Lighting the Refugium

There are plenty of reasons that you might consider installing a refugium in your marine aquarium system. Yours might be to add water volume to compensate for an increasingly crowded main tank. You might want to boost your copepod production. Most probably, you’re aiming to reduce dissolved nutrient concentrations. Particularly in the latter case, you’ll be looking at doing a little marine gardening on the side. That is, you’ll be looking to use some form (or several forms) of macroalgae in the refugium. And you can’t even casually consider macroalgae without seriously considering lighting.

Like all plants, macroalgae require some minimal amount of light to survive. Their actual rates of growth depend upon the intensity of light that is available to them. Thus, when using macro for nutrient control, it makes no sense to skimp on this critical aspect. To get this right, when selecting/installing a lighting system, an aquarist must address the following:

  1. Emitted Intensity.
  2. Received Intensity.
  3. Spectrum.
  4. Photoperiod.

Bringing in the Sun

Our eyes are well equipped to control the amount of light that passes through our pupils. The result is that the artificial lighting in our homes or on our tanks might appear to be close in intensity to the natural sunlight outdoors. Well, get this: Marine plants (and of course all plants) are adapted to life under real sunlight–bright light. And they don’t have pupils.

First off, do you want red algae (e.g. Gracilaria)? Green algae (e.g. Chaetomorpha)? Some combination of the two? Know that this choice affects how much light you’ll need. Every single plant species on this planet has its own preferred light level. But, as a general rule, red algae require a slightly lower light intensity than the greens; as you might have guessed, this means that they also tend to grow more slowly, and so remove nutrients at a slower pace. Red algae usually will grow under lighting that is roughly equivalent to what you’d use to grow, say, green star polys. On the other hand, if you want to meet the minimal requirements for healthy green algae growth, you’ll need to install a fixture with an output that is more suitable for stuff like SPS corals. When culturing a combination of reds and greens, opt for very intense light.

Like us, plants respire. They metabolically burn fuel using oxygen as an oxidizing agent. Unlike us, they can’t eat. Rather, they make their own fuel. This (as you probably already know) occurs through the process of photosynthesis. The minimal amount of light a plant needs to survive (referred to as the compensation point) is the amount it needs to make just enough energy to cover respiration. Additional intensity, to a point, allows for growth and reproduction. That point (referred to as the saturation point) is the maximum amount of light that a plant can absorb before photosynthetic activity levels out or decreases.

So you can have too much light? Yes. At this intensity, photoinhibition occurs as the photosynthetic machinery in the plant’s chloroplasts are operating beyond their maximal efficiency. Exceeding this cannot increase photosynthetic rates and can even injure the plant. But don’t worry much about applying too much light under captive conditions with artificial illumination. With most seaweed species, this is unlikely using even the brightest available light fixtures. However, it is quite common for hobbyists to underpower their light output. Using a “good enough” light for their crop (which then kind of just hangs on around the compensation point), they wonder why their macros fail to grow and reduce nutrient levels as intended.

We get it, lighting is expensive. But what’s the point going through the trouble and expense of building a refugium, only to render it incapable of growing macro because you cheaped out on the lighting? When finances are limiting (as they are for most of us!) one should really have a good idea of what their lighting needs will be and what meeting them will cost. If you find that the cost of the lighting you’ll need for the refugium you want is too prohibitive, you’ll simply have to go with a smaller refugium.

Something in the way

When calculating how powerful the lighting system will need to be, remember to account for light loss. Not all light emitted from your fixture will fall upon the macros! This is because there is substantial loss of intensity along the way. The amount of this loss depends upon several factors. The first is reflection. Yes, some light (sometimes a lot) is reflected away before it even begins to penetrate the water column. Know that this increases with both (1) distance of the fixture from the water surface and (2) surface agitation (those little waves from the water currents). Then, some light extinction occurs as the light passes through the water. The water itself absorbs some light energy, but additional absorption takes place due to substances in the water. Therefore, if you don’t have high water clarity, expect a significant loss of intensity. Finally, there will be some shading due to crowding, being covered with film algae or sediments, etc.

Living color

Different algae prefer different light spectra. This is because they naturally occur at different water depths. Of all visible light spectra, blue light penetrates water most deeply. Red algae are frequently outcompeted by greens in the brightly lit shallows. Thus, in their retreat to deeper, subtidal habitats, they have adapted to mainly harvest and utilize blue light. A nice bright actinic is therefore sufficient for growing reds. Greens will appreciate “full-spectrum” light, particularly that which has peaks in both the blueish and reddish spectral regions. When culturing a combination of reds and greens, opt for full-spectrum lighting.

Night and day

Finally, you’ll want to plan on establishing a photoperiod. This refers to the lighting schedule. You certainly may run the lights exactly while the main tank lights are on. You can also run them 24/7. Some aquarists swear by daily cycles that include at least some short period of darkness, but suggest you schedule a reverse cycle. In other words, you run the refugium lights when the lighting on the main tank is off, and vice versa. The reason for this is to stabilize pH, which may fluctuate somewhat between periods of varying photosynthetic activity.

You can choose your macroalgae to suit a particular type of refugium lighting, or you can select a light to best match a specific species of macro. Either way, by optimizing your light intensity, spectrum and photoperiod, you will get the very most growth and nutrient control capacity out of your planted refugium!

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