The bottom layer should be a saturated solution, with some solid salt on the bottom to ensure that it stays saturated. Because of its high salt content, it will remain denser than an upper layer of fresh water even with the bottom saturated-salt layer approaching the boiling point 100 degrees C and the top fresh-water layer at typical ambient temperatures of say 20 degrees to 30 degrees C. This will prevent mixing by convection. Mixing by diffusion will still occur, but it is much slower than mixing by convection. Of course all mixing can be prevented by using only a saturated salt layer with a cover such as glass or some clear plastics that admits visible light but blocks infrared radiation from the pond. This cover will also reduce heat losses to the air.
Maintaining the density differences in the three zones Upper Convective Zone (UCZ ), NCZ and LCZ because Temperature is also proportional to density variations