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The thermocline depth is defined by the depth in the water column where the temperature changes most rapidly. The depth of the thermocline has important consequences on the biology and chemistry of a lake.
Thermal Stratification
Thermal stratification refers to the layering that occurs, particularly in the warm months, during which a warmer, less dense layer (the epilimnion) overlies a colder denser layer (the hypolimnion). Thermal stratification is a common occurrence in deep northern temperate lakes such as Onondaga Lake.Between these two layers is a third layer (the metalimnion) where strong vertical differences (gradients) in temperature, and therefore, density prevail. The vertical temperature profile shown below depicts this layering for a hypothetical summer profile.
Thermal stratification is widely considered to be an important regulator of the overall metabolism of a lake. The epilimnion is usually relatively well mixed, as it is subject to mixing induced from the wind. In contrast, mixing is much more limited in the hypolimnion because these deeper lake layers are isolated from energy inputs imparted to the lake’s surface. Exchange of dissolved substances between the epilimnion and hypolimnion (across the metalimnion) is quite limited because of the low level of turbulence/mixing. Generally the greater the temperature/density gradient of the metalimnion, the less exchange across this layer. This limited mixing has important implications for the cycling of critical constituents such as nutrients and dissolved oxygen.
The thermocline depth is defined by the depth in the water column where the temperature changes most rapidly
 Lakes in this climate experience major seasonal changes in the thermal stratification regime that are coupled to the seasonality of meteorological conditions. In early spring, after the loss of ice-cover, vertically uniform low temperatures are observed top to bottom. The absence of vertical temperature gradients allows mixing to occur throughout the water column with only modest energy (wind) input. This interval of uniform temperatures is referred to as spring turnover. Thermal stratification develops when surface waters are heated more rapidly (from increasing air temperature and solar radiation) than the heat  can be distributed by vertical mixing. Progressive increases in the temperature of the epilimnion occur during summer, accompanied by increases in the temperature/density gradient in the metalimnion. Cooling of the epilimnion starts in late summer or early fall as air temperatures and solar radiation inputs decrease. The dimensions of the epilimnion deepen progressively until vertically isothermal conditions develop – the onset of fall turnover.
The features of thermal stratification of a particular system, such as the timing of turnover and the onset of stratification, the vertical dimensions of the layers, and the temperature of the layers, are manifestations of a number of system-specific characteristics and the influence of various environmental (forcing) conditions. In particular, these features are regulated by basin morphometry (size, shape, depth), and setting, attendant meteorological conditions, hydrology, and the extent of light penetration. Substantial year-to-year variations in the features of stratification can occur as result of natural variations in meteorological conditions.
Estimated Thermocline Depth Time-Series
Current year’s
2000
2001
2002
2003
2004
2005
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