A thermocline is a zone of rapid temperature change with depth in a body of water. It is the boundary between two layers of water that have different temperatures, in a lake, estuary, or an ocean. The thermocline is marked by a dramatic change in temperature, where the water temperature changes at least one Celsius degree with every meter of depth. Because of density differences associated with a change in temperature, thermoclines can prevent mixing of nutrients from deep to shallow water, and can therefore cause the surface waters of some lakes and ocean to have very low primary productivity, even when there is sufficient light for phytoplankton to grow. In other areas, the thermocline can prevent mixing of oxygen-rich surface waters with bottom waters in which oxygen has been depleted as a result of high rates of decomposition related to eutrophication. In temperate freshwater lakes, the thermocline is disrupted in fall when the surface waters become denser as decreasing air temperatures cool the surface of the lake. Cooler water is denser and sinks, thereby causing warmer bottom water to rise to the surface and mix nutrients throughout the water column.