Shedding Light On The Reef
by: Richard Harker
While a reefkeeper can be very successful without ever having seen an actual coral reef, the experience of seeing a natural reef can have a profound affect on how he or she views an artificial reef. When observing a coral reef for the first time most hobbyists are struck by how different the light striking the reef appears, and how difficult it is to realistically simulate it in an artificial reef.
Unfortunately, only a small proportion of reefkeepers will ever see a natural reef, let alone a Pacific reef where the animals we keep are collected. So, I write this article for those of you who have not seen a natural reef and who want to better understand the lighting in these areas.
As it travels underwater, light is affected in many ways. It is scattered and absorbed by dissolved and particulate material, so intensity declines. Water also acts as a selective filter, removing colors as the light penetrates deeper water. Light also reacts with the coral reef environment in unpredictable ways, reflecting off organisms, coral rubble and substrate.
Light duration and intensity over coral reefs;
For the most part, coral reefs are found close to the equator, generally within 15 degrees north or south. While the proximity of most coral reefs to the equator is generally attributed to more favorable conditions found there, the reason corals grow in this area is only indirectly related to temperature or lighting conditions (Johannes 1983). Nevertheless, because most corals grow near the equator, a starting point for understanding coral reef lighting is to examine light found at the equator.
Those of us living north or south of the equator experience longer summer days and shorter winter days as the earth, tilted on its axis, rotates around the sun. The length of day at the equator is constant, so daylight on the natural reef varies by only a few minutes from month to month. Thus, corals are used to days of equal length throughout the year. In this respect, artificial light can more easily simulate conditions found on the natural reef than natural light at a high latitude location, a place far north or south of the equator. Daylight at the equator lasts slightly less than 14 hours.
Anyone who has watched a sunrise knows that sunlight gradually increases as the sun rises above the horizon until it reaches its brightest point, when the sun is directly overhead, and then gradually declines as the sun sets. While the process is obvious, the intensities may surprise most people.
From total darkness, it only takes a few minutes for light to reach levels normally found in a reef tank. Measuring light over a shallow reef off Sulawesi, Indonesia, I found that intensity reached 200 microEinsteins per square meter per second (µE/m2/sec) by 6:30 a.m. This is a light intensity level that exceeds that found on many reef tanks. By 8:00 a.m. light intensity exceeded the intensity found over a tank lighted by 400-watt metal halide bulbs. This raises the question of whether slowly “cycling†lights on gradually is of any value.
On the natural reef, light exceeds that of a typical reef tank in less than an hour. If a hobbyist wants to re-create sunrise on a natural reef, a cycling time well less than an hour would be more realistic than the more typical several hour cycling.
As the sun rises, light intensity increases more or less linearly until noontime, where it reaches over 2000 µE/m2/sec on a day without clouds. This level of intensity is virtually impossible to recreate in a captive system with lighting commonly available to the hobby. A 400-watt metal halide bulb generates 2000 µE/m2/sec within a few inches of the envelope.
Reefkeeping discussions regarding proper lighting for photosynthetic animals often distinguish between “low light†corals and “high light†corals. Provided the terms allude to a coral’s ability to adapt to low light conditions, the distinction is valid. Some corals can successfully adapt to low light conditions better than others. Some hobbyists believe, however, that low-light corals are harmed by too much light.
While there is research that suggests that some corals are harmed by full sun irradiance, the light levels encountered in virtually all reef tanks fall well short of the 2000 µE/m2/sec full sun irradiance the authors are speaking of. The corals normally encountered in the hobby can adapt to conditions found in virtually all brightly lighted (e.g., 400-watt metal halides) reef tanks in the hobby. The Indonesian reef mentioned above was home to a wide range of soft and stony corals from Xenia to Euphyllia ancora, all growing under intensive light.
While the 2000 µE/m2/sec peak noontime light intensity is an unreachable number, peak intensity is far less important than total integrated irradiance. Integrated irradiance is the total amount of sunshine over the entire day. The total integrated irradiance is the critical determinant of whether corals and other photosynthetic organisms have received enough light to meet their metabolic needs. Integrated irradiance can be calculated by multiplying the intensity of light by the duration of the light.
