Pycnocline
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A pycnocline is a rapid change in water density with depth. In freshwater environments such as lakes this density change is primarily caused by water temperature, while in seawater environments such as oceans the density change may be caused by changes in water temperature and/or salinity.
Pycnoclines tend to disappear at around 50 or 60 degrees North or South Latitude. This is due to lowered salinity and temperature change near the poles.