The lowest zone of a lake is known as the benthic zone. This zone encompasses the very bottom of the lake, including the sediment surface and the sub-surface layers. In very deep lakes, the portion of the benthic zone that lies below the depth where light can penetrate is often referred to as the profundal zone. Because light does not reach these depths, photosynthesis cannot occur, meaning there is no plant life. Instead, the benthic zone is populated by "decomposers" such as bacteria, fungi, and detritivores (like certain types of worms, snails, and insect larvae) that feed on the "organic rain" of dead matter drifting down from the upper layers. In deep, cold-water lakes, this zone is characterized by low oxygen levels and constant, chilly temperatures. The life forms found here are specifically adapted to high pressure and low-resource environments. This zone is ecologically vital because it acts as the lake's "recycling center," breaking down organic waste into nutrients that are eventually cycled back to the upper layers (the limnetic and littoral zones) during seasonal "turnovers," where the lake's water layers mix due to temperature changes.