Coral growth, bioerosion, and secondary accretion of living orbicellid corals from mesophotic reefs in the US Virgin Islands
Abstract
Growth rates of the major Caribbean reef framework-building coral Orbicella spp decrease with increasing water depth and light attenuation. However, reliable, spatially distribute growth rates for corals deeper than 30 m are rare, and the combined influence of coral framewor accretion, secondary framework accretion, and framework macroboring on net framework calcificatio in these habitats is poorly constrained. To better understand the growth limits and spatia variability of Orbicella-dominated mesophotic coral reef ecosystems, live platy samples of Orbicell franksi were collected at 3 upper mesophotic reef habitats with varying structural characteristics o the Puerto Rican Shelf, south of St. Thomas, US Virgin Islands. Average mesophotic coral linea extension rates, determined by standard X-radiographic techniques and confirmed by stable isotopi analyses, were 0.80 ± 0.03 mm yr?1 (±SE), slower than previously recorded for Orbicella spp at shallower US Virgin Island reefs. However, coral cover at 2 of the mesophotic reefs was considerabl higher than at nearby shallow-water St. Thomas reefs, implying that fast coral growth rate are not necessarily needed to maintain Caribbean reefs with high coral cover and structural complexity A probable reason for this is reduced bioerosion with depth. Rates of net coral framewor calcification were significantly different between the neighboring mesophotic coral reef habitats likely a result of the complex interplay between site variability in irradiance, nutrient availability and other factors. Analysis also indicated that the influence of El Niño-Southern Oscillation o water circulation and salinity as well as on water clarity and nutrient distribution in the Caribbea is reflected in the annual variability of mesophotic O. franksi growth rates. Site differences in ne coral framework calcification suggest a potential long-Term mechanism for the production or maintenanc of heterogeneous reef-scale geomorpho logy along broad-sloping carbonate shelves colonize with mesophotic coral reef systems.