Mesophotic Coral Ecosystems

bleaching event, than adjacent shallow reefs (Bongaerts et al. 2010a, Bridge et al. 2014). MCEs may have the potential to act as refugia over longer timescales in some circumstances, particularly to provide lineage continuation for key coral reef taxa (Muir et al. 2015). Currently, few long-term datasets exist to enable quantitative evaluation of the deep reef refugia hypothesis, particularly over longer temporal scales (years to decades), primarily due to the logistical difficulties involved inmonitoringmesophotic habitats. There is evidence that mesophotic reef populations can mitigate against local extinction following disturbance (e.g. Sinniger et al. 2013, Smith et al. 2014). However, it is also clear that MCEs are not immune from natural and human threats, such as coral bleaching and tropical storms (see Chapter 6), and should not be considered as a panacea to addressing the threats faced by coral reef ecosystems. For example, bleaching of MCEs is known to occur where internal waves or vertical mixing brings over-heated surface waters or cooler deep waters into contact with mesophotic corals (Bak et al. 2005, Smith et al. 2015). In addition to serving as a refuge, a second premise of the deep reef refugia hypothesis is whether MCEs can provide a source of larvae to repopulate adjacent shallow reefs following a disturbance on ecologically significant timescales. The viability of MCEs to serve as a source to reseed or replenish shallow reef species is dependent on several factors, including

whether the same species are present at both depths, the extent of species adaptation at particular depths, and whether there is oceanographic connectivity between the reefs. Studies addressing this question for coral species have, to date, generally looked at genetic connectivity between mesophotic and shallow populations, and have revealed complex patterns. In general, deeper mesophotic coral populations (> 60–70 m in depth) appear to be isolated from shallower populations (Bongaerts et al. 2015b). In contrast, coral connectivity between populations shallower than 60–70 m appears to be both species and location-specific and dependent on oceanographic connectivity (van Oppen et al. 2011, Serrano et al. 2014). For fish species, connectivity has been evaluated using genetics and ecology (presence of the same species at both depths). In the case of the common coral reef damselfish, Chromis verater , no genetic differences were found among shallow and mesophotic populations (Tenggardjaja et al. 2014), meaning they constitute a single population and should be managed as such. Meanwhile, ecological connectivity has been shown for fish species between shallow reefs and MCEs off La Parguera in southwest Puerto Rico. These MCEs serve as a refuge, particularly for exploited large groupers and snappers, and 76 per cent of species present at mesophotic depths were common inhabitants of shallow reefs, indicating that connectivity exists between shallow reefs and MCEs (Bejarano et al. 2014). Irrespective of their potential to repopulate shallow-water reefs, MCEs support unique biodiversity and warrant appropriate attention from managers.

Interconnection between land and shallow-water and mesophotic reefs - the impacts of human and natural disturbances on coral reefs tend to diminish with depth and distance from shore

0m

Storms diminish with depth

Sediment plume

Sedimentation (e.g. from rivers, coastal development) and shing pressure diminish with distance from shore

60m

Source: Adapted from Bridge et al. 2013

Figure 1.1. Impacts of human and natural disturbances tend to decrease with depth and distance from the coast, making shallow reefs generally more vulnerable than MCEs.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 10

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