As all serious reef aquarium hobbyists know, light intensity is an important consideration for a successful captive reef. This short article will examine how light intensity decreases as water depth increases in an aquarium the depth often maintained by hobbyists. The Swedish researcher Nils Jerlov was a pioneer in defining the optical qualities of oceanic and coastal waters. His work involved a scale of meters and could be applied to very large private and public aquaria. However, most hobbyists maintain aquaria less than one meter deep.

First, I’ll offer definitions of terms used in this article:

Attenuation:
For our purposes, reduction in light intensity due to absorption by water and dissolved organics or scattering by suspended particles.

Coefficients:
A multiplier or factor that allows determination of light intensity at a given depth. For example, if the coefficient at a depth of 6 inches is 0.384 and the PPFD value just below the surface is 1601, then 1601 x 0.384 = 614.

Color of Water:
A decrease of water clarity caused by dissolved substances. In aquaria, this is usually a yellow color caused by decaying organic materials. Color is reported in zero to 500 Platinum-Cobalt (Pt-Co) units. There is True Color (where suspended particles are removed through fine filtration) and Apparent Color, where suspended particles remain.

Line Source: An idealized source of light consisting of an infinitely long line from which light is emitted with uniform intensity. For our purposes, fluorescent lamps and LED strip lights could be regarded as line sources although intensity does tend to drop some at the end of these lights.

Photosynthetic Photon Flux Density (PPFD): The amount of Photosynthetically Active Radiation (PAR) that falls upon a given surface area in one second and is reported in units of micromole per square meter per second (µmol/m²/sec.)

Point Source:
A single identifiable localized source of light. Arc tubes in metal halide lamps and LED pucks could be considered as point sources.

Turbidity: Cloudiness or haziness of water caused by suspended particles.

Procedure

A jig to hold the ‘improved’ sensor of an Apogee Instruments’ MQ-510 quantum meter was built of dimension lumber and ½” CPVC pipe. This held the sensor in place with only depth being the only variable dimension. Measurements were taken just below the water surface and then at 1-inch increments within a 120-gallon aquarium, with the sensor being a maximum of 12-inches from any glass pane (See Figure 1.) Pumps were turned off and water surface motion was minimal. No corrections for the immersion effect were necessary as this is automatically made by the firmware. Data were entered into an Excel worksheet and a polynomial trend line was determined (See Figure 2.) Apparent Color of water was determined through use of a Hach DR890 colorimeter and was found to be minimal (14 Pt-Co units.)

Figure 1. A portion of the jig built for this procedure.


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Results

Figure 2 shows the results.

Figure 2. PPFD at various depths in an aquarium. A point light source (LED puck light) was used.

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Table One: The same data pictured above.

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Table Two. Apply these coefficients to a PPFD measurement made immediately under the water surface. See comments in the Discussion section below.


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Discussion

Obviously, this information could be used as a shortcut since determination of light intensity at the surface must be made for the coefficients to be applied. Even then, caveats apply. Use the factors if the sub-surface measurement is made directly under a point source of light. More latitude is allowed if a line source, or natural sunlight, is used. Turbidity should not be an issue if no suspended particles are apparent (including micro-bubbles if in quantity.) Color of water is a different issue. Since yellow water preferentially absorbs ultraviolet/violet/blue wavelengths, it is advisable to routinely use activated carbon combined with water changes to minimize water color. Geometry of the aquarium (square, bow front, round) could also affect the results due to reflections from the glass panes. Reflection from aqua-scaping could influence the numbers, although likely to a minimal degree. Color of the background might be a consideration as well.

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About the Author: Dana Riddle

Dana Riddle is Reef2Reef's resident lighting expert, and he has his own lighting forum here. He has been keeping saltwater livestock since he was a child the 1960's.

He has published over 250 articles and a book called The Captive Reef. He regularly speaks at aquatic clubs and conferences, and was MASNA's Aquarist of the Year in 2011. Dana has a very sophisticated laboratory at home that includes an analytical balance, centrifuge, spectrometer, colorimeter, data loggers, Ocean Optics spectrometers for analyses of light, two PAM fluorometers, drying oven, incubators, water bath, chlorophyll meters, and electronic water velocity meter. His research into aquaria lighting is ongoing.

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