I realize this is a very open ended question, but how much of a "PAR" level is required for SPS corals to thrive in a home aquarium.
Phew, so we're swinging for the fences today eh? Unfortunately, this is really an impossible question to answer. The needs of different corals and different zooxanthellae (remember, it's really the zoox. that are most directly affected by varied light intensity, though the corals end up being affected by the health of the zoox.). Asking what is required in terms of intensity for a coral to do well is a bit like asking how big big a tank a saltwater fish needs. A little Neon goby would do fine in a 10 gal tank and be happy forever. A Sohal tang wouldn't even fit in a 10 gal tank as an adult. A Sohal might be content in a 300 gal tank though, but a Neon goby would be content in that 300 gal tank too.
With sufficient food corals can be maintained indefinitely in the dark. That is, no light is required if the animals are fed heavily. Of course, this isn't a realistic situation for out in the world or in a reef tank. Corals of the same species and with the same or different zoox. can and do adapt to a wide range of light intensities in nature and captivity though. You might find the same coral in 1 m of water in uber-bright light and at 30 m in very dim light and it is growing and doing ok in both locations.
Coral calcification and photosynthesis are both saturated from about 20 m depth up to close to the surface on most reefs. Over this depth range you might see a difference in max. intensity of as little as 250 - 300 PAR units up to 1000 - 1800 PAR units. That is a broad spawn, but due to the adaptive nature of the corals and zoox. they are able to do fine over this range. Below about 20 m on most reefs and the rate of calcification tends to start dropping. Typically the max. intensity is < 300 PAR units below ~ 20 m (this all assumes nice clear water--the zonation is moved higher if the water is turbid). You'll still find a lot of the same corals on the deeper parts of the reef, but some grow a bit slower while others just increase their predatory activity and actually maintain a similar growth rate despite lower photosynthesis. Now, the corals are not receiving 12 hrs a day at the max. intensity. On a really clear day (few clouds) they get ~ 4 hrs a day of intense light, another 4 - 6 hrs a day of intermediate intensity and ~ 2 hrs of dimmer light. If there are clouds (which there usually are, then those hours are reduced. In a shallow area the light intensity at noon might be 800 PAR units only to drop down to 200 units as a cloud passes and then jump back up to 800 again. The light field in nature is highly variable, especially in shallow water.
Is the higher the PAR the better?
No, and assuming that brighter light is inherently better can and does get folks into 'hot water' so to speak. Photosynthesis is a very energetic process. A lot of energy is being captured and transferred to electrical potential in the cells and eventually chemical potential energy as carbon is fixed. If something happens to disrupt the flow of energy to C-fixation, or if there is more light absorbed than can be adequately quenched photochemically (i.e., if the energy absorbing system--photosystem II can't dump energy fast enough) then the excess energy can cause really nasty damage to the cell. Think of a car overheating... When light intensity becomes high enough to significantly impede photosynthesis we say that the organism is experiencing photoinhibition. Corals in shallow water normally do experience chronic photoinhibition. For a few hours a day, in the shallowest water, the zoox. take a beating from the sun. As long as they are adapted to this regime and the damage isn't too bad they recover and continue growing. Many of these corals in shallower water DO grow faster and are generally in better condition at intermediate depth where they do not experience chronic photoinibition. Now, the processes that cause photoinhibition are always happening no matter what level of light there is, but at lower levels of light the damage is so minimal that photosynthesis is not impeded. At very high light levels there is so much photodamage that it overwhelms the repair systems.
The intensity necessary to induce significant photoinhibition varies dramatically from coral to coral due to inherent differences in the zoox/coral, due to adaptation (e.g., previous light regimes), and due to a variety of environmental conditions (temp, UV, water flow, etc.). Corals adapted to really high light generally aren't significantly photoinhibited until you start talking intensities of 1000 PAR units or so. Corals adapted to dimmer light or with zoox. that are more tolerant of dim light might be significantly harmed by light intensities of only 300 - 500 PAR units. The same coral in stagnant water might be fried with 400 PAR units but is fine up to 1200 PAR units with strong flow... It's all depends on so many factors generalizations really can't be drawn.
Now, at lower light levels more light definitely can increase calcification/photosynthesis. A coral only getting 50 PAR units will probably calcify faster if given 100 or 150 PAR units, though it depends on other factors as well. Many corals are good at switching between more autotrophy (photosynthesis) vs. more hetertrophy (prey capture) depending on what's available while some other corals are not very good at this. A Goniastrea growing in shallow, clear water will rely heavily on light and not very much on capturing prey while the same coral in deeper or more turbid water will capture more prey since less light is available and end up with about the same growth rate either way. A Porites will calcify quickly with lots of light and slowly with less light and probably won't change its prey capture efforts much either way.
Having said all of this, for most corals, in the neighborhood of 200 - 300 umol photons / m2 / s for 10 or 12 hrs a day is going to be pretty good. For corals that will take higher lighting, 400 - 500 umol photons / m2 / s is probably fine, provided flow is strong. For coral that prefer lower lighting, 50 - 150 umol photons / m2 / s is probably more preferable. With other factors, such as feeding, taken into account most corals will do fine at or below the 'preferred' intensity range.
cj
