[Witfull]

another refference on sunrise and light penetration on reefs.

here

The sun begins the day’s journey toward its zenith and the clear ocean water begins to assume the color of the reflected light. What was once a black ocean turns to gray; when the sun is higher, the sea will turn to blue. The morning sun falls upon the coral. Within the tissues, the zooxanthellae awaken and begin to use the small amount of energy. They begin to produce small amounts of oxygen through the process we call photosynthesis. Radiant energy increases as the solar disk climbs higher in the sky. The photosynthetic pigments within the coral’s zooxanthellae work harder and produce more oxygen. At some point, the oxygen produced by the zooxanthellae will meet their and the coral’s respiratory needs and they will no longer gather dissolved oxygen from the water (Scientists would say that they have reached the Compensation Point)..... As the sun begins its slow march to dusk, less energy penetrates the warm ocean and once again the coral’s zooxanthellae photosynthesize at a rate proportional to the amount of light falling upon them. At sunset, just as earlier in the day, they will again reach the Compensation Point and begin to extract oxygen from the water in order to survive. They will continue to do so until the next morning’s dawn and the process will begin anew.