Figure 1 shows the irradiance pattern according to time of day and sun elevation on an Australian coral reef. Phases of dawn, day and dusk are depicted. To follow this pattern in an aquarium setting, dimmable lights are required.
Corals located near the water surface receive a large amount of solar radiation and are forced to reduce their zooxanthellae population to reduce oxidative stress. This reduction results in a much lighter coloration. However, corals in deep zones, in order to maintain the same photosynthesis rate, must develop a higher density of zooxanthellae, presenting much darker shades. Another factor that significantly affects the coloration of corals, anemones and giant clams are the protective pigments they develop in response to UV and intense light radiation. These pigments provide them with very attractive colors, in shades of blue, mauve, yellow or pink, and their function is to reduce oxidative stress. In the reef aquarium, intense lighting, within reasonable limits, favors the formation of these pigments and the corresponding associated colors. These colors are a natural response and are consolidated when environmental conditions are extraordinarily stable. Frequent changes in the location of corals into a reef tank greatly limit the development of these pigments.
Light is an electromagnetic radiation that has the same nature as radio or cell phone signals. The energy carried by a light source is inversely proportional to its wavelength, i.e., the longer the wavelength the lower the energy. If an electromagnetic radiation has a wavelength between 400 nm and 700 nm (1 nm being one millionth of a millimeter), it is perceived by the human eye in the form of light and colors. Each color corresponds to exactly one wavelength. This electromagnetic spectral range is known as the “visible spectrum”. Light from the sun carries all the wavelengths of the visible spectrum and additionally those corresponding to infrared and ultraviolet. Ultraviolet radiation is located before 400 nm and infrared radiation after 700 nm, i.e., at the borders of the visible spectrum above and below. The approximate equivalences between wavelengths and colors are described in Table 1.
Figure 5. Absorption response for photosynthetic pigments.
The type of light depicted in Figure 4 has everything biologically necessary for the health of fish, corals, invertebrates and the associated food web of a reef aquarium. However we can optimize this spectrum by reducing greens, yellows, oranges and reds, without any detrimental effect on fish, corals and invertebrate’s health. The reduction of wavelengths near to red, not only protects our corals from oxidative stress, but also minimizes the probability of occurrence of unwanted algae. In tide pools, close to the reef, the light received has a large amount of red color, which is used by hair and macro algae for colonization and growth. The same thing can happen in the aquarium if a lot of red-light radiation is used.
Figure 6 shows what the reference light spectrum would look like for reef tank application. HQI bulbs, fluorescent tubes and commercially available LEDs fixtures are designed to provide this spectrum to a greater or lesser extent, with slight modifications. Taking this spectrum as a baseline, if we wish to recreate shallow areas or even intertidal pools, it is recommended to increase the content of green, yellow and red colors. In the case of recreating deeper areas, we can reduce the greens, yellows and reds as much as necessary, without limiting the health or growth of the corals, giant clams and invertebrates.
As an example, Figures 7 to 10 show the spectrum provided by the manufacturer Giesemann in four HQI bulb models, together with their corresponding color temperature. We verify that the color temperature increases with blue light content, decreasing when there is a greater amount of red and yellow components.
Figure 7. HQI Geisemann Megachrome Marine. 12.000 ºK. With "rippling sun reflection effect”. There are a lot of red and yellow components.
Figure 9. HQI Geisemann Megachrome Crystal. 17.500 ºK. “Great growth in SPS and LPS corals effect”. The decrease in yellow components increases the color temperature.
Figure 10. Geisemann Megachrome Blue. 22,000 °K. “Accentuates the blue components effect”. There is a significant decrease in the red, yellow and green components, producing a blue-dominant light with high color temperature.
In Figure 11 (below) the spectrum provided by a Ecotech Radion XR30w G5 LED is also depicted. We can see that the spectrum is similar to the one in figure 6, which is the reference spectrum, as explained.
Figure 11. Spectrum provided by Ecotech XR30w G5 PRO LED fixture.
When an aquarist wonders what type of light is appropriate for a reef aquarium, the immediate tendency is to mimic as closely as possible the natural light that fish, corals and invertebrates receive in nature. The spectrum of this light includes many of the colors that the human eye is capable of perceiving, for example violets, blues, greens, yellows, oranges and reds. However, zooxanthellae do not use all of these wavelengths equally, but mainly colors close to blue and red. It is therefore possible to decrease the intensity of colors close to green, yellow and orange, without having a negative impact on their health. On the other hand, we know that intense red light causes great oxidative stress in corals and additionally, and favors the appearance of unwanted algae in the aquarium. Also, blue light produces in corals a higher rate of photosynthesis and a higher density of zooxanthellae than red light. The optimal spectrum for a reef aquarium therefore has a large amount of blue light, with other colors such as green, yellow, orange and red, significantly reduced with respect to natural light.
Angel Cegarra is the author of "Reefkeeping Fundamentals". He has traveled to destinations such as the Red Sea, Indonesia, the Philippines, Australia and the Pacific Ocean teaching underwater photography at diving centers. He is passionate about anemones, algal refuges, deep sand beds and cryptic zones.