The aphotic zone (aphotic from Greek prefix ἀ- + φῶς "without light") is the portion of a lake or ocean where there is little or no sunlight. 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 in this zone 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 ocean
Depending on how aphotic zone is defined, the aphotic zone of the ocean begins between depths of roughly 200 m (660 ft) to 1,000 m (3,300 ft), and extends to the ocean floor. Temperatures can range from roughly 0 °C (32 °F) to 6 °C (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 further divided into additional zones: the bathyal zone, the abyssal zone, and the hadal zone. The bathyal zone extends from 200 metres (656 ft) to 2,000 metres (6,562 ft). The abyssal zone extends from 2,000 metres (6,562 ft) to 6,000 metres (19,685 ft). The hadal zone spans from depths of 6,000 metres (20,000 ft) to the ocean floor. Creatures in these areas must be able to live in complete darkness.