Mesophotic Coral Ecosystems

2.3. Geomorphology of mesophotic coral ecosystems

MCE habitats may be broadly characterized as either platforms or slopes (Locker et al. 2010). Low-gradient platform MCE habitats include outer continental and insular shelves, relic terraces and isolated banks with relatively flat tops. Slope habitats include the steep margins of continental and insular shelves and banks that extend from the platform break to the adjacent basin. MCEs are often extensions of shallow coral ecosystems, located directly below shallow reefs. However, not all MCEs have a shallow-water counterpart, for example Pulley Ridge and Gulf of Carpentaria MCEs, described in Chapter 3, are not adjacent to shallow reefs and are located offshore. Platform habitats that dip gently into the mesophotic zone can include relict ridges, terraces and banks that formed during periods of lower sea level (Harris and Davies 1989, Macintyre et al. 1991, Beaman et al. 2008, Harris et al. 2008; see text box). These features may be the result of erosional processes (e.g. wave cut platforms), constructional processes (i.e., relict reefs) or a combination of the two. Importantly, 2.3.1. Platform habitats

they are hard substrates that are topographically high or prominent slope breaks that are conducive to colonization by MCEs. Examples include extensive areas (> 25,000 km 2 ) of submerged banks in the Great Barrier Reef (Harris et al. 2013), submerged ridges off the south coast of Barbados, and relict terraces on many Pacific Islands (Bare et al. 2010). Often, a series of terraces can be found off a given stretch of coastline (e.g. Barbados), with the terraces at different mesophotic depths being colonized by different species and growth forms of corals (Rooney et al. 2010). MCEs in slope habitats are influenced by slope gradient and geomorphology (Sherman et al. 2010). Optimal slope habitats for MCEs are stable, rocky protrusions affording access to light and away from gullies and submarine canyons in which sediment and debris are transported downslope (Sherman et al. 2010). In the Caribbean, many islands and banks have steep outer slopes within the mesophotic zone, and in the tropical Indian and Pacific Oceans, both barrier and fringing reefs may have MCEs on their lower slopes. 2.3.2. Slope habitats

MCEs established after the last ice age

All MCE habitats were established under rising global sea levels after the last ice age (Figure 2.12). Sea level was 120 m below its present position at around 18,000 years before present (BP) when Pleistocene reefs lived on the continental slope. Sea level rose to 50 m by around 12,000 years BP and corals colonized relict limestone platforms and other rocky surfaces on the outer shelf (or on atoll rims), leaving the Pleistocene reefs stranded below rising sea levels on the slope.

Sea level rose rapidly to 30 m by around 10,500 years BP. Some reefs were able to keep up with sea level rise but others, for reasons that are not fully understood, were not (Montaggioni 2005, Harris et al. 2008, Woodroffe and Webster 2014). By the time sea level reached its present position around 6,500 years BP, only some reefs had kept pace with rising sea levels; those that had not are sites of many of today’s MCEs (sensu Macintyre 1972).

Mesophotic coral ecosystems (MCEs) established under rising sea levels

Reef growth continues

Relict reef limestone Pleistocene reef

Holocene reef growth initiated

Shallow reef

Reef growth initiated

Reef growth stalled

Shallow reef

MCEs

shelf

Reef growth stalled

slope

atoll

Sea level -120 metres

Sea level -50 metres

Sea level -30 metres

Present Sea level

18000 years before present

12000 years before present

10500 years before present

6500 years to present

Source: GRID-Arendal

Figure 2.12. MCEs established under rising sea level.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 17