reduction was correlated with decreasing bioeroding parrotfish biomass and increasing seawater depth. Comparison between mesophotic sites revealed significant mesophotic bioerosion rate variability between shallow mesophotic reefs (-19.6 g/year) and mid-depth mesophotic reefs (maximum of 3.7 g/year) after two years of exposure. Macroborer removal of carbonate in coral rubble was lower at shallow sites compared to all mesophotic sites except at a deep rhodolith mesophotic patch reef. Relatively similar initial substrate bioerosion rates of mid-depth mesophotic reefs lead us to suggest that the structural variability of reefs at these depths is dependent on the quantity and location of available substrate and the exposure time of both in situ coral framework and coral rubble within a local habitat. This study provides one of the first comparisons of substrate modification within multiple deep reef systems, and extends the depth gradient of reef bioerosional processes. However, more research is required to understand how differences in substrate production, availability, and distribution effect mesophotic coral recruitment and ultimately the overall carbonate system.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013