Sunlight is modified by a number of factors from the time it hits the Earth's atmosphere until it reaches organisms living underwater on a reef. Even before it reaches the ocean surface, sunlight is modified due to the Earth's orientation and by the atmosphere. The ocean surface itself can affect the penetration of sunlight and various factors in the water itself, such as depth and turbidity, can influence the intensity and spectrum of light.

Before I discuss these influences, I feel it is important to note that while our vision is sensitive to various intensities and spectra of light, it is not very good at judging either intensity or color in isolation. Our eyes adjust to a huge range of intensities, over a number of orders of magnitude, but it is not easy for us to estimate the intensity with our eyes alone. Similarly, our brains interpret images we see and may adjust the perceived colors such that we may be unaware of a color shift. For example, when I walk into a room lit by incandescent lamps, I rarely notice that everything has a red or yellow shift due to the spectrum output by the lamps. Therefore, the influences discussed in this article may not be obvious to our eyes. For example, when diving, the attenuation of light with depth may not be noticeable until a depth of 15 or 20m is reached.

Celestial and Atmospheric Effects to Lighting

Solar radiation that reaches the outside of the Earth's atmosphere is fairly constant. It is worth noting that the orbit of the Earth around the Sun is such that the distance between the two is not always constant. The Earth is at its closest to the Sun (147.5 million kilometers) in early January, and at its furthest (152.5 million kilometers) in early July. This will only have a very slight impact (<5%) on light intensity.

Solar radiation is made up of far more than just visual light (~400-700nm) and includes shorter wavelengths (ultraviolet) and longer wavelengths (infrared).

Despite the near constant amount of solar radiation hitting the atmosphere, the amount that reaches the surface of the oceans varies considerably based on seasons, latitude, and time of day. This is largely due to the angle at which light from the Sun strikes the Earth. If the Sun is perpendicular to the ocean surface, the light is spread over a relatively small area. As the angle of the Sun decreases, the same amount of light will be spread over an increasingly wider area, reducing the intensity at any one point. Figure 1 shows this relationship.

Figure 1: A "beam" of sunlight that hits the Earth perpendicular to the ocean surface is spread over a smaller area than a "beam" hitting the ocean at a more acute angle relative to an ideal horizon.

As light passes through the atmosphere, it is by both scattered and absorbed and these processes reduce the amount of light reaching the ocean surface. Scattering occurs mainly due to air molecules and aerosols, and absorption is largely due to ozone, water vapor, oxygen and carbon dioxide (Alados-Arboledas et al. 2000). The scattering and absorption act differently on different wavelengths, so the quality or color of light is also affected. The greater the distance of atmosphere through which the light passes, the greater the effect; conversely, the shorter the distance, the greater the transmittance. When sunlight passes through the atmosphere at an angle perpendicular to the ocean surface, it will travel through less atmosphere than when it travels through the atmosphere at an angle, as shown in Figure 2.

Figure 2: A "beam" of sunlight that is perpendicular to the Earth passes through less atmosphere than a "beam" at a more acute angle relative to an ideal horizon. The blue line shows the depth of atmosphere through which the light must pass.

The combined effect of the spreading, scattering, and absorption is a variation of illumination with solar elevation. An example of the variation in illumination with solar elevation is shown in Figure 3.

Figure 3: Direct illuminance variation with the solar altitude for clear skies (Robledo and Soler, 2000).

Not all wavelengths of solar radiation penetrate the Earth's atmosphere to the same degree. Shortwave ultraviolet radiation below around 290nm (UV-C) does not penetrate the ozone layer in the outer atmosphere (Odum, 1971). Thinning of the ozone layer may lead to greater transmittance of UV-C which is lethal to organisms. Infrared radiation (700-10,000nm) is irregularly attenuated due to absorption by the atmosphere (Odum, 1971).

Seasons, latitude, and time of day all affect the angle between the ocean surface and the Sun. Between sunrise and sunset, the angle of the Sun increases from an angle of zero degrees at sunrise to its highest point at around noon, and then decreases to zero degrees at sunset. Figures 4 and 5 show the hourly solar elevations for two tropical locations at three different times of the year.

Figure 4: Solar elevation over the course of three days for a location on the Equator (Data from Geoscience Australia: National Mapping Division).

