Photosynthesis: describe question

A student investigated the effect of light intensity on the rate of photosynthesis of the aquatic plant Elodea canadensis. The rate was measured by counting the number of oxygen bubbles released per minute at different distances from a lamp. The results showed that at low light intensities, the bubble count increased as light intensity increased. However, at high light intensities, the bubble count levelled off and did not increase further despite further increases in light intensity.

Describe why the rate of photosynthesis plateaus at high light intensities.