This is the intensity of light where the plant is using all of it's pigments and is not wasting energy as heat and fluorescence. OVER saturation, and the plant has to work to get rid of the energy. UNDER saturation, and the plant will re-arrange the pigments to be more efficient with limited light.
120uE/cm2 is a good general saturation intensity to maximize growth.
This image is from the above experiment:
Water temperature was 38C with only glass protecting the culture, and 32C with the glass and the window screen.