The mesopelagic zone includes the region of sharp changes in temperature, salinity and density called the thermocline, halocline, and pycnocline. The temperature variations are large; from over 20 °C (68 °F) at the upper layers to around 4 °C (39 °F) at the boundary with the bathyal zone. The variation in salinity is smaller, typically between 34.5 and 35 psu. The density ranges from 1023 to 1027 g/kg of seawater. These changes in temperature, salinity, and density induce stratification which create ocean layers. These different water masses affect gradients and mixing of nutrients and dissolved gasses. This makes this a dynamic zone.