For a rising and setting sun, the calculation can be difficult, but for a reef tank it is much easier. Reef tanks with artificial lighting have constant lighting, so it is a simple matter to multiply the total intensity of the lights by the length of time the lights are on. For example, if the lights over a reef tank are on 10 hours and generate 200 µE/m2/sec of light, the total integrated irradiance is 200 x 10 x 3600 (the number of seconds in an hour) or 7,200,000 µE/m2/sec. This is normally converted to Einsteins, so the final number is 7.2 E/m2. Equatorial sunlight over coral reefs typically generates over 50 E/m2 on a typical sunny day. This is at the water’s surface, however.
Because light intensity declines underwater, a more important question is what a typical coral receives in daily integrated irradiance. In a study of Pocillopora damicornis, Montipora verrucosa and Porites lobata, scientists found that at 3 meters in somewhat turbid water, a typical sunny day generated a total of 14.4 E/m2 day, while a cloudy day generated 6.2 E/m2 day (Davies 1994). The study found that a sunny day generated sufficient energy to meet the needs of all three corals, but a cloudy day left P. damicornis and P. lobata in an energy-deficient state. This suggests that for the reef tank example shown above, extending the photoperiod would be beneficial. Increasing the photoperiod to 13 hours would increase total integrated irradiance to 9.4 E/m2, a number closer to natural reef conditions.
The sun over the natural reef sweeps across the sky. A coral is not evenly lighted over the entire colony over the course of a day. Portions of a colony will self-shade themselves part of the day, and be in direct sunlight other parts of the day. In contrast, corals in a reef tank receive light from the same direction every hour of every day. This increases the likelihood that captive corals will morph into unusual shapes, as colonies adjust to stationary light. It also increases the likelihood that lower portions of a colony will receive inadequate light.
Light intensity at different depths varies according to the clarity of the water. Jerlov (1976) identified a number of different types of water depending on turbidity and degree of light absorption at various wavelengths. Water flowing over a fore reef is generally classified “Oceanic I†or “II†water, some of the clearest water in his categories. In contrast, lagoons can be turbid and absorb much more light. However, even turbid lagoons are brighter than most reef tanks.
While the rate of light fall-off depends on the clarity of the water, generally speaking, light falls off exponentially as one goes deeper in the water. A doubling in the depth of water translates into a reduction of light to only one quarter of the available light. This means light drops off very quickly as one goes deeper in the water. At only 10 meters or 33 feet, light intensity is only 20 percent of surface irradiance (Crossland 1987).
(CONT)
by: Richard Harker
While a reefkeeper can be very successful without ever having seen an actual coral reef, the experience of seeing a natural reef can have a profound affect on how he or she views an artificial reef. When observing a coral reef for the first time most hobbyists are struck by how different the light striking the reef appears, and how difficult it is to realistically simulate it in an artificial reef.
Unfortunately, only a small proportion of reefkeepers will ever see a natural reef, let alone a Pacific reef where the animals we keep are collected. So, I write this article for those of you who have not seen a natural reef and who want to better understand the lighting in these areas.
As it travels underwater, light is affected in many ways. It is scattered and absorbed by dissolved and particulate material, so intensity declines. Water also acts as a selective filter, removing colors as the light penetrates deeper water. Light also reacts with the coral reef environment in unpredictable ways, reflecting off organisms, coral rubble and substrate.
Light duration and intensity over coral reefs;
For the most part, coral reefs are found close to the equator, generally within 15 degrees north or south. While the proximity of most coral reefs to the equator is generally attributed to more favorable conditions found there, the reason corals grow in this area is only indirectly related to temperature or lighting conditions (Johannes 1983). Nevertheless, because most corals grow near the equator, a starting point for understanding coral reef lighting is to examine light found at the equator.
Those of us living north or south of the equator experience longer summer days and shorter winter days as the earth, tilted on its axis, rotates around the sun. The length of day at the equator is constant, so daylight on the natural reef varies by only a few minutes from month to month. Thus, corals are used to days of equal length throughout the year. In this respect, artificial light can more easily simulate conditions found on the natural reef than natural light at a high latitude location, a place far north or south of the equator. Daylight at the equator lasts slightly less than 14 hours.
Anyone who has watched a sunrise knows that sunlight gradually increases as the sun rises above the horizon until it reaches its brightest point, when the sun is directly overhead, and then gradually declines as the sun sets. While the process is obvious, the intensities may surprise most people.
From total darkness, it only takes a few minutes for light to reach levels normally found in a reef tank. Measuring light over a shallow reef off Sulawesi, Indonesia, I found that intensity reached 200 microEinsteins per square meter per second (µE/m2/sec) by 6:30 a.m. This is a light intensity level that exceeds that found on many reef tanks. By 8:00 a.m. light intensity exceeded the intensity found over a tank lighted by 400-watt metal halide bulbs. This raises the question of whether slowly “cycling†lights on gradually is of any value.
