The profundal zone is a deep zone of an inland body of freestanding water, such as a lake or pond, located below the range of effective light penetration. This is typically below the thermocline, the vertical zone in the water through which temperature drops rapidly. The temperature difference may be large enough to hamper mixing with the littoral zone in some seasons which causes a decrease in oxygen concentrations. The profundal is often defined, in accordance with Thienemann (1925), as the deepest, vegetation-free, and muddy zone of the lacustrine benthal. The profundal zone is often part of the aphotic zone.
The lack of light and oxygen in the profundal zone determines the type of biological community that can live in this region and this is distinctly different from the community in the overlying waters. The profundal macrofauna is therefore characterized by physiological and behavioural adaptations to low oxygen concentration. Chironomidae and Oligochaetae often dominate the benthic fauna of the profundal zone because they possess hemoglobin like molecules to extract oxygen from poorly oxygenated water.