Vertical stratification including an aerobic surface layer, an anaerobic bottom layer, and a facultative intermediate layer is essential to proper functioning of a facultative lagoon ecosystem.  Stratification is maintained by a thermal gradient of cool, dense water at the bottom of the lagoon overlain by warmer, less dense water on the surface.  This thermal gradient becomes unstable when water reaches its maximum density at 4 degrees Celsius (39 degrees Fahrenheit).  Facultative lagoons are impractical in cold climates, because the lagoons become non-functional when cooler air temperatures depress water temperatures below this critical value.