1. ecology, hydrology the complete mixing of a lake that occurs when the epilimnion and hypolimnion become isothermal 2. ecology, soils the production and death of roots (usually fine roots) over a certain period of time 3. wildlife the movement of year classes into, through, and out of a population —note the rate of movement is commonly specified by the time required for a given year class to die, e.g., a three-year turnover —see mortalityThis definition last updated 10/10/2008