Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

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

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.

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

Sedimentation (e.g. from rivers,

coastal development) and

shing pressure diminish with

distance from shore

Storms diminish

with depth

60m

Sediment plume

Source: Adapted from Bridge et al. 2013