Figure 5: Solar elevation over the course of three days for a Heron Island, Great Barrier Reef (23º26' S) (Data from Geoscience Australia: National Mapping Division).

The highest point of the Sun on any given day is determined by a combination of latitude and season. The Earth's axis is inclined by 23º30' in relation to the Sun, and this means that the northern hemisphere is tilted towards the Sun in late June and tilted away from the Sun in late December. At "true noon"* on the Tropic of Cancer (23º30'N) on the Summer solstice (June 21/21), the Sun is at 90º to the ocean surface. The same condition occurs at noon on the Tropic of Capricorn (23º30'S) on December 21/22, on the equator (0º) on both equinoxes (March 21/22 and September 22/23) and at other latitudes in the tropics at two days per year on, before, and after the Summer solstice. Outside the tropics, the Sun will never be at an elevation of 90º. Figure 6 shows the maximum elevations for three tropical locations over the course of a year.

* By "true noon" I mean the time of day when the Sun reaches its highest point (zenith); this may or may not coincide with noon local time.

Figure 6: Annual cycle of maximum solar elevation for three locations, Heron Island, GBR (23º26' S); Lizard Island, GBR (14º40' S) and a location on the Equator (Data from Geoscience Australia: National Mapping Division).

Whilst the elevation of the Sun determines the maximum possible intensity, atmospheric conditions such as cloud cover and haze may directly reduce the amount of light hitting the ocean surface. Cloud cover and generally humid conditions are quite common in the tropical locations of reef systems, and tropical storms are a common occurrence during the summer months. Clouds affect both the quantity and quality of light reaching the ocean surface by reflecting, absorbing, and transmitting the incoming solar radiation. The loss of intensity will vary depending on the thickness and type of cloud cover, and it is difficult to quantify the effect. Cloud cover attenuates shorter wavelengths (UV, violet, blue and green) almost equally, but causes greater attenuation of longer wavelengths (yellow, orange, red and infrared) (Odum, 1971).

Figure 7 shows readings for irradiance over the course of a single day at One Tree Island, Great Barrier Reef. The irradiance curve is not smooth due to the effect of atmospheric conditions such as cloud cover and haze. The irradiance curve can be compared to the solar elevation for the same day. The differences in the curves are largely due to atmospheric conditions.

Figure 7: Irradiance and solar elevation for September 2, 1998 at One Tree Island, Great Barrier Reef (23°30'S, 152°06'E) (A. Salih, unpublished data).

Latitude and time of the year also affect the length of the day; that is, the number of hours between sunrise and sunset. The days are longest at the same time of the year when the sun reaches its highest maximum elevation. Interestingly, locations on the equator have a nearly constant day length of just over 12 hours. Figure 8 shows the annual cycle of day length from three tropical locations.

Figure 8: Annual cycle of day length from three locations, Heron Island, GBR (23º26' S); Lizard Island, GBR (14º40' S) and a location on the Equator (Data from Geoscience Australia: National Mapping Division).

The long term effects of hourly and seasonal variations in irradiance, day length and atmospheric conditions results in a annual cycle of the amount of light reaching the ocean surface. The differences in the mean and the total irradiance striking the ocean surface over different times of a year can be significant. Figure 9 shows the variation in mean monthly irradiance over 6 years.

Figure 9: Monthly means of photosynthetically available radiation (PAR) at the surface (Surface PAR) and underwater (Underwater PAR) near Ko Phuket, Thailand (7°53'N, 98°24'E) (Dunne and Brown, 2001).

Effects of the Air-Water Interface

When light hits the water surface, some light penetrates, but some light is reflected. The amount of light that is reflected is greater when the angle between the light rays and the water surface is small. When light is coming from a point source, such as the Sun, the percentage of reflectance can be determined using Fresnel's law (Weinberg, 1976). Figure 10 shows the theoretical reflection of sunlight based on the elevation of the Sun. However, Fresnel's law assumes an optically flat water surface which would be rare for the ocean surface, and so actual reflectance may be greater. Figure 10 also includes measured values of reflectance for both smooth and rough water surfaces.

Figure 10: Reflectance of sunlight in relation to solar radiation. Theoretical and measured percentage of sunlight reflected off a completely smooth water surface in relation to solar elevation (based on calculations in Weinberg, 1976; Grichenko in Weinberg, 1976).

