Mesophotic Coral Ecosystems

2.4. Differences between shallow-water and mesophotic coral ecosystems

While MCEs are viewed as extensions of shallow-water coral reef ecosystems, there are some notable differences between them (Table 2.1). It is important to note that the transition from shallow-water reefs to MCEs does not occur at a specific depth; rather the depth of transition varies between locations depending on water clarity (depth of light penetration), temperature, substrate type and other factors. In general, in tropical and subtropical areas, coral reefs shallower than approximately 30– 40 m are considered to be shallow-water reefs. For example, in the Great Barrier Reef, shallow reefs cover an area of 20,680 km 2 and have a mean depth of 14.9 ± 15.4 m (Harris et al. 2013). MCEs generally occur below a depth of approximately 30–40 m and may extend to over 150 m in clear waters. There is no specific lower depth limit of MCEs because this also varies by location. Shallow reefs may occur adjacent to land, as in the case of fringing reefs, or they may be located a distance offshore, such

as in the case of platform reefs, shelf-edge barrier reefs and atolls. MCEs may be located close to shore in areas with steep bathymetry, but are also found a distance from land, either independently or as deep-water extensions of shallow reefs. Overall, distance from land is not a reliable predictor of reef occurrence for either shallow coral reefs or MCEs. The hydrodynamic environment of surface coral reefs is quite different from that of MCEs. Breaking waves over surface reefs induce flow and circulation within the reef (Gourlay and Colleter 2005). Surface reefs may locally amplify tidal currents such that they are accelerated through narrow, inter-reef channels, a process which controls their geomorphic evolution (Hopley 2006). Finally, shallow lagoon waters may become thermally stratified (e.g. Andrews et al. 1984). These processes are much reduced or non-existent on MCEs.

Table 2.1. General differences between shallow-water coral reef ecosystems and MCEs.

Shallow-water coral reef ecosystems

Mesophotic coral ecosystems (MCEs)

Depth range

• 0 to approx. 30–40 m. • Lower depth corresponds to a moderate faunal transition. • Detectable in satellite images.

• From approx. 30–40 m to deeper than 150 m. • Lower depth limit varies by location due to differences in light penetration and other abiotic factors. • Not detectable in satellite images. • Dominant species are plate-like and encrusting zooxanthellate scleractinian corals, octocorals, antipatharians, calcareous and foliose macroalgae and sponges. • Generally temperatures are cooler and naturally more variable on MCEs than on shallower reefs, especially those located on the continental slope, which are subject to internal waves. • Deeper water column may protect MCEs from extreme (warm) thermal events. • Below the depth affected by breaking waves. • Seafloor generally unaffected by wave motion. Powerful storms can directly and indirectly impact MCEs (resuspend sediment or cause a debris avalanche), especially in the upper mesophotic zone (30–50 m). • Generally middle- to low-light environments.

Dominant habitat- building taxa

• Dominant species are zooxanthellate scleractinian corals, octocorals, calcareous and foliose macroalgae and sponges.

Light levels

• Generally well-lit environments. • Shallow reefs can become light-limited in turbid waters (e.g. near estuaries). • Generally stable thermal regime. • Shallow, stratified waters with high residence time may be subject to extreme thermal events causing coral bleaching.

Thermal regime

Hydrodynamic regime

• Subject to breaking waves and turbulence, except in sheltered lagoons. • Wave-induced shear stress and mobilization of seafloor sediments. • High residence times within lagoons.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 19