The mesopelagic zone, also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; depth ranges from approximately 200 to 1,000 meters below the ocean surface. It is largely nutrient-rich in comparison to the euphotic zone, making the biogeochemical processes of the mesopelagic particularly important to nutrient cycling in the ocean.