Mesophotic Coral Ecosystems

2.2. Light reaching the mesophotic zone

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).

Reef environment and light reaching the mesophotic zone

300

400

500

600

700

800

Wavelength (nm)

Depth in metres

Fringing reef on atoll

0

Fringing reef

Mesophotic zone

100

Patch reef

Barrier reef

200

300

400

Shallow-water coral reefs Mesophotic coral ecosystems

500

Deep-sea or cold-water coral ecosystems

Source: GRID-Arendal

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.

Depth in metres Deepest observations of zooxanthellate scleractinian coral

0

50

Barbados

West Florida Shelf

Curaçao

Bermuda

Northern Gulf of Mexico

Puerto Rico

100

Jamaica

American Samoa

Okinawa

Belize

Marshall Islands

Bahamas

Great Barrier Reef

150

Gulf of Aqaba

Marianas Islands

Hawai‘i

Johnston Atoll

South Paci c Ocean MCEs North Paci c Ocean MCEs

North Atlantic Ocean MCEs Gulf of Mexico MCEs

Red Sea MCEs Caribbean Sea MCEs

200

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

Figure 2.5. The depth range of zooxanthellate mesophotic scleractinian corals is location-dependent due to differences in light penetration and other abiotic factors.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 13

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