Mesophotic Coral Ecosystems

learned that MCEs are not protected from all threats, and that oceanographic processes are perhaps more important than depth in terms of buffering corals from the impact of thermal stress (Neal et al. 2014, Nir et al. 2014). 6.3.1. Coral bleaching MCEs are not immune to thermal stress (Neal et al. 2014) and coral bleaching. Coral bleaching, as evidenced by coral skeletons that are white or bleached in appearance, occurs when zooxanthellate corals become stressed and lose or expel their symbiotic algae (Figure 6.4). Corals can regain their algae and recover from coral bleaching, but prolonged stressful conditions may cause corals to die from starvation. Coral bleaching often results from an interaction between high water temperatures and high light irradiance (Baird et al. 2009). The thresholds at which bleaching occurs vary among individual species, populations and geographic locations

(Baker et al. 2008). In some cases, previous exposure to stressors is also a factor affecting resilience. For example, corals that inhabit areas with fluctuating environmental conditions are often more tolerant than those from stable environments (Oliver and Palumbi 2011). The mechanisms influencing thermal tolerance include photo-protective pigments, a natural coral sunscreen (Salih et al. 2000), and switching of algal symbionts to more light-tolerant strains (Bongaerts et al. 2015b). However, since corals are thermally adapted or acclimated to their local environment, their ultimate response to fluctuating temperatures will be dictated by how quickly the external environment changes relative to their recent history (Oliver and Palumbi 2011). Such dependence is evident in the observed seasonal bleaching in mesophotic corals in the Red Sea as summer months bring higher light levels and temperatures (Nir et al. 2014). Consequently, as observed in the U.S. Virgin Islands (Smith et al. 2015), mesophotic corals may exhibit lower bleaching thresholds than corals in shallower habitats. However, other mesophotic corals occur in dynamic

Temperature at 57 metres water depth

18 19 20 21 22 23 24 25 26 27 28 29 30 31

Date photographs taken

Water temperature o c

“La Nina” like

Temperature at 57 m depth recorded every 30 minutes. Location, short drop o , Palau

“El Nino” like

Jan Feb Mar

Apr

May

Jun

Jul

Aug Sep Oct

Nov

Dec

Source: Colin 2010

Figure 6.5. Temperature at 57 m in depth recorded every 30 minutes during 2010 off Palau. The rapid shift from a highly stratified El Niño type water column to a uni-thermal (at near 30 o C) water column, with associated coral bleaching seen on the mesophotic reef (photos below graph). The bleached corals are platy agariciid corals within 30 m of the thermograph station. These and other similar corals did not survive the bleaching event of 2010 (photos Patrick L. Colin).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 71