The zone lacking oxygen, muddy bottoms, and sediments with absolutely no oxygen are said to be anoxic.

This anoxic zone has more organic material to decay and use up oxygen. Secondly, the flow of water that brings in new oxygen is reduced. If you go very far down at all, the oxygen is completely used up. Anoxic conditions are no problem for bacteria that are capable of anaerobic respiration, in which they break down organic matter without oxygen. A poisonous gas hydrogen sulphide is produced in this zone. This gas turns things black and smells like rotten eggs. Animals living in this anoxic zone have adapted to low-oxygen environments.