"Mesophotic Coral Ecosystems (MCEs) are characterised by the presence of light-dependent corals and associated communities typically found at depths ranging from 30-40 m (98-131 ft) and extending to over 150 m (492 ft) in tropical and subtropical regions. The dominant communities providing structural habitat in the mesophotic zone can be comprised of coral, sponge, and algal species." (Puglise et al. 2009).
Figure 1: Mesophotic Coral Ecosystems (MCEs; 30 to >150 m) schematic. The first 30 m is where the majority of reef-research has been conducted. However, technological advances in closed circuit rebreather diving is increasing our exploration and research capabilities of depths beyond conventional SCUBA.
This formal definition was established after an international workshop held in Jupiter, Florida - 12-15th July, 2008. The research priorities were "divided into three themes: characterisation, ecology, and values and threats." (Puglise et al. 2009). Prior to this time the deeper reefs of tropical and subtropical regions were predominately dubbed the 'Twilight Zone' taken from an article by Starck and Starck (1972) in the National Geographic magazine.
Figure 2: National Geographic article by Starck and Starck (1972) where, the first reference to the term twilight was reported to have been used.
Even though popular and fun, the term 'Twlight Zone' is scientifically incorrect. It may also be a little confusing, as not only was it popularised by the 1959 Rod Serling Sci-Fi TV series, it is also used as an alternative term for the Mesopelagic Zone, which is an area of the open ocean between 200-1,000 m (656-3,281 ft) depth. Therefore, the terms mesophotic (meso - middle, photic - light), mesophotic coral ecosystem, or mesophotic reefs will be used from hereon in.
In general, MCEs are characterised by strong attenuation gradients in environmental variables such as water clarity and wave surge. By definition however, light or more precisely, solar irradiance (hereon in, irradiance for simplicity) is the critical parameter of the mesophotic environment. Irradiance is a function of depth, water quality, and/or geomorphology. Taken together, or in isolation, the reef communities particularly of the upper-mesophotic depths are dependent on irradiance levels, whereas other factors, such as physical oceanography, increase their influence with depth.
Figure 4: Gorgonian Octocorals of the genus Subergorgia, Gray, 1857, 12 m (40 ft.) depth, populating the underside of overhangs and small caves at Ant Atoll, Federated States of Micronesia. Image by SJ. Rowley.
The attenuation of irradiance is due to the absorption, reflection, and scattering of light by sediment, dissolved organic matter, and phytoplankton. Therefore, the greater the concentration of particulates in the water column results in an increased attenuation in light availability for photosynthesis.
Figure 5: Schematic diagram of the vertical profile of photosynthesis in the water column and the attenuation of light (solar irradiance). The vertical axis is the natural log (ln) of irradiance (i.e., optical depth (ς)). Ze is the compensation depth or base of the euphotic zone (see text below). E depicts euphotic zone, and P depicts photosynthesis. Figure adapted from Falkowski & Raven, 2007.
Light attenuation values are independent of water depth, being vertical profiles of the rate of the attenuation light (see Aquatic Photosynthesis, by Falkowski and Raven, 2007). Therefore, light attenuation is defined as optical depth (ς), which is particularly useful when comparing values with other locations.
Writing in progress! Please come back in a few of days.....
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