Answer: Pycnocline separates layers of different density.
The pycnocline is a layer having the greatest density gradient in a water body. Ocean currents are generated due to temperature and salinity differences, wind, gravitational force, Coriolis Effect etc. The density gradient also affects the vertical profile and flow in an ocean. The pycnocline comprises both halocline which is the salinity gradient and thermocline which is the temperature gradient and the density changes rapidly with depth. As density depends on salinity and temperature, pycnocline is a function of both halocline and thermocline.