When the celestial and atmospheric effects are considered with reflectance, solar elevation has a significant effect on the amount of light penetrating the ocean surface. While sunrise will bring some light to the submarine organisms, the amount of light reaching the organisms will not be significant until the solar elevation is at least 10º or even higher.

Effect of Depth

Light penetrating the water surface is absorbed and scattered by the water molecules, plus suspended and dissolved particles. Even very clear water attenuates light at a significant rate. Even in very clear water less than one tenth of the light that penetrates the water surface still remains at a depth of 30m (Dustan, 1982), and in less clear water, 90% attenuation may occur at shallower depths, such as 15m on an inshore reef (Fabricius and Alderslade, 2001). Figure 11 shows measured irradiance at various depths. Note that the irradiance scale is logarithmic. Figure 12 shows irradiance attenuation as a percentage for different depths on different reefs. Additionally, different wavelengths of light are attenuated at different rates. In clear waters, blue light between 440 and 490nm is attenuated the least, while orange and red light (590 to 700nm) is attenuated the most. In coastal waters, more blue light is absorbed due to more suspended matter and phytoplankton in the water, and green light penetrates the furthest. Figure 13 gives examples of the irradiance of various wavelengths at different depths.

Figure 11: Irradiance versus depth for an "average sunny day" on Dancing Lady Reef, Jamaica between February and April, 1978 (Dustan, 1982).

Figure 12: Light profile measured on 17 inner-, mid- and outer-shelf reefs on the central Great Barrier Reef (Fabricius and Alderslade, 2001).

Figure 13: Spectral Irradiance versus depth for an "average sunny day" on Dancing Lady Reef, Jamaica between February and April, 1978 (Dustan, 1982). * 40, 52, and 60m are calculated values.

Infrared radiation is rapidly absorbed and does not penetrate into the water to any appreciable degree (Falkowski et al. 1990). UV-B (290-320nm) appears to be attenuated quite rapidly, but UV-A (320-400nm) can penetrate to depths over 100m, with attenuation rates similar to light in the range 425-560nm (Schlichter et al. 1986).

Other Factors Affecting Quantity and Quality of Light

A number of other factors may affect the quantity and quality of light reaching reef organisms. Turbid water can significantly increase both absorption and scattering, generally resulting in less transmitted light. The turbidity may come from rough weather stirring up sediment or coastal runoff. Additionally, turbid water differentially affects the wavelengths of light that are absorbed and scattered, changing the spectra of available light at different depths, with more blue light being absorbed as previously mentioned. Plankton blooms may also affect the transmittance of light.

Sessile organisms may be attached to substrates at various angles. Horizontally oriented surfaces receive more light than surfaces on an angle, and vertical surfaces may receive only 25% of the light that is available to a horizontal surface (Falkowski et al. 1990). The orientation of the organism alters the amount of light received because much of the underwater light is directional and penetration is mostly vertical. The directional nature increases with depth. When the light hits an angled or vertical surface, the same amount of light (that would have hit a horizontal surface) is spread over a greater area, reducing the intensity.

Sessile organisms may be in shaded situations and this reduces the light they receive. The shading may come from other sessile organisms growing above them, or from fixed caves or shelves in the underwater terrain. In shallower water, where the direction of the light may be more influenced by the elevation of the sun, the amount of shading on a sessile organism may vary throughout the day or year.

In shallow water, surface water movement such as waves can act like a lens, focusing the light from the Sun and resulting in flashes of light at more than twice the intensity of light transmitted through a smooth surface (Falkowski et al. 1990).

Summary

I have shown the great number of influences on the intensity and spectra of light reaching underwater organisms. While it is not necessary to understand the nature of all these influences, it is useful to realize how variable the underwater lighting regime can be. The location (both latitude and depth) of an organism will determine the maximum intensity the organism can receive; however, at any particular time of day or from day to day, month to month, both the intensity and spectrum of light an organism receives varies greatly.

Knowing the latitude and depth from where a light requiring organism was collected would go a long way towards providing information about the lighting requirements for that organism. While this information is generally not available for the collected organisms we keep, it would only provide part of the picture, and as most photosynthetic organisms are photoadaptive (the subject of a future article), the precise intensity values are not needed.