Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

Light attenuation in the ocean rapidly reduces both the

amount and quality of visible light with depth, so that only

a portion of the light spectrum is available at mesophotic

depths. Attenuation is due to absorption and scattering of

light by seawater, dissolved constituents and suspended

particles. Long wavelength colours such as red, orange and

yellow are most quickly absorbed, so that by the time the light

reaches the mesophotic zone, only the blue wavelengths of the

spectrum remain (Figure 2.4). This zone of light penetration

in the water column is referred to as the euphotic zone, and it

extends to the depth where light diminishes to approximately

1 per cent of its surface value. The depth of the euphotic zone

depends on the concentration of dissolved and suspended

light-absorbing and light-scattering materials in the water

column. In the clearest ocean water, zooxanthellate (light-

dependent) scleractinian corals have been documented at

depths as great as 165 m at Johnston Atoll in the Pacific Ocean

(Maragos and Jokiel 1986; Figure 2.5).

2.2.

Light reaching the mesophotic zone

Figure 2.5.

The depth range of zooxanthellate mesophotic scleractinian corals is location-dependent due to differences in light

penetration and other abiotic factors.

Figure 2.4.

Conceptual model of light penetration in the ocean. Blue light dominates the photic zone below 30 m, but the actual depth

of light penetration is site-specific and dependent on a variety of physical factors, such as suspended particulate matter.

300

400

500

600

700

800

Wavelength (nm)

100

200

300

400

500

Depth

metres

Source: GRID-Arendal

Reef environment and light reaching the mesophotic zone

Mesophotic zone

Deep-sea or cold-water coral ecosystems

Fringing

reef

Shallow-water coral reefs

Patch reef

Mesophotic coral ecosystems

Barrier reef

Fringing reef on atoll

Johnston

Atoll

Hawai‘i

Gulf of Aqaba

Bahamas

Marshall

Islands

Belize

Jamaica

Puerto

Rico

American

Samoa

Okinawa

Barbados

Northern

Gulf of

Mexico

Bermuda

Curaçao

West

Florida

Shelf

South Paci c Ocean MCEs

Red Sea MCEs

Caribbean Sea MCEs

Marianas

Islands

Great Barrier

Reef

North Paci c Ocean MCEs

North Atlantic Ocean MCEs

Gulf of Mexico MCEs

100

150

200

Depth in metres

Deepest observations of zooxanthellate scleractinian coral

Source:Table 4 in Kahng et al. 2010 and references therein, Blythe-Skyrme et al. 2013 and Englebert et al. 2014.