Oxygen enters the water through diffusion. The amount of oxygen that can be dissolved in water depends on the atmospheric pressure, the water temperature and whether the water is salty. For example, at 20 °C (68 °F) and one atmosphere of pressure, a maximum of 8 mg/l of oxygen can dissolve in sea water (35 mg/l salinity) while a maximum of 9 mg/l of oxygen can dissolve in fresh water. The amount of oxygen that can be dissolved in the water decreases by about 1 mg/l for each 10 °C increase in water temperature above 20 °C.