hipolimnion

The hypolimnion, sometimes spelled hipolimnion, is the lowest layer of water in a stratified lake or reservoir during the warm season. It lies beneath the metalimnion (thermocline) and the epilimnion, forming a relatively stable, cold, and dark water body as the upper layers warm and stratify.

Physically, the hypolimnion is characterized by low temperatures that are often near 4°C, the temperature at which freshwater is densest. Its depth varies with season, lake size, and climate, expanding during summer stratification and thinning during autumn and spring overturns. In permanently stratified (meromictic) systems, the hypolimnion can remain isolated from the surface for long periods, potentially developing distinct chemical conditions.

Chemically and biologically, the hypolimnion is commonly low in dissolved oxygen, especially in productive or deep lakes where respiration and decomposition consume oxygen faster than it is replenished by mixing. Under hypoxic or anoxic conditions, reduced compounds such as hydrogen sulfide and methane may accumulate, and nutrient release from sediments can intensify—a process known as internal loading. The limited light and cold temperatures slow photosynthesis and influence microbial communities and nutrient cycling within this layer.

Seasonal dynamics often involve mixing events. In spring and autumn, wind-driven turnover homogenizes the water column, re-oxygenates the hypolimnion, and redistributes nutrients. In winter, ice cover and reduced mixing alter its temperature and chemistry. Understanding the hypolimnion is important for assessing lake oxygen regimes, nutrient dynamics, and the ecological responses of aquatic systems.