How do lake waters mix seasonally?
In temperate lakes, temperature-dependent water density can create summer and winter stratification with spring and autumn turnover.
Formation process and geographic mechanism
In summer, solar heating leaves a light epilimnion above a cold hypolimnion, with a thermocline limiting mixing. Autumn cooling increases surface density and wind can mix the column. Because freshwater is densest near four degrees Celsius, inverse stratification occurs beneath ice. Spring warming equalizes temperatures and can trigger another turnover.
Spatial outcomes, measurement and management
Turnover carries oxygen downward and nutrients upward. Excess nutrient input can increase bottom decomposition and late-summer oxygen loss, and turnover may temporarily stress fish. Depth, wind exposure, salinity and climate alter mixing. Not every lake turns over twice yearly; tropical, shallow and saline lakes behave differently.