Lighting is one of many important factors in keeping a healthy reef aquarium and yet there is still a huge lack of understanding as well as misguided information being given to hobbyists. In this article I hope to clear up some of the myths about lighting and provide some practical and useful information that you will be able to put to use when looking after your aquarium inhabitants.
Firstly when we are talking about light we need to get the terminology right or more specifically the way that we measure light intensity and the units we use. Many hobbyists talk about watts per gallon, and as I hope to show in this article that this way of measuring the light intensity in our aquariums can be very misleading. Our interest in light, as far as our corals go is mostly to do with the zooxanthellae’s ability to photosynthesise and as such we are mainly considering the amount of PAR that is bombarding any given coral. PAR stands for phtosynthetically available radiation and is measured in Micro Einstein’s per Meter squared per Second (µE·m²·sec) or Micro Mols per Meter squared per Second (µmol·m²·sec). As I will show, by using terms such as watts per gallon we are only considering the overall amount of light that is being put into our aquariums and the intensity of light at various given points can vary enormously, whereas when we refer to PAR and µmol·m²·sec then we are talking about the exact amount of light that is hitting a given spot at any one time.
We measure PAR with light meters and these are very easy to use and available to hobbyists from various aquatic internet companies and have been specifically designed for use in aquariums. Although they are not cheap my own quantum meter is one of the most useful pieces of kit that I have ever bought and worth every penny of the £300 that it cost me. Since then light meters have come down in price and you can now get a good Apogee quantum meter for £200. They consist of a hand held light meter that has a digital readout and a sensor connected to a long flexible cable.
For the rest of this article I will be referring to light intensities in µmol·m²·sec but so that you can have some perspective on light intensities here are some facts: When the sun is directly overhead on any cloudless day the intensity of the sunlight at ground level in the tropics is around 2,000 µmol·m²·sec. Under the same cloudless day at a depth of 1m on a coral reef that intensity will have dropped to around 1600 µmol·m²·sec, at 5m depth it is around 950 µmol·m²·sec, 10m it is around 600 µmol·m²·sec and by the time you get to 20m depth it has dropped to around 300µmol·m²·sec.
Now that you have some idea of the variation in intensity of light that is found on a coral reef lets have a look at how that compares to our aquariums. There are a number of factors that determine the intensity of light that reaches any given point in an aquarium. Obviously the type of light source that you are using has a great effect, for now we will be concentrating mainly on metal halide lamps. The wattage of the halide lamp that you are using is understandably one of the main factors, however there are many other factors governing the intensity of the light that reaches a coral including the Kelvin rating of the lamp, the manufacturer, the design of the reflectors, the distance from the lamp to the surface of the water, the clarity of the water, the depth under the water that the coral is, the ballast and the effect of any ripples on the surface of the water.
Firstly lets take a look at the effect that the distance you position the lamp away from the surface of the water has. As we all know water absorbs light, however within our aquariums we are usually only passing light through 18-30 inches of water and the amount of light that the water absorbs through this distance is actually quite small, far more relevant is the distance between the light source and the coral itself. The main cause of diminishing light intensity in our aquariums is due to the distance that the lamp is away from the subject and the drop in light intensity follows the “inverse square law”. You don’t really need to understand the workings of this law but it’s effects are very relevant. To understand how the distance between a metal halide lamp and a given point within our aquariums effects the intensity of light at that point I took some readings using a light meter. The light used was a 250w 10,000 Kelvin single ended metal halide with a reflector. The lamp was 6 months old and suspended over an aquarium. I took the readings 2″ beneath the surface of the water which had no ripples on it using an Apogee quantum meter. By raising the light unit I was able to take readings with varying distances of between 5 and 15 inches between the lamp and the light sensor The graph in figure 1 shows the results.
As you can see when the lamp was 5 inches above the surface of the water the light intensity directly under the centre of the lamp and 2 inches beneath the surface of the water was 1070 µmol·m²·sec. By the time I had raised the light so that there was 9 inches between the lamp and the surface of the water that intensity had dropped drastically to 520 µmol·m²·sec. This change in distance of only 4 inches is equivalent to a change in depth from roughly 4 to 14 meters on a coral reef, so you can see that the distance between a metal halide lamp and the surface of the water has a dramatic effect on the light intensity that is being provided to your corals.
