Water clarity is known to have quite a large impact on the transmission of light over depth in a natural reef situation (Dustan, 1982; Fabricius and Alderslade, 2001; Falkowski et al., 1990; Weinberg, 1976). On a natural reef, the differences caused by water clarity are seen at depths of 5 metres or more. The effect of water clarity on light transmission in an aquarium is frequently discussed and assumptions made that it has a large impact, but without measurements, the impact is subject to speculation.

The purpose of this simple study was to take measurements under controlled conditions through air and water of different clarity in an attempt to quantify the effect.

Materials and Methods

The readings were take on the bottom of a black plastic 40 litre garbage bin. Black was chosen to reduce the impact of reflection. The distance from the bottom of the bin to the top was 45 centimetres.

The light source for the readings was a 150W Metal Halide fitting with a BLV 10000K double-ended lamp. The lamp had previously been used on an aquarium for just over 12 months. The UV filter on the light fitting was mounted exactly 15 centimetres above the top of the garbage bin. The lamp was switched on for 10 minutes before any readings were taken.

The light readings were taken with a quantum meter with separate sensor (QMSS-ELEC) made by Apogee Instruments.

Readings were taken with:

• Air
• Tap water
• Freshly collected seawater
• Seawater from an existing aquarium

For the readings with air, the sides of the bin were thoroughly dried to remove the influence of reflection on the wet surface. For the water readings, the bin was filled to the very top so that the water depth was 45 centimetres. The seawater from an existing aquarium had an obvious yellow tint to it from the decomposition of organic matter.

Results

The light readings from the study are shown in Table 1.

Table 1: Light readings with various media

Medium	Reading (μE.m-2.s-1)
Air	147
Tap water	140
New seawater	135
Old seawater	118

Discussion

The results show that water clarity does have an impact on the transmission of light through the water. In the setup for the study, 12.6% less light reached the sensor in old seawater compared to new seawater and when compared to air, 19.7% less light reached the sensor in old seawater. As the new seawater had not been filtered or treated in any way, it is possible that with ozone and/or carbon that the water could have had more clarity, but it would still have less clarity than air. It can be assumed that the loss of light due to organic compounds in the water is going to be less than 20% although maybe more than 10%.

The results can be analysed from the other direction. Given a tank with yellowed water, how much more light could reach organisms in the tank if the water was treated with ozone and/or carbon? With a starting point of
118 μE.m^-2.s^-1, the best possible improvement would be to 147 μE.m^-2.s^-1, which is an increase of 24.6%.

In nature, most species of corals are found over a range of depths and this leads to corals experiencing a wide range of irradiance. Even a species of coral that has a natural distribution restricted to the top 3 metres will experience light from 100% surface irradiance to 60%. Of course, most corals have a much wider depth range. A coral species found between 0 and 5 metres would receive between 100 and 30% of surface radiation. Stylophora pistillata live in areas where the lighting can be as high as 100% down to 0.5% surface radiation (Titlyanov et al, 2001). Based on the wide range of light intensities experience by corals in the wild, a increase of 25% in intensity by improving water quality is going to make little difference.

References

Dustan P. 1982. Depth-dependent photoadaption by zooxanthellae of the reef coral Montastrea annularis.. Mar. Biol. 68:253-264.

Fabricius K. and Alderslade P. 2001. Soft Corals and Sea Fans: A comprehensive guide to the tropical shallow-water genera of the Central-West Pacific, the Indian Ocean and the Red Sea. AIMS, Townsville, Australia. 264pp.

Falkowski P.G., Jokiel P.L. and Kinzie R.A. 1990. Irradiance and corals. In: Dubinsky Z. (Ed.) Ecosystems of the world. 25. Coral Reefs. Elsevier, Amsterdam, pp 89-108.

Titlyanov E.A., Titlyanova T.V., Yamazato K. and Woesik R.v. 2001. Photo-acclimation dynamics of the coral Stylophora pistillata to low and extremely low light. J. Exp. Mar. Biol. Ecol. 263(2):211-225.

Weinberg S. 1976. Submarine daylight and ecology. Mar. Biol. 37:291-304.

Last updated: December 23, 2004