It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone. Most food comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters.
The depth of the aphotic zone can be greatly affected by such things as turbidity and the season of the year. The aphotic zone underlies the photic zone, which is that portion of a lake or ocean directly affected by sunlight.
The aphotic zone of the ocean is 0.9 - 4.6 km (3,000 - 15,000 feet) deep and the temperatures range from 0 - 6°C (32 - 43°F). Unusual and unique creatures dwell in this expanse of pitch black water, such as the gulper eel, the giant squid, the anglerfish, and the vampire squid.
The aphotic zone is divided into two parts- the bathyal zone and the abyssal zone. The bathyal zone extends from 200 meters to 2000 meters. The abyssal zone extends from 2000 meters to the bottom. Creatures in this area must be able to live in complete darkness.