It extends from the atmosphere-water interface downwards to a depth where light intensity falls to 1 percent of that at the surface (also called euphotic depth), so its thickness depends on the extent of light attenuation in the water column.
It extends from the atmosphere-water interface downwards to a depth where light intensity falls to one percent of that at the surface, called the euphotic depth. Accordingly, its thickness varies widely on the extent of light attenuation in the water column.