Light penetrates into water.
The photographic zone (also called euphotic zone or epipelagic zone) is the upper zone of the water column in a lake or ocean, where enough sunlight penetrates photosynthesis. The depth of the photo zone may vary considerably by seasonal water depletion. The word euphotic is derived from the ancient Greek for well-lit: εὖ: good, comprehensive and φῶς: light.
The photo zone is considered to hold at a depth where the light intensity is only 1% of the water surface value. This depth is called the eupotic depth and depends on the amount of light attenuation in the water column. The eupotic depth can range from only a few cm deep into cloudy eutrophic lakes to around 200 m depth in open ocean waters.
At deep-watered hydrothermal sources at mid-oceanic ridges, the photo zone is the only place in the water where primary organic material production takes place. Eupotic depth is usually dependent on primary production in a particular surface water. The eupotic depth is determined by measuring the light transmittance of the water. This can, for example, be very simple with a sec. Disc. Also see
wiki
