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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