The height that a lighting unit is suspended at is by no means the only factor. Those readings shown in the graph above only take into account the distance of a subject directly underneath the light source, it does not show how dramatically the horizontal positioning of corals can effect the amount of light they receive. To show this I took a large range of light readings from various positions along the centre line within an aquarium. The tank measured 48x24x24 inches and was lit with two 250w 10,000 Kelvin single ended metal halide lamps. These lamps were suspended eight inches above the surface of the water and positioned so that they pointed towards the front of the aquarium with the screw fittings pointing towards the back rather than orientated lengthways along the aquarium. There was a 13 inch space between the two lamps. The diagram in figure 2 shows the readings grouped to give an overall pattern of light intensity throughout the aquarium. Front view (Lamps were centred in the middle of the aquarium and readings were taken along the central length line) (fig 2)
This diagram clearly shows something that is so often overlooked when positioning corals and considering light intensity in aquariums. It is the horizontal positioning that is just as relevant, if not more so than the vertical positioning in your aquarium! The actual data figures show this even more clearly. Directly beneath the right hand lamp, at the surface of the water the light intensity was 1,100 µmol·m²·sec, but only 4 inches to the right the light intensity at the surface was only 550 µmol·m²·sec and this trend follows throughout the aquarium. As soon as you move out from directly underneath your source of light the intensity drops drastically. However you should also note that if you have two sources of light as in this case then it is often the position between the two lamps that is the brightest due to the overlap of light. This however does all depend on how far apart your lamps are positioned and the depth and angle of incidence.
Another factor that is so often overlooked when considering metal halide lamps and light intensity in aquariums is the Kelvin rating of the lamps. The Kelvin rating refers to the colour temperature of the light emitted, with higher Kelvin rated lamps the blue part of the spectrum prevails while in lower Kelvin rated lamps it is the red part of the spectrum that prevails. The fact that is so often neglected is that the higher Kelvin rate a lamp has the less light it emits. That’s right, a 250w 14,000 Kelvin metal halide lamp is not as bright as a 250w 10,000 Kelvin lamp and a 20,000 Kelvin 250w lamp emits less light than a 14,000 Kelvin lamp! Not only do they emit less light but it is quite significantly less too. To show this I took some more readings with the quantum meter. I compared a 250w single ended 10,000 Kelvin BLV lamp with a 250w single ended 14,000 Kelvin Coralvue lamp. Both of the lamps were six months old and had been burning for the same amount of time each day and both of the bulbs were suspended 9 inches above the surface of the water and had the same reflectors. The readings were taken directly beneath the lamps and 2 inches beneath the surface of the water. At this distance the light at that given point beneath the 10,000 Kelvin lamp was 515 µmol·m²·sec whereas beneath the 14,000 Kelvin lamp it was 320 µmol·m²·sec . That’s a big difference, in fact it’s over 35% difference! In this instance the 10,000 Kelvin lamp was over 35% brighter than the 14,000 Kelvin lamp. Although there may be some slight differences in ballast performance or between bulb manufacturers, when comparing 10,000 Kelvin lamps to 14,000 Kelvin lamps I have always found the 14,000 Kelvin lamps to emit at least 25% less PAR than the 10,000 Kelvin Lamps.
But wait, wont the light from the higher Kelvin rated lamps which is bluer penetrate further through the water column and so be brighter at the bottom of the aquarium than the 10,000 Kelvin lamps? Well it is true that the wavelengths from the blue part of the spectrum will penetrate further but don’t forget that we are only dealing with a very shallow depth of water in our aquariums and so this effect is fairly insignificant in comparison to the difference in intensity between the two lamps.
Hang on I hear all you 14,000 Kelvin lamp users shout, don’t corals need the blue part of the spectrum and so grow faster under 14,000 Kelvin lamps than 10,000 Kelvin lamps? No, and for a couple of reasons. Firstly the light intensity as we have seen is less from 14,000 Kelvin lamps than it is for 10,000 Kelvin lamps so unless you have reached the saturation point for a given coral then the rate of photosynthesis achievable under the 14,000 Kelvin lamps is gong to be less than could be achieved under the 10,000 Kelvin. Secondly and very importantly corals are very adaptable. Although zooxanthellae do naturally absorb light from the blue part of the spectrum (as well as other wavelengths) the amount of blue light that is emitted from even a 10,000 Kelvin lamp is often four or five times more than is found on reefs and some lamps such as Radium 20,000 Kelvin bulbs have been shown to emit 8-12 times the radiation at 454 nm than would be found in nature.
