Density of water

Fresh water and seawater have very different physical properties.

Seawater: Seawater is saline and the salt affects the density. The density of surface seawater ranges from about 1020 to 1029 kg/m$^3$, which is higher than the density of fresh water. The density depends on the temperature and salinity and it increases with depth and the water is stratified all the way to the sea floor, although in >2000 m depth there is only a small increase. In the Atlantic Ocean, the density ranges from 1024 at the surface to 1028 kg/m$^3$ in a depth of >2000 m.
Freshwater: As the air temperature falls the temperature of the water at the surface of a freshwater lake decreases and its density changes. The maximum density is at 3.98°C. As a lake cools to this temperature in winter the surface waters will sink through convection and warmer water rises to the surface. With continued cooling this warmer surface water also becomes dense and sinks. As the surface water is being cooled the lake will become stratified. That is, the density will increase with depth to almost 1000 kg/m$^3$, and the deep water will have a temperature of 3.98°C. Imagine the convection continuing until all the water has reached 3.98°C. Eventually the surface layer of water will be cooled to the freezing point (0°C) and ice will form on the surface. The temperature structure throughout the lake will show three layers: a cold surface layer with 0°C, an intermediate layer and a warm deeper layer with 4°C. The Density structure is mirroring the shape: a surface layer with lower density of 999.85 kg/m$^3$, an intermediate layer and a a deeper layer with higher density of 999.97 kg/m$^3$.

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