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 concetrations. 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.