However this large difference between the spectral output of the lamps we use and what is found on natural reefs is not as problematic as you might first think due to the great adaptability of corals. It has been known for a long time that corals are able to change the abundance of different strains of zooxanthellae that are adapted to different light spectrums. The zooxanthellae are able to change their content of light absorbing pigments as well as the activity of various enzymes to make use of the particular wavelengths of light that are available. If you moved a coral on a reef gradually from 15 meters depth to 10 meters depth and examined the abundance and light absorbing spectrum of the light absorbing pigments such as chlorophyll a, chlorophyll c2 and peridinin you would find that they have changed so that they can make the most use of their new light environment. This is great news for us as hobbyists as it means our corals can adapt greatly to the different types of light that we provide and still thrive and grow.
Another factor that should be mentioned when examining light intensities on corals is the effect of light flashes or “glitter lines”. Naturally waves can act as lenses and focus light into high intensity flashes. In shallow water reef environments it has been shown that these flashes of light can double the intensity of light for short bursts. These glitter lines will only occur when spot lights are used over aquariums and will not occur when using diffuse types of lighting such as fluorescent tubes. By using metal halide lamps and surface agitation of the water glitter lines will occur although their intensity will not be as great when compared to natural reefs.
Now that we have all this information on light intensities within our aquariums, how can we apply what we know to what our corals require? Fortunately there has been some very good research done on coral compensation points and saturation points. The compensation point is the minimum amount of light a given coral needs to survive and describes the point at which the oxygen production by the zooxanthellae is equal to the oxygen requirements of the zooxanthellae/coral host. At the other end of the scale is the saturation pint, this is the point at which the rate of photosynthesis is at it’s maximum and providing more light will not increase photosynthesis any further. Another factor that needs to be considered is photoinhibition, this is when there is so much light bombarding a coral that it becomes damaging and photosynthesis begins to shut down.
Thanks to the work of some great researchers we now have the compensation points, saturation points and levels at which photoinhibition occur in some corals. By comparing these requirement of our corals with the light intensities that we find at given points within our aquarium we are able to determine which corals we are able to keep and where they should be positioned within the aquarium. You may also be surprised to find that it is quite easy to cause photoinhibition to occur on many species of coral if they are positioned incorrectly.
* = information not available
Although the information available is by no means complete yet, it does show some interesting facts. It should be noted however that corals do have the ability to adapt as previously mentioned and the species mentioned here are likely to have slightly different saturation points if they had been living at different depths on the reef.
However, that having been said for the vast majority of corals the saturation point rarely seems to exceed 350 µmol·m²·sec and for some families it is regularly much lower. Exceeding the intensity of light above a corals saturation point provides the coral with no benefit at all, it will not increase the rate of photosynthesis or growth any further and when you reach the point where photoinhibition occurs then you are actually reducing the rate of photosynthesis and growth.
The saturation point of corals does also depend on a number of other factors and it has been shown that by increasing the water flow rate around corals the saturation point can be increased within certain limits. You can also see from the information in the table that it is actually quite easy to cause photoinhibition in some of your corals in aquaria, don’t forget that a coral may not die due to photoinhibition even if the start point has been exceeded. The figures give the start point for photoinhibition which means that the optimal light intensities for photosynthesis have been exceeded and the excess light is causing the coral stress, however corals have many adaptations to deal with changes in light intensity including polyp retraction and zooxanthellae expulsion. But if the light intensity is too high then it can be quite easy to cause long term health problems in your coral and death. Hopefully in this article I have given some practical advice and information that will help you choose and manage your lighting correctly and by comparing some of the light requirements of the corals in the table you may have a better idea of where and where not to position them within your aquarium.
References: Fossa and Nilsen. The Modern Coral Reef Aquarium. Vol. 1 Sanjay Joshi. Underwater Light Field and it’s Comparison to Metal Halide Lighting Dana Riddle. How Much Light?! Analyses of Selected Shallow Water Invertebrates’ Light Requirements Dana Riddle. Lighting the Reef Aquarium Dana Riddle. Technical and comparison data on various light bulbs used in aquarium lighting applications