Thermocline

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.

[Marie H. Bundy ]

thermocline A steep temperature gradient that exists in the middle zone (the metalimnion) of a lake and gives rise to thermally induced vertical stratification of the water. The metalimnion lies between the relatively warm epilimnion above and the cold hypolimnion below. The thermocline may represent a temperature change of 1°C for every incremental depth of 1 metre of water. It may be short-lived, especially in shallow lakes where wind action can mix the water from different levels. However, it can exist for most of the summer period in temperate lakes and sometimes nearly all year in tropical lakes. A thermocline can speed up the process of eutrophication by preventing the diffusion of oxygen from the epilimnion to the hypolimnion (see eutrophic).

thermocline Generally, a gradient of temperature change, but applied more particularly to the zone of rapid temperature change between the warm surface waters and cooler waters at depth. In the oceans, this zone of rapid temperature change starts at 10–500m below the surface and can extend down to more than 1500m. In polar regions the thermocline is generally absent since the ocean surface is covered with ice in winter and solar radiation is small in summer. In thermally stratified lakes in summer the thermocline separates the warm surface waters (epilimnion) from the cooler deep waters (hypolimnion).

thermocline Generally, a gradient of temperature change, but applied more particularly to the zone of rapid temperature change between the warm surface waters (epilimnion) and cooler deep waters (hypolimnion) in a thermally stratified lake in summer. In the oceans this zone of rapid temperature change starts 10–500 m below the surface and can extend down to more than 1500 m. In polar regions the thermocline is generally absent, because the ocean surface is covered with ice in winter and solar radiation is small in summer.

thermocline Generally, a gradient of temperature change, but applied more particularly to the zone of rapid temperature change between the warm surface waters (epilimnion) and cooler deep waters (hypolimnion) in a thermally stratified lake in summer.