1. That depth at which density differences occurring in the earth’s crust are compensated isostatically; calculated to be between 62 and 70-73 miles (100 and 113-117 kilometers). 2. The depth in a body of water at which illuminance has diminished to the extent that oxygen production through photosynthesis and oxygen consumption through respiration by plants are equal; it is the lower boundary of the euphotic zone.
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