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 distributed growth rates for corals deeper than 30 m are rare, and the combined influence of coral framework accretion, secondary framework accretion, and framework macroboring on net framework calcification in these habitats is poorly constrained. To better understand the growth limits and spatial variability of Orbicella-dominated mesophotic coral reef ecosystems, live platy samples of Orbicella franksi were collected at 3 upper mesophotic reef habitats with varying structural characteristics on the Puerto Rican Shelf, south of St. Thomas, US Virgin Islands. Average mesophotic coral linear extension rates, determined by standard X-radiographic techniques and confirmed by stable isotopic 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 considerably higher than at nearby shallow-water St. Thomas reefs, implying that fast coral growth rates 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 framework 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 on water circulation and salinity as well as on water clarity and nutrient distribution in the Caribbean is reflected in the annual variability of mesophotic O. franksi growth rates. Site differences in net coral framework calcification suggest a potential long-term mechanism for the production or maintenance of heterogeneous reef-scale geomorpho logy along broad-sloping carbonate shelves colonized with mesophotic coral reef systems.

Mesophotic coral ecosystems are threatened and ecologically distinct from shallow water reefs

“…Many of these reefs may have already been degraded and/ or eliminated by destructive fishing, mining, and sedimentation, and many species and potential natural resources may disappear before we have the chance to discover and study them. Because of the generally slower growth of reef-building http://science.sciencemag.org/ invertebrates at depth (24), recovery of these regions may depend on long-term management of disturbed areas. Thus, measures to protect deeper ecosystems should be prioritized in environmental policy for global marine conservation.…”

Ecological insights from environmental disturbances in mesophotic coral ecosystems

“…Mesophotic coral reefs show lower bioerosion (Weinstein et al. , ) and mortality (Laverick and Rogers ) rates than shallow reefs, and these plate‐like, foliose, or tabular corals commonly cover extensive areas on deep reefs (Kahng et al. ).…”

“…, Kahng , Weinstein et al. , Groves et al. ), a high frequency and intensity of natural disturbances and their association with anthropogenic stressors could undermine the resilience of the system and cause phase shift scenarios in degraded assemblages.…”

“…in the Caribbean (Hoeksema et al 2017). Mesophotic coral reefs show lower bioerosion (Weinstein et al 2014(Weinstein et al , 2016 and mortality (Laverick and Rogers 2018) rates than shallow reefs, and these plate-like, foliose, or tabular corals commonly cover extensive areas on deep reefs (Kahng et al 2010). In this context, we propose that disturbance of these communities may actually provide the microhabitat and space necessary for highly successful recruitment ) and occurrence of different species.…”

“…However, when weakened by stressors such as overfishing, sedimentation, pollution, or invasive species, MCEs may become less resilient to disturbance, as is also the case for shallow coral reefs (Hughes et al 2007). Since MCE coral species appear to have slow growth and/or calcification rates due to decreased metabolic rates (Kahng et al 2010, Kahng 2013, Weinstein et al 2016, Groves et al 2018), a high frequency and intensity of natural disturbances and their association with anthropogenic stressors could undermine the resilience of the system and cause phase shift scenarios in degraded assemblages.…”