On the natural reef, light exceeds that of a typical reef tank in less than an hour. If a hobbyist wants to re-create sunrise on a natural reef, a cycling time well less than an hour would be more realistic than the more typical several hour cycling.
As the sun rises, light intensity increases more or less linearly until noontime, where it reaches over 2000 µE/m2/sec on a day without clouds. This level of intensity is virtually impossible to recreate in a captive system with lighting commonly available to the hobby. A 400-watt metal halide bulb generates 2000 µE/m2/sec within a few inches of the envelope.
Reefkeeping discussions regarding proper lighting for photosynthetic animals often distinguish between “low light†corals and “high light†corals. Provided the terms allude to a coral’s ability to adapt to low light conditions, the distinction is valid. Some corals can successfully adapt to low light conditions better than others. Some hobbyists believe, however, that low-light corals are harmed by too much light.
While there is research that suggests that some corals are harmed by full sun irradiance, the light levels encountered in virtually all reef tanks fall well short of the 2000 µE/m2/sec full sun irradiance the authors are speaking of. The corals normally encountered in the hobby can adapt to conditions found in virtually all brightly lighted (e.g., 400-watt metal halides) reef tanks in the hobby. The Indonesian reef mentioned above was home to a wide range of soft and stony corals from Xenia to Euphyllia ancora, all growing under intensive light.
While the 2000 µE/m2/sec peak noontime light intensity is an unreachable number, peak intensity is far less important than total integrated irradiance. Integrated irradiance is the total amount of sunshine over the entire day. The total integrated irradiance is the critical determinant of whether corals and other photosynthetic organisms have received enough light to meet their metabolic needs. Integrated irradiance can be calculated by multiplying the intensity of light by the duration of the light.
For a rising and setting sun, the calculation can be difficult, but for a reef tank it is much easier. Reef tanks with artificial lighting have constant lighting, so it is a simple matter to multiply the total intensity of the lights by the length of time the lights are on. For example, if the lights over a reef tank are on 10 hours and generate 200 µE/m2/sec of light, the total integrated irradiance is 200 x 10 x 3600 (the number of seconds in an hour) or 7,200,000 µE/m2/sec. This is normally converted to Einsteins, so the final number is 7.2 E/m2. Equatorial sunlight over coral reefs typically generates over 50 E/m2 on a typical sunny day. This is at the water’s surface, however.
Because light intensity declines underwater, a more important question is what a typical coral receives in daily integrated irradiance. In a study of Pocillopora damicornis, Montipora verrucosa and Porites lobata, scientists found that at 3 meters in somewhat turbid water, a typical sunny day generated a total of 14.4 E/m2 day, while a cloudy day generated 6.2 E/m2 day (Davies 1994). The study found that a sunny day generated sufficient energy to meet the needs of all three corals, but a cloudy day left P. damicornis and P. lobata in an energy-deficient state. This suggests that for the reef tank example shown above, extending the photoperiod would be beneficial. Increasing the photoperiod to 13 hours would increase total integrated irradiance to 9.4 E/m2, a number closer to natural reef conditions.
The sun over the natural reef sweeps across the sky. A coral is not evenly lighted over the entire colony over the course of a day. Portions of a colony will self-shade themselves part of the day, and be in direct sunlight other parts of the day. In contrast, corals in a reef tank receive light from the same direction every hour of every day. This increases the likelihood that captive corals will morph into unusual shapes, as colonies adjust to stationary light. It also increases the likelihood that lower portions of a colony will receive inadequate light.
Light intensity at different depths varies according to the clarity of the water. Jerlov (1976) identified a number of different types of water depending on turbidity and degree of light absorption at various wavelengths. Water flowing over a fore reef is generally classified “Oceanic I†or “II†water, some of the clearest water in his categories. In contrast, lagoons can be turbid and absorb much more light. However, even turbid lagoons are brighter than most reef tanks.
While the rate of light fall-off depends on the clarity of the water, generally speaking, light falls off exponentially as one goes deeper in the water. A doubling in the depth of water translates into a reduction of light to only one quarter of the available light. This means light drops off very quickly as one goes deeper in the water. At only 10 meters or 33 feet, light intensity is only 20 percent of surface irradiance (Crossland 1987).
(CONT)
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