Originally Posted by Hoppy
Light reflects off the water surface in increasing amounts as the angle between the vertical and the light increases - at 0 degrees, straight down into the water, virtually no light is reflected, but at 90 degrees, parallel to the water, or close to it, most of the light is reflected. When the surface is rippled, a lot of the surface is at lower angles to the incident light even way off to the side from the light. So, that might lead to a higher PAR. But, I haven't tested this idea.
On my former reef tank I actually measured my par using an apogee meter with still water and aggressive surface movement. At a depth of 24" I would measure approximately 350 uMol with still water but during aggressive surface aggitation it would fluctuate between 345-355. From a practical standpoint it averages out and wasn't something I was concerned with.
For reference this was with 108 leds and 16 x 39 watt t5s over a 265 gallon tank. These results may not scale well with less intensity.