profundal zone

The profundal zone is the deep-water area of a lake that lies beyond the reach of sunlight. This region is characterized by darkness and is home to organisms adapted to these conditions. In thermally stratified lakes, it extends from the lower part of the hypolimnion to the lake bottom, or down to 600 meters in very deep lakes. Unlike the upper zones where light supports photosynthesis, the profundal zone relies on a food supply that sinks from the surface waters. Organisms here are adapted to low temperatures and limited oxygen.123