Final answer:
Deep water circulation is driven by density, impacted by both temperature and salinity, with temperature changes causing density variations crucial to aquatic systems.
Step-by-step explanation:
The circulation of water at great depths is primarily driven by density, which is a function of both temperature and salinity. Factors such as temperature change and gravitational acceleration play a significant role in density variances. When water cools, it becomes denser until it reaches approximately 4°C, at which point it is the densest. This is why during winters, ice forms only on the surface, and liquid water below can sustain aquatic life. The pressure in a static fluid, like water, only depends on the depth due to the weight of the fluid above a particular level, assuming constant density, which is true for most liquids under standard conditions.