Density stratification refers to the separation of water into separate and distinct horizontal layers according to differences in density. This process generally occurs due to variances in temperature or the presence of dissolved and suspended solids.
Density stratification serves as an important environmental function. This naturally occurring phenomenon prevents the conditions that create an anoxic event from forming in the ocean. An anoxic event refers to certain times throughout Earth's history when large geographical portions of the ocean became depleted of oxygen.
During these anoxic events, euxinia can develop. Euxinia refers to anoxic waters that contain hydrogen sulfide. These anoxic events, although not present in recent history, coincide with several mass extinctions. Scientists believe that past anoxic events contributed to the mass extinctions via an apparent link to the slowing of ocean circulation, climatic warming and an increased level of greenhouse gases. Enhanced volcanic activity/eruption is believed to be the predominant external factor for anoxic events.
Density stratification also applies to lakes. Lakes are separated into three layers: the top of the lake is referred to as the epilimnion, the middle layer is called the metalimnion and the bottom layer is referred to as the hypolimnion. Lake stratification is attributed to changes in the water's density at different temperatures.
Lake stratification can cause environmental and natural resource problems including diminishing fish populations, contributing to excessive plankton growth and adversely affecting the quality